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{{short description|Plant considered undesirable in a particular place or situation}}
{{About|plants specifically called weeds}}
{{About|plants specifically called "weeds"|the psychoactive plant commonly called "weed"|Cannabis (drug)|other uses|Weed (disambiguation)}}
{{pp-semi-indef}}
{{pp-move-indef}}
]'', '']'', '']'', '']'')]]
A '''weed''' is a ] considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals.<ref name="bridges">{{cite journal |last1=Bridges |first1=David C. |title=Impact of Weeds on Human Endeavors |journal=Weed Technology |date=1994 |volume=8 |issue=2 |pages=392–395 |doi=10.1017/S0890037X00038987 |jstor=3988124 |s2cid=90116503 }}</ref><ref name="JRHarlan">Harlan, J. R., & deWet, J. M. (1965). Some thoughts about weeds. ''Economic botany'', ''19''(1), 16-24.</ref><ref>{{cite web |title=Define the term weed. |url=https://forages.oregonstate.edu/nfgc/eo/onlineforagecurriculum/instructormaterials/availabletopics/weeds/definition |website=forages.oregonstate.edu |date=June 2009 |publisher=Oregon State University Forage Information System}}</ref><ref name="SnohomishEx">{{cite web |title=What is a Weed |url=https://extension.wsu.edu/snohomish/garden/gardening-resources/weed-id-and-management/what-is-a-weed/ |publisher=Snohomish County Extension Office}}</ref> Plants with characteristics that make them hazardous, aesthetically unappealing, difficult to control in managed environments, or otherwise unwanted in ], ], ]s, ]s, ], recreational spaces, residential and industrial areas, may all be considered weeds.<ref name="SnohomishEx" /><ref name="JRHarlan"/><ref name="Bio&EcoWeeds">Holzner, W., & Numata, M. (Eds.). (2013). ''Biology and ecology of weeds'' (Vol. 2). Springer Science & Business Media.{{pn|date=July 2022}}</ref> The concept of weeds is particularly significant in ], where the presence of weeds in fields used to grow ]s may cause major losses in yields.<ref name="GrandChallenge">{{cite journal |last1=Chauhan |first1=BS |title=Grand Challenges in Weed Management |journal=Front. Agron. |date=2020 |volume=1 |issue=3 |doi=10.3389/fagro.2019.00003 |doi-access=free }}</ref> ], plants introduced to an environment where their presence negatively impacts the overall functioning and biodiversity of the ecosystem, may also sometimes be considered weeds.<ref>{{cite book |last1=Nakano |first1=Michelle |title=Red Seal Landscape Horticulturist Identify Plants and Plant Requirements |publisher=Kwantlen Polytechnic University |url=https://kpu.pressbooks.pub/plant-identification/chapter/characteristics-of-weedy-species/ |chapter=21: Characteristics of weedy species|date=13 February 2020 }}</ref><ref name="Janick 1979 308">{{cite book | isbn = 0-7167-1031-5 | last = Janick | first = Jules | title = Horticultural Science | location = San Francisco | publisher = W.H. Freeman | year = 1979 | page = | edition = 3rd | url = https://archive.org/details/horticulturalsci0000jani_x4s4/page/308 }}</ref>


], the term "weed" has no botanical significance, because a plant that is a weed in one context, is not a weed when growing in a situation where it is wanted. Some plants that are widely regarded as weeds are intentionally grown in gardens and other cultivated settings. For this reason, some plants are sometimes called ]s. Similarly, ] from a previous crop are regarded as weeds when growing in a subsequent crop. Thus, alternative nomenclature for the same plants might be hardy pioneers, cosmopolitan species, volunteers, "spontaneous urban vegetation," etc.<ref>{{Cite book |last=Stromberg |first=Juliet C. |title=Bringing Home the Wild: A Riparian Garden in a Southwest City |publisher=University of Arizona Press |year=2023 |isbn=978-0-8165-5028-9 |location=Tucson |pages=29 |language=en-us}}</ref>
{{See also|Invasive species}}
{{TOCright}}


Although whether a plant is a weed depends on context, plants commonly defined as weeds broadly share biological characteristics that allow them to thrive in ] environments and to be particularly difficult to destroy or eradicate. In particular, weeds are adapted to thrive under human management in the same way as intentionally grown plants.<ref name="bridges"/> Since the origins of agriculture on Earth, agricultural weeds have co-evolved with human crops and agricultural systems, and some have been domesticated into crops themselves after their ] in agricultural settings became apparent.<ref name="Co-evolution">{{cite journal |last1=Guglielmini |first1=A.C. |last2=Ghersa |first2=C.M. |last3=Satorre |first3=Emilio Horacio |title=Co-evolution of domesticated crops and associated weeds |journal=Ecologia Austral |date=2007 |volume=17 |issue=1}}</ref>
] is a common weed all over the world, especially in Europe, Asia and the Americas.]]
A '''weed''' in a general sense is a ] that is considered by the user of the term to be a nuisance, and normally applied to unwanted plants in human-controlled settings, especially ] and ]s, but also ]s, ]s, ], and other areas. More specifically, the term is often used to describe ] or nonnative plants that grow and reproduce aggressively.<ref>{{cite book | isbn = 0-7167-1031-5 | last = Janick | first = Jules | title = Horticultural Science | location = San Francisco | publisher = W.H. Freeman | year = 1979 | page = 308 | edition = 3rd}}</ref> Generally, a weed is a plant in an undesired place.


More broadly, the term "weed" is occasionally applied pejoratively to species outside the plant kingdom, species that can survive in diverse environments and reproduce quickly; in this sense it has even been applied to ]s.<ref name=Quammen>{{citation |title=Planet of Weeds |author=David Quammen |magazine=Harper's Magazine |date=October 1998 |url=http://sep.csumb.edu/class/ESSP645/readings/Quammen%201998.pdf |access-date=November 15, 2012}}</ref>
In ''Weeds of the West'',<ref>{{cite book | isbn = 0-941570-13-4| last = Tom | first = Whitson, ed. | title = Weeds of the West | publisher = Western Society of Weed Science | year = 1992 | page = ix}}</ref> the authors determined which plants to include in the book based on the following criterion, attributed to J.M. Torrell:


] is important in agriculture and ]. Methods include hand cultivation with ], powered cultivation with ]s, smothering with ] or ], lethal ] with high heat, burning, or chemical attack with ]s and cultural methods such as crop rotation and fallowing land to reduce the weed population.<ref>Blackshaw, R. E., Anderson, R. L., & Lemerle, D. E. I. R. D. R. E. (2007). Cultural weed management. ''Non-Chemical Weed Management: Principles, Concepts and Technology, Wallingford, UK: CAB International'', 35-48.</ref>
<blockquote>A plant that interferes with management objectives for a given area of land at a given point in time.</blockquote>


==History==
Weeds may be unwanted for a number of reasons: they might be unsightly, or crowd out or restrict light to more desirable plants (especially crop plants) or use limited ]s from the ]. They can harbor and spread plant ]s that infect and degrade the quality of ] or ] plants. Some weeds are a nuisance because they have thorns or prickles, some have chemicals that cause skin irritation or are hazardous if eaten, or have parts that come off and attach to fur or clothes.
It has long been assumed that weeds, in the sense of rapidly-evolving plants taking advantage of human-disturbed environments, evolved in response to the ] approximately 12,000 years ago. However, researchers have found evidence of "proto-weeds" behaving in similar ways at ], a 23,000-year-old archeological site in ].<ref>{{cite journal|last1=Ainit Snir|title=The Origin of Cultivation and Proto-Weeds, Long Before Neolithic Farming|journal=]|date=22 July 2015|doi=10.1371/journal.pone.0131422|display-authors=etal|volume=10|issue=7|pages=e0131422|pmc=4511808|pmid=26200895|bibcode=2015PLoSO..1031422S|doi-access=free}}</ref>


The idea of "weeds" as a category of undesirable plant has not been universal throughout history. Before 1200 A.D., little evidence exists of concern with weed control or of agricultural practices solely intended to control weeds. A possible reason for this is that for much of human history, women and children were an abundant source of cheap labor to control weeds, and not directly acknowledged.<ref>{{cite web |last1=Bell |first1=Carl E. |title=A Historical View of Weed Control Technology |url=https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=17593 |website=UC Weed Science}}</ref> Weeds are assumed to have existed since the beginning of agriculture, and accepted as an "inevitable nuisance."<ref>{{cite book |last1=Holm |first1=LeRoy G. |last2=Plucknett |first2=Donald L. |last3=Pancho |first3=Juan V. |last4=Herberger |first4=James P. |title=The World's Worst Weeds: distribution and biology |date=1977 |publisher=University Press of Hawaii |location=Honolulu |page=ix |isbn=0-8248-0295-0}}</ref>
The term ''weed'' in its general sense is a subjective one, without any classification value, since a "weed" is not a weed when growing where it belongs or is wanted. Indeed, a number of "weeds" have been used in gardens or other cultivated-plant settings. An example is the ], '']'', which was a common field weed exported from ] along with ], but now sometimes grown as a garden plant.<ref>{{cite web | url = http://davesgarden.com/pf/go/1341/ | work = PlantFiles | title = Detailed information on Corn Cockle (Agrostemma githago) | accessdate = February 15, 2009}}</ref>


Though the plants are not named using a specific term denoting a "weed" in the contemporary sense, plants that may be interpreted as "weeds" are referenced in the ]:<ref name="Janick 1979 308"/>
Professor ] of ] defines weeds as plants that create environmental conditions in which they themselves cannot reproduce. He takes the example of pine trees that crowd out sunlight such that their own offspring cannot grow. Weeds continue to exist, because the environment is continually being disturbed to create open conditions for new generations, such as forest fires and human activity.<ref>A speech given on the radio program ''Big Ideas'', (5/11/2003): "A weed is literally a plant... which comes into a disturbed habitat, which then changes the nature of the soil, the shading, and everything, and the moisture, ectcetera, in such a way that it cannot reproduce itself in that habitat."</ref>


{{blockquote|Cursed is the ground because of you; through painful toil you will eat of it all the days of your life. It will produce thorns and thistles for you, and you will eat the plants of the field. By the sweat of your brow you will eat your food until you return to the ground.<ref>Genesis 3:17-19 New International Version</ref>}}
==Distribution==
], a ] native to southern Europe and the Middle East that is an invasive weed in parts of ].]]
Weeds generally share similar ]s that give them advantages and allow them to proliferate in disturbed environments whose soil or natural vegetative cover has been damaged. Naturally occurring disturbed environments include ]s and other windswept areas with shifting soils, ] flood plains, ]s and ]s, and areas that are often burned. Since human agricultural practices often mimic these natural environments where weedy species have evolved, weeds have adapted to grow and proliferate in human-disturbed areas such as agricultural fields, lawns, roadsides, and construction sites. The weedy nature of these species often gives them an advantage over more desirable crop species because they often grow quickly and ] quickly, have seeds that persist in the ] for many years, or have short lifespans with multiple generations in the same growing season. Perennial weeds often have underground stems that spread out under the soil surface or, like ground ivy ('']''), have creeping stems that root and spread out over the ground.<ref>{{cite web | url = http://employees.csbsju.edu/ssaupe/biol327/Lecture/Plant_Foraging_case_study.pdf | title = Plant Foraging: Two Case Studies | first = Stephen G. | last = Saupe | accessdate = February 15, 2009}}</ref>


Some early Roman writers referenced weeding activities in agricultural fields, but weed control in the pre-modern era was probably an incidental effect of plowing.<ref>{{cite journal |last1=Timmons |first1=F.L. |title=A History of Weed Control in the United States and Canada |journal=Weed Science |date=2005 |volume=53 |issue=6 |pages=748–761 |doi=10.1614/0043-1745(2005)0532.0.CO;2 |jstor=4046973 |s2cid=86059980 |url=https://www.jstor.org/stable/4046973}}</ref> Ancient Egyptians, Assyrians, and Sumerians had no specific word for "weeds," seeing all plants as having some use. The English word "weed" can be traced back to the Old English ''weod'', which refers to ], rather than a category of plant as in the modern usage; in early medieval European herbals, each plant is regarded as having its own "virtues".<ref name="Clayton">{{cite journal |last1=Clayton |first1=Neil |title=Weeds, People, and Contested Places |journal=Environment and History |date=2003 |volume=9 |issue=3 |page=304, 306, 308-309 |doi=10.3197/096734003129342863 |jstor=20723295 |url=https://www.environmentandsociety.org/sites/default/files/key_docs/clayton-9-3.pdf|url-access= |url-status= |archive-url= |archive-date= }}</ref>
Some plants become dominant when introduced into new environments because they are freed from specialist consumers; in what is sometimes called the “natural enemies hypothesis,” plants freed from these specialist consumers may increase their competitive ability. In locations were predation and mutual competitive relationships no longer exist, some plants are able to increase allocation of resources into growth or reproduction. The weediness of some species that are introduced into new environments can be caused by the introduction of new chemicals; sometimes called the "novel weapons hypothesis," these introduced ]ic chemicals, which indigenous plants are not yet adapted to, may limit the growth of established plants or the germination and growth of seeds and seedlings.<ref name=Willis2007>{{Cite book| title = The History of Allelopathy| url = http://www.google.com/books?id=C-nPBYjDAjYC&pg=PA3&| year = 2007| author = Willis, Rick J.| journal = Springer| page = 8| isbn = 140204092X| accessdate = 2009-08-17}}</ref><ref>{{cite web|url=http://plantecology.dbs.umt.edu/Full%20text%20papers%20and%20abstracts/2004%20papers/Callaway&Ridenour2004NOVEL.pdf |title=Callaway.qxd |format=PDF |date= |accessdate=2010-03-20}}</ref>


By the sixteenth century, the concept of a "weed" was better defined as a "noxious" or undesirable type of plant, as referenced metaphorically in ]'s works.<ref name="Clayton" /> An example of a ] reference to weeds is found in ]:
== Relation to humans ==
{{blockquote|To thy fair flower add the rank smell of weeds: / But why thy odour matcheth not thy show, / The soil is this, that thou dost common grow.<ref name="knox">{{cite book |title=The Works of Shakespeare: Sonnets |editor-first=C Knox |editor-last=Pooler |series=The Arden Shakespeare |location=London |publisher=Methuen & Company |date=1918 |url=https://archive.org/details/sonnetseditedbyc00shakuoft |oclc=4770201}}</ref>}}
As long as humans have cultivated plants, weeds have been a problem. Weeds have even been mentioned in religious and literature texts like the following quotes from ] and a ] ]:


In London during this period, poor women were paid low wages to weed gardens and courtyards.<ref name="morethanweeds">{{cite web |title=A history of weeding |url=https://morethanweeds.co.uk/a-history-of-weeding/ |website=More than Weeds|date=24 April 2020 }}</ref>
<blockquote>"Cursed is the ground because of you; through painful toil you will eat of it all the days of your life. It will produce thorns and thistles for you, and you will eat the plants of the field. By the sweat of your brow you will eat your food until you return to the ground,"<ref>Genesis 3:17-19 New International Version</ref></blockquote>


After the Reformation, Christian theology that emphasized the degradation of nature after the ], and humankind's role and duty to dominate and subdue nature, became more developed and widespread. Various European writers designated certain plants as "vermin" and "filth," though many plants identified as such were valued by gardeners or by herbalists and apothecaries, and some questioned the idea that any plant could be without purpose or value.<ref name="Clayton" /> Laws mandating the control of weeds emerged as early as the seventeenth century; in 1691 a law in New York required the removal of "poysonous and Stincking Weeds" in front of houses.<ref name="Falck">{{cite journal |last1=Falck |first1=Zachary J.S. |title=Controlling the Weed Nuisance in Turn-of-the-Century American Cities |journal=Environmental History |date=2002 |volume=7 |issue=4 |pages=613, 616, 621 |doi=10.2307/3986059 |jstor=3986059 |url=https://www.jstor.org/stable/3986059}}</ref>
<blockquote>"To thy fair flower add the rank smell of weeds: But why thy odour matcheth not thy show, The soil is this, that thou dost common grow."<ref>{{cite book | authorlink = William Shakespeare | first = William | last = Shakespeare | url = http://www.infoplease.com/t/lit/shakespeare-sonnets/69.html | title = Those parts of thee that the world's eye doth view | publisher = ] | accessdate = February 15, 2009}}</ref></blockquote>


In the nineteenth century, manual labor was used to control weeds in European towns and cities, and chemical methods of weed control emerged. For example, a French journal in 1831 documented a mixture of sulfur, lime and water boiled in an iron cauldron as an effective herbicide to prevent grass from growing among cobblestones.<ref name="morethanweeds"/>
]


The cultural association between weeds and moral or spiritual degradation persisted into the last nineteenth century in American cities. Urban expansion and development created ideal habitats for weeds in nineteenth-century America.<ref name="Falck" /> Reformers consequently saw weeds as a part of the larger problem of filth, disease, and moral corruption that plagued the urban environments, and weeds were seen as refuge for "tramps" and other criminal or undesirable people. The St. Louis Post-Dispatch credited weeds as causing ], ], and ].<ref name="Falck" /> In St. Louis between the years of 1905-1910, weeds became viewed as a major public health hazard, believed to cause typhoid and malaria, and legal precedents were set in order to control weeds that would help facilitate the adoption of weed control laws throughout the country.<ref name="Falck" />
Weed seeds are often collected and transported with crops after the harvesting of ]s. Many weed species have moved out of their natural geographic locations and have spread around the world with humans. (See ].) Not all weeds have the same ability to damage crops and horticultural plants or cause harm to animals. Some have been classified as ]s by governmental authorities because if left unchecked, they often dominate the environment where crop plants are to be grown or cause harm to ]. They are often foreign species mistakenly or accidentally imported into a region where there are few natural controls to limit their population and spread. Many weeds have ideal locations for growth and reproduction because of the large areas of open soil created by the conversion of land to field agriculture. Farming practices that produce unvegetated soils part of the year and human distribution of food crops mixed with seeds of weeds from other parts of the world have facilitated the colonization of vast new areas for many weedy species; humans are the vector of transport and the producer of disturbed environments, thus many weedy species have an ideal association with humans.


==Ecological significance==
A number of weeds, such as the ] '']'', are edible, and their leaves and roots may be used for ] or ]. ] is common weed over much of the world, and is sometimes used to make ] and other medicine in ]. These so-called "]s" may have other beneficial effects, such as drawing away the attacks of crop-destroying ]s, but often are breeding grounds for insects and pathogens that attack other plants. Dandelions are one of several species which break up ] in overly cultivated fields, helping crops grow deeper root systems. Some modern species of domesticated ] actually originated as weeds in cultivated fields and have been bred by people into garden plants for their flowers or foliage.
] is a common plant ], especially in Europe, Asia, and the Americas. It is considered a weed in some contexts (such as ]s) but not others (such as when it is grown as a ] or ]).]]


"Weed" as a category of plant overlaps with the closely related concepts of ] and ].<ref name="TenWays">{{cite journal |last1=Clements |first1=David R. |last2=Jones |first2=Vanessa L. |title=Ten Ways That Weed Evolution Defies Human Management Efforts Amidst a Changing Climate |journal=Agronomy |date=2021 |volume=11 |issue=2|page=284 |doi=10.3390/agronomy11020284 |doi-access=free }}</ref> Pioneer species are specifically adapted to disturbed environments, where the existing plant and soil community has been disrupted or damaged in some way. Adaptation to disturbance can give weeds advantages over desirable crops, pastures, or ornamental plants. The nature of the habitat and its disturbances will affect or even determine which types of weed communities become dominant.<ref>{{Cite book|author=Bell, Graham|title=The Permaculture Garden|publisher=Chelsea Green Publishing|year=2005|isbn=9781856230278|pages=–64|url=https://archive.org/details/permaculturegard0000bell|url-access=registration}}</ref> In weed ecology some authorities speak of the relationship between "the three Ps": plant, place, perception. These have been very variously defined, but the weed traits listed by H.G. Baker are widely cited.<ref>{{Cite journal |last=Baker |first=Herbert G. |date=November 1974 |title=The Evolution of Weeds |url=https://www.annualreviews.org/doi/10.1146/annurev.es.05.110174.000245 |journal=Annual Review of Ecology and Systematics |language=en |volume=5 |issue=1 |pages=1–24 |doi=10.1146/annurev.es.05.110174.000245 |issn=0066-4162}}</ref><ref>Baker H. G. "Characteristics and modes of origin of weeds". In The Genetics of Colonizing Species. H. G. Baker, G. L. Stebbins. eds. New York, Academic Press, 1965, pp. 147-172</ref>
Some people have appreciated weeds for their tenacity, their wildness and even the work and connection to nature they provide. As ] wrote in ''The Well-Tempered Garden''


Examples of such ruderal or ] include plants that are adapted to naturally-occurring disturbed environments such as ]s and other windswept areas with shifting soils, ] flood plains, ]s and ], and areas that are burned repeatedly.<ref name="LambersIII2008">{{cite book|author1=Hans Lambers|author2=F Stuart Chapin III|author3=Thijs L. Pons|title=Plant Physiological Ecology|url=https://books.google.com/books?id=PXBq6jsT5SYC&pg=PA507|date=8 October 2008|publisher=Springer|isbn=978-0-387-78341-3|pages=507–}}</ref> Since human agricultural and horticultural practices often mimic these natural disturbances that weedy species have adapted for, some weeds are effectively ] to grow and proliferate in human-disturbed areas such as agricultural fields, lawns, gardens, roadsides, and construction sites. As agricultural practices continue and develop, weeds evolve further, with humans exerting ] upon weeds through manipulating their habitat and attempting to control weed populations.<ref name="Co-evolution" />
"Many gardeners will agree that hand-weeding is not the terrible drudgery that it is often made out to be. Some people find in it a kind of soothing monotony. It leaves their minds free to develop the plot for their next novel or to perfect the brilliant repartee with which they should have encountered a relative's latest example of unreasonableness."<ref>Christopher Lloyd, The Well-Tempered Garden, 1973</ref>


Due to their ability to survive and thrive in conditions challenging or hostile to other plants, weeds have been considered ]s.<ref>{{cite journal |last1=Sharma |first1=Gourav |last2=Barney |first2=Jacob N. |last3=Westwood |first3=James H. |last4=Haak |first4=David C. |title=Into the weeds: new insights in plant stress |journal=Trends in Plant Science |date=2021 |volume=26 |issue=10 |pages=1050–1060 |doi=10.1016/j.tplants.2021.06.003 |pmid=34238685 |doi-access=free |bibcode=2021TPS....26.1050S }}</ref>
Shunryu Suzuki, the Zen master, is credited with proclaiming, "For Zen students, a weed is a treasure."


===Adaptability===
Perhaps the greatest defense of weeds is contained in the last stanza of Gerard Manley Hopkins' poem ''Inversnaid:''


Due to their evolutionary heritage as disturbance-adapted pioneers, most weeds exhibit incredibly high phenotype plasticity, meaning that individual plants hold the potential to adapt their morphology, growth, and appearance in response to their conditions.<ref name="TenWays" /> The potential within a single individual to adapt to a wide variety of conditions is sometimes referred to as an "all-purpose genotype."<ref name="clements">{{cite journal |last1=Clements |first1=David R. |last2=Jones |first2=Vanessa |title=Rapid Evolution of Invasive Weeds Under Climate Change: Present Evidence and Future Research Needs |journal=Frontiers in Agronomy |date=2021 |volume=3|doi=10.3389/fagro.2021.664034 |doi-access=free }}</ref> Disturbance-adapted plants typically grow rapidly and ] quickly, with some annual weeds having multiple generations in a single growing season. They commonly have seeds that persist in the ] for many years. Perennial weeds often have underground stems that spread under the soil surface or, like ground ivy ('']''), have creeping stems that root and spread out over the ground.<ref>{{cite web | url = http://employees.csbsju.edu/ssaupe/biol327/Lecture/Plant_Foraging_case_study.pdf | title = Plant Foraging: Two Case Studies | first = Stephen G. | last = Saupe | access-date = February 15, 2009}}</ref> These traits make many disturbance-adapted plants highly successful as weeds.<ref name="TenWays" />
"What would the world be, once bereft,<br>
of wet and wildness? Let them be left.<br>
O let them be left; wildness and wet;<br>
Long live the weeds and the wilderness yet."


On top of the ability of individual plants to adapt to their conditions, weed populations also evolve much more quickly than older models of evolution account for.<ref name="clements"/> Once established in an agricultural setting, weeds have been observed to undergo evolutionary changes to adapt to selective pressures imposed by human management. Some examples include changes in seed dormancy, changes in seasonal life cycles, changes in plant morphology, and the evolution of resistance to ]s.<ref name="Co-evolution" /> Rapid life cycles, large populations, and ability to spread large numbers of seeds long distances also allow weed species with these general characteristics to evolve quickly.<ref>{{cite journal |last1=Ziska |first1=Lewis H. |last2=Blumenthal |first2=Dana M. |last3=Franks |first3=Steven J. |title=Understanding the nexus of rising CO2, climate change, and evolution in weed biology |journal=Invasive Plant Science and Management |date=2019 |volume=12 |issue=2|pages=79–88 |doi=10.1017/inp.2019.12 |s2cid=199632010 |doi-access=free }}</ref>
==Examples==

*'']''
===Dispersal===
*] - perennial, spreading by runners, rhizomes and seeds.
The concept of weeds also overlaps with the concept of ], both in the sense that human activities tend to introduce weeds outside their native range, and that an introduced species may be considered a weed. Many weed species have moved out of their natural geographic ranges and spread around the world in tandem with human migrations and commerce. Weed seeds are often collected and transported with crops after the harvesting of ]s, so humans are a vector of transport as well as a producer of the disturbed environments to which weed species are well adapted, resulting in many weeds having a close association with human activities.<ref name="HassanScholes2005">{{cite book|author1=Rashid M. Hassan|author2=Robert Scholes|author3=Neville Ash|title=Ecosystems and Human Well-Being: Current State and Trends: Findings of the Condition and Trends Working Group|url=https://books.google.com/books?id=UFVmiSAr-okC&pg=PA570|date=14 December 2005|publisher=Island Press|isbn=978-1-55963-228-7|pages=570–}}</ref><ref name="Geographic2011">{{cite book|author=National Geographic|title=National Geographic Answer Book: 10,001 Fast Facts about Our World|url=https://books.google.com/books?id=m-vOkEXEARAC&pg=PA175|year=2011|publisher=National Geographic Society|isbn=978-1-4262-0892-8|pages=175–}}</ref>
*]

*] – perennial, spreads by seeds that persist in the soil for many years
Some plants become dominant when introduced into new environments because the animals and plants in their original environment that compete with them or feed on them are absent; in what is sometimes called the "natural enemies hypothesis", plants freed from these specialist consumers may become dominant. An example is ], which threatened millions of hectares of prime grain and grazing land in North America after it was accidentally introduced. The ], a species that specializes in consuming the plant, was imported during World War II. Within several years Klamath weed was reduced to a rare roadside weed.<ref>{{Cite web|url=http://faculty.ucr.edu/~legneref/biotact/ch-66.htm|title=Biological Control of Klamath Weed < ''Hypericum perforatum''|website=faculty.ucr.edu}}</ref><ref>{{cite web |title=Chrysolina quadrigemina |url=https://biocontrol.entomology.cornell.edu/weedfeed/Chrysolina.php |website=biocontrol.entomology.cornell.edu |publisher=College of Agriculture and Life Sciences |access-date=25 July 2023}}</ref> In locations where predation and mutually competitive relationships are absent, weeds have increased resources available for growth and reproduction. The weediness of some species that are introduced into new environments may be caused by their production of ] chemicals which indigenous plants are not yet adapted to, a scenario sometimes called the "novel weapons hypothesis". These chemicals may limit the growth of established plants or the germination and growth of seeds and seedlings.<ref name=Willis2007>{{Cite book| title = The History of Allelopathy| url = https://books.google.com/books?id=C-nPBYjDAjYC&pg=PA3| year = 2007| author = Willis, Rick J.| publisher = Springer| page = 8| isbn = 978-1-4020-4092-4| access-date = 2009-08-17}}</ref><ref>{{cite web |url=http://plantecology.dbs.umt.edu/Full%20text%20papers%20and%20abstracts/2004%20papers/Callaway&Ridenour2004NOVEL.pdf |title=Callaway.qxd |access-date=2010-03-20 |url-status=dead |archive-url=https://web.archive.org/web/20060910062836/http://plantecology.dbs.umt.edu/Full%20text%20papers%20and%20abstracts/2004%20papers/Callaway%26Ridenour2004NOVEL.pdf |archive-date=September 10, 2006 }}</ref> Weed growth can also inhibit the growth of later-successional species in ].<ref>{{Cite journal |last1=Van der Putten |first1=W. H. |last2=Mortimer |first2=S. R. |last3=Hedlund |first3=K. |last4=Van Dijk |first4=C. |last5=Brown |first5=V. K. |last6=Lepä |first6=J. |last7=Rodriguez-Barrueco |first7=C. |last8=Roy |first8=J. |last9=Diaz Len |first9=T. A. |last10=Gormsen |first10=D. |last11=Korthals |first11=G. W. |last12=Lavorel |first12=S. |last13=Regina |first13=I. Santa |last14=Smilauer |first14=P. |date=2000-07-01 |title=Plant species diversity as a driver of early succession in abandoned fields: a multi-site approach |url=https://doi.org/10.1007/s004420050028 |journal=Oecologia |language=en |volume=124 |issue=1 |pages=91–99 |doi=10.1007/s004420050028 |pmid=28308417 |bibcode=2000Oecol.124...91V |s2cid=38703575 |issn=1432-1939}}</ref>
*] – biennial

*]<ref>{{cite web | url = http://plants.usda.gov/java/profile?symbol=CASA3 | title = PLANTS Profile for Cannabis sativa (marijuana) | publisher = ] | work = ] | accessdate = February 15, 2009}}</ref><ref name=By1997>{{Cite book | editor-first = Mary Lynn | editor-last = Mathre | year = 1997 | title = Cannabis in medical practice: a legal, historical and pharmacological overview of the therapeutic use of Marijuana | page = 208 | url = http://books.google.com/?id=1AWGDhIOvk0C&pg=PA208&dq=Cannabis+is+a++weedy+species | isbn = 9780786403615 | publisher = McFarland | location = Jefferson, NC | accessdate = February 15, 2009}}</ref>
] have been observed to undergo rapid evolutionary change to adapt to their new environments, with changes in plant height, size, leaf shape, dispersal ability, reproductive output, vegetative reproduction ability, level of dependence on the ], and level of phenotype plasticity appearing on timescales of decades to centuries.<ref>{{cite journal |last1=Buswell |first1=Joanna |last2=Moles |first2=Angela |author-link2=Angela Moles |last3=Hartley |first3=Stephen |date=2010 |title=Is rapid evolution common in introduced plant species? |journal=Journal of Ecology |volume=99 |issue=1}}</ref> Invasive species can be more adaptable in their new environments than in their native environments, occupying broader ranges in areas where they are invasive than in areas where they are native. Hybridization between similar species can produce novel invasive plants that are better adapted to their surroundings. ] is also observed to be strongly selected for among some invasive populations, such as '']'' in ]. Many weed species are now found almost worldwide, with novel adaptations that suit regional populations to their environments.<ref name="clements"/>
*] - annual, biennial, or perennial<ref>On lawns and elsewhere, some people consider clover a weed, and some do not, as it has some beneficial effects.</ref>

*] – perennial, fast-spreading plants with long creeping stems
==Negative impacts==
*] – perennial, wind-spread, fast-growing, and drought-tolerant
] as a roadside weed in Poland]]
*] – perennial
] | location = Willougby | year = 1989 }}</ref>]]
*]

*] – perennial
Some negative impacts of weeds are functional: they interfere with food and fiber production in ], wherein they must be controlled to prevent lost or diminished ] yields. In other settings, they interfere with other cosmetic, decorative, or recreational goals, such as in ]s, ], ]s, and ]s. In the case of ], they can be of concern for environmental reasons, when introduced species outcompete ] and cause broader damage to ecosystem health and functioning.<ref name="AshrafÖztürk2012">{{cite book|author1=Muhammad Ashraf|author2=Münir Öztürk|author3=Muhammad Sajid Aqeel Ahmad|author4=Ahmet Aksoy|title=Crop Production for Agricultural Improvement|url=https://books.google.com/books?id=VytWKxw3M_IC&pg=PA525|date=2 June 2012|publisher=Springer|isbn=978-94-007-4116-4|pages=525–}}</ref>
*] – perennial, with underground stems
Some weed species have been classified as ]s by government authorities because, if left unchecked, they often compete with native or ] plants or cause harm to ].<ref name="Office1985">{{cite book|author=United States. Bureau of Land Management. Oregon State Office|title=Northwest area noxious weed control program: environmental impact statement : final|url=https://archive.org/details/northwestareanox21unit|year=1985|publisher=U.S. Dept. of the Interior, Bureau of Land Management, Oregon State Office|pages=–}}</ref> They are often foreign species accidentally or imprudently imported into a region where there are few natural controls to limit their population and spread.<ref name="Revision2008">{{cite book|author=House (U S ) Office of the Law Revision|title=United States Code, 2006, V. 3, Title 7, Sections 701-End|url=https://books.google.com/books?id=DB1ep621fRsC&pg=PA1230|date=25 April 2008|publisher=Government Printing Office|isbn=978-0-16-079998-3|pages=1230–}}</ref>
*] – annual or biennial

*] – perennial
In a range of contexts, weeds can have negative impacts by:
*] – annual
* competing with the desired plants for the resources that a plant typically needs, namely, direct sunlight, ] nutrients, water, and (to a lesser extent) space for growth,
*] – annual
* providing hosts and vectors for plant ]s, giving them greater opportunity to infect and degrade the quality of the desired plants;
*] - perennial
* providing food or shelter for animal pests such as seed-eating birds and ] fruit flies that otherwise could hardly survive seasonal shortages;<ref name="isbn0-409-08398-4">{{cite book |author1=Annecke, D. R. |author2=Moran, V. C. |title=Insects and mites of cultivated plants in South Africa |publisher=Butterworths |location=London |year=1982 |isbn=0-409-08398-4 }}</ref>
*] – woody perennial
* offering irritation to the skin or digestive tracts of people or animals, either physical irritation via ], or ]s, or chemical irritation via natural poisons or irritants in the weed (for example, the poisons found in '']'' species);<ref name="WattPP">{{cite book|last1=Watt|first1=John Mitchell|last2=Breyer-Brandwijk|first2=Maria Gerdina|title=The Medicinal and Poisonous Plants of Southern and Eastern Africa|edition=2nd|publisher=E & S Livingstone|year=1962}}</ref>
*] – biennial
* causing root damage to engineering works such as drains, road surfaces, and foundations,<ref>{{cite book|last1=Roberts|first1=John|last2=Jackson| first2=Nick|last3=Smith|first3=Mark|title=Tree Roots in the Built Environment|year=2006|publisher=The Stationery Office |isbn=978-0117536203}}</ref>
*] – perennial
* in the case of aquatic plants, obstructing or clogging streams and waterways, which interferes with boating, irrigation systems, fishing, and hydroelectric power.<ref>{{cite web|url=http://www.weeds.org.au/cgi-bin/weedident.cgi?tpl=plant.tpl&ibra=all&card=T04|website=Weeds Australia|archive-url=https://web.archive.org/web/20150711120521/http://www.weeds.org.au/cgi-bin/weedident.cgi?tpl=plant.tpl&ibra=all&card=T04 |archive-date=2015-07-11 |title=Black Willow}}</ref><ref>{{cite book |last1=Holm |first1=LeRoy G. |last2=Plucknett |first2=Donald L. |last3=Pancho |first3=Juan V. |last4=Herberger |first4=James P. |title=The World's Worst Weeds: distribution and biology |date=1977 |publisher=University Press of Hawaii |location=Honolulu |page=xi |isbn=0-8248-0295-0}}</ref>
The five plants designated "injurious weeds" under UK law are:<ref>{{cite web | url = http://www.defra.gov.uk/rds/weeds-act.htm | title = Weeds Act 1959 | archiveurl = http://web.archive.org/web/20070926235042/http://www.defra.gov.uk/rds/weeds-act.htm | archivedate = 2007-09-26 | publisher = ] (Defra), UK | accessdate = February 15, 2009}}</ref>

* ]
==Positive impacts==
* ]
{{see also|companion plant|beneficial weed|List of beneficial weeds|list of edible flowers}}
* ]
{{quotebox
* ]
| width = 22em
* ]
| align = right
In addition, it is an offence under section 14(2) of the Wildlife and Countryside act 1981 to "plant or otherwise cause to grow in the wild" any plant listed in Schedule nine, Part II to the Act, which includes ].
| quote = ''"What would the world be, once bereft,<br />
''of wet and wildness? Let them be left.''<br />
''O let them be left; wildness and wet;''<br />
''Long live the weeds and the wilderness yet."''
| source = — Gerard Manley Hopkins' poem ''Inversnaid''
}}

While the term "weed" generally has a negative connotation, many plants known as weeds can have beneficial properties. A number of weeds, such as the ] ('']'') and ], are edible, and their leaves or roots may be used for food or ]. ] is common over much of the world, and is sometimes used to make soup and medicine in ].<ref>{{cite web|title=Burdock Root|url=http://www.chinesesouppot.com/tag/burdock-root|website=Chinese Soup Pot|access-date=29 May 2015}}</ref> Some weeds attract ]s, which in turn can protect crops from harmful pests. Weeds can also prevent pest insects from finding a crop, because their presence disrupts the incidence of positive cues which pests use to locate their food. Weeds may also act as a "living mulch", providing ground cover that reduces moisture loss and prevents erosion. Weeds may also improve soil fertility; dandelions, for example, bring up nutrients like calcium and nitrogen from deep in the soil with their tap root, and clover hosts nitrogen-fixing bacteria in its roots, fertilizing the soil directly. The dandelion is also one of several species which break up ] in overly-cultivated fields, helping crops grow deeper root systems. Some garden flowers originated as weeds in cultivated fields and have been selectively bred for their garden-worthy flowers or foliage. An example of a crop weed that is grown in gardens is the ], ('']''), which was a common weed in European wheat fields, but is now sometimes grown as a garden plant.<ref name=Preston2002>Preston, Pearman & Dines. (2002). New Atlas of the British Flora. Oxford University Press.</ref>

===Ecological role===
As ], weeds begin the process of ] after a disturbance event. The rapid, aggressive growth of weeds rapidly prevents erosion in newly exposed bare soil, and has substantially slowed ] due to anthropogenic disturbances.<ref>{{cite web |last1=Schonbeck |first1=Mark |title=An Ecological Understanding of Weeds |url=https://eorganic.org/node/2314 |website=eOrganic}}</ref>

===In climate change adaptation===
It has been suggested that weeds, with their aggressive ability to adapt, could provide humans with vital tools and knowledge for ]. Some researchers argue that researching weed species could offer valuable insights for crop breeding, or that weeds themselves hold potential as hardy, climate-change-resistant crops. Adaptable weeds could also be a source of ] genes which could confer useful traits upon crops.<ref name="TenWays" />

Weed species have been used in the restoration of land in ] using a method called natural sequence farming. This method allows non-native weeds to stabilize and restore degraded areas where native species are not yet capable of regenerating themselves.<ref>{{cite web |last1=Kenyon |first1=Georgina |title=How weeds help fight climate change |url=https://www.bbc.com/future/article/20190507-weeds-a-surprising-way-to-fight-climate-change |website=bbc.com |publisher=BBC Future}}</ref>

==Weeds as adaptable species==
{{quotebox
| width = 22em
| align = right
| quote = "We've got to be one of the most bomb-proof species on the planet."
| source = — Paleontologist David Jablonsky<ref name=Quammen/>
}}

An alternate definition often used by biologists is any species, not just plants, that can quickly adapt to any environment.<ref name=Quammen/> Some traits of weedy species are the ability to reproduce quickly, disperse widely, live in a variety of habitats, establish a population in strange places, succeed in disturbed ecosystems and resist eradication once established. Such species often do well in human-dominated environments as other species are not able to adapt. Common examples include the ], ] and the ]. Other weedy species have been able to expand their range without actually living in human environments, as human activity has damaged the ecosystems of other species. These include the ], the ] and the ].<ref name=Quammen/>

In response to the idea that humans may face extinction due to ], paleontologist David Jablonsky counters by arguing that humans are a weed species. Like other weedy species, humans are widely dispersed in a wide variety of environments, and are highly unlikely to go extinct no matter how much damage the environment faces.<ref name=Quammen/>

==Plants often considered to be weeds==
{{More citations needed section|date=February 2021}}
]] is considered by some to be a weed in lawns, but in many other situations is a desirable source of fodder, honey and soil nitrogen.<ref>{{cite book|last=Voisin|first=Andre|title=Grass Productivity|publisher=Island Press|year=1988|isbn=978-0933280649}}</ref><ref>Woodfield, Derek R. White clover, New Zealand's competitive edge. Symposium NZ Agronomy Society and Grassland Association at Lincoln University, New Zealand, November, 1995</ref>

A short list of some plants that often are considered to be weeds follows:
* ] – ("pigweed") annual with copious long-lasting seeds, also a highly edible and resilient food source
* ] – perennial, spreading by runners, rhizomes and seeds.
* ]
* ] – perennial, spreads by seeds that persist in the soil for many years
* ] – biennial
* ] – annual
*] - ''Imperata cylindrica'' - One of the most damaging pest weeds in the world, infesting vast areas in the tropics.<ref>{{Cite book|last=Nations|first=Food and Agriculture Organization of the United|url=https://books.google.com/books?id=vVhRAAAAMAAJ|title=Agrochemicals, Fate in Food and the Environment: Proceedings of an International Symposium on Agrochemicals: Fate in Food and the Environment Using Isotope Techniques|date=1982|publisher=International Atomic Energy Agency|isbn=978-92-0-010382-7|language=en}}</ref><ref>{{Cite web
|title=Weed Risk Assessment for Imperata cylindrica (L.) P. Beauv. (Poaceae) – Cogongrass
|url=https://www.aphis.usda.gov/plant_health/plant_pest_info/weeds/downloads/wra/imperata-cylindrica.pdf|website=Animal and Plant Health Inspection Service U.S. Department of Agriculture}}</ref>
* ] – perennial, fast-spreading plants with long creeping stems
* ] – perennial, wind-spread, fast-growing, and drought-tolerant
* ] – perennial
* ]
* ] – perennial
* ] – perennial, with underground stems
* ] – annual or biennial
* ] – perennial
* ] – annual
* ] – annual or perennial
* '']''
* ] – perennial
* ] – woody perennial
* ] – woody perennial
* ] – biennial
* ] – perennial
* ] – perennial

Many ] weeds were ] deliberately in the first place, and may have not been considered nuisances at the time, but rather beneficial.

==Weed control==
] being weeded in Colorado, United States, in 1972]]

Weed control encompasses a range of methods used by humans to stop, reduce or prevent the growth and reproduction of weeds within agricultural or other managed environments. Some weed control is preventative, implementing protocols to stop weeds from invading new areas. Cultural weed control involves shaping the managed environment to make it less favorable for weeds.<ref>{{cite web |title=Describe the five general categories of weed control methods. |url=https://forages.oregonstate.edu/nfgc/eo/onlineforagecurriculum/instructormaterials/availabletopics/weeds/control |website=forages.oregonstate.edu |date=June 2009 |publisher=Oregon State University}}</ref><ref name="GenMethods">{{cite web |title=General Methods of Weed Management |url=https://ipm.ucanr.edu/agriculture/floriculture-and-ornamental-nurseries/general-methods-of-weed-management/ |website=ipm.ucanr.edu |publisher=University of California Agriculture and Natural Resources}}</ref> Once weeds are present in an area, a wide variety of means to destroy the weeds and their seeds can be employed. Since weeds are highly adaptable, relying on a single method to control weeds soon results in the invasion or adaptation of weeds that are not susceptible. ] as it applies to weeds refers to a plan of controlling weeds that integrates multiple methods of weed control and prevention.<ref>{{cite web |last1=Pittman |first1=Kara |last2=Flessner |first2=Michael |last3=Rubione |first3=Claudio |last4=Ackroyd |first4=Victoria |last5=Mirsky |first5=Steven |title=What Is Integrated Weed Management? |url=https://growiwm.org/what-is-integrated-weed-management/ |website=growiwm.org}}</ref>

Methods of preventative weed control include cleaning equipment, stopping existing weeds in nearby areas from producing seed, and avoiding seed or manure that could be contaminated with weeds.<ref>{{cite web |title=Weed prevention |url=https://www.alberta.ca/weed-prevention.aspx |website=Alberta.ca|date=24 April 2024 }}</ref> A wide variety of cultural weed control methods are used, including ], ], selecting the most competitive cultivars of crops, mulching, planting with optimal density, and ].<ref>{{cite web |last1=Mohler |first1=Charles L. |last2=Teasdale |first2=John R. |last3=DiTommaso |first3=Antonio |title=Ch 3. Cultural Weed Management |url=https://www.sare.org/publications/manage-weeds-on-your-farm/cultural-weed-management/ |website=sare.org |publisher=Sustainable Agriculture Research and Education}}</ref>

Mechanical methods of weed control involve physically cutting, uprooting, or otherwise destroying weeds. On small farms, hand weeding is the dominant means of weed control, but as larger farms dominate agriculture, this method becomes less feasible.<ref name="GrandChallenge" /> On many operations, however, some hand-weeding may be an unavoidable component of weed control.<ref name="GenMethods" /> ], ], and burning are common examples of mechanical weed control on larger scales. New technology increases the range of mechanical weed control options. One newly emerging form of mechanical weed control uses electricity to kill weeds.<ref>{{cite web |last1=Claver |first1=Hugo |title=Electric weeding with high frequency electricity gives better weed control with lower energy use |url=https://www.futurefarming.com/crop-solutions/weed-pest-control/electric-weeding-with-high-frequency-electricity-gives-better-weed-control-with-lower-energy-use/ |website=futurefarming.com|date=12 September 2022 }}</ref>

Mechanical weed control has been increasingly replaced by the use of ]s.<ref>{{cite journal |last1=Gianessi |first1=Leonard P |title=The increasing importance of herbicides in worldwide crop production |journal=Pest Manag. Sci. |date=2013 |volume=69 |issue=10 |pages=1099–1105 |doi=10.1002/ps.3598 |pmid=23794176 |url=https://pubmed.ncbi.nlm.nih.gov/23794176/}}</ref> The reliance on herbicides has resulted in the rapid evolution of ] in weeds, making previously effective herbicide treatments useless for the control of weeds.<ref name="10.1111/wre.12153">{{cite journal|first1=Abed|last1=Forouzesh|first2=Eskandar|last2=Zand|first3=Saeid|last3=Soufizadeh|first4=Sadegh|last4=Samadi Foroushani|title=Classification of herbicides according to chemical family for weed resistance management strategies–an update|journal=Weed Research|date=2015|doi=10.1111/wre.12153|volume=55|issue=4|pages=334–358|bibcode=2015WeedR..55..334F }}</ref> In particular, ], which was once considered a revolutionary breakthrough in weed control, was relied upon heavily when it was first introduced to agriculture, resulting in rapid emergence of resistance.<ref name=s1309>{{Cite journal | doi = 10.1126/science.341.6152.1329| title = What Happens when Weed Killers Stop Killing?| year = 2013| last1 = Service | first1 = R. F.| journal = Science| volume = 341| issue = 6152| pages = 1329| pmid=24052282}}</ref> As of 2023, 58 weed species have developed resistance to glyphosate.<ref>{{cite web |title=Number of Resistant Species to Individual Active Herbicides |url=https://www.weedscience.org/Pages/Graphs/activebyspecies.aspx |website=weedscience.org}}</ref>

Herbicide resistance in weeds has rapidly developed into new, increasingly challenging forms as the plants continually evolve. Non-target site resistance, or NTSR, is particularly difficult to counteract, since it may confer resistance to multiple herbicides at once, including herbicides the plants' ancestors were never exposed to.<ref>{{cite news |last1=Brown |first1=H. Claire |title=Attack of the Superweeds |work=The New York Times |date=18 August 2021 |url=https://www.nytimes.com/2021/08/18/magazine/superweeds-monsanto.html }}</ref> Various methods of adjusting herbicide application to avoid resistance, such as rotating herbicides used and tank mixing herbicides, have all been questioned in terms of their efficacy for preventing resistance from arising.<ref>{{cite web |last1=Quinn |first1=Lauren |title=Tank mixing herbicides may not be enough to avoid herbicide resistance |url=https://www.farmersadvance.com/story/news/2023/06/07/tank-mixing-herbicides-may-not-be-enough-to-avoid-herbicide-resistance/70281618007/ |website=farmers advance.com}}</ref>

Understanding the habit of weeds is important for non-chemical methods of weed control, such as plowing, surface scuffling, promotion of more beneficial cover crops, and prevention of seed accumulation in fields. For example, amaranth is an edible plant that is considered a weed by mainstream modern agriculture. It produces copious seeds (up to 1 million per plant) that last many years, and is an early-emergent fast grower. Those seeking to control amaranth quote the mantra "This year’s seeds become next year’s weeds!".<ref>{{cite web|title=The Biology and Ecology of Palmer Amaranth: Implications for Control|url=http://extension.uga.edu/publications/detail.cfm?number=C1000|publisher=UGA extension|access-date=29 May 2015|archive-date=27 April 2015|archive-url=https://web.archive.org/web/20150427215404/http://extension.uga.edu/publications/detail.cfm?number=C1000|url-status=dead}}</ref> However, another view of amaranth values the plant as a resilient food source.<ref>{{cite web|title=Rethinking a Weed: the Truth about Amaranth|url=http://ourworld.unu.edu/en/rethinking-a-weed-the-truth-about-amaranth|website=Our World|publisher=United Nations University}}</ref>

Some people have appreciated weeds for their tenacity, their wildness and even the work and connection to nature they provide. As ] wrote in ''The Well-Tempered Garden'':

{{blockquote|Many gardeners will agree that hand-weeding is not the terrible drudgery that it is often made out to be. Some people find in it a kind of soothing monotony. It leaves their minds free to develop the plot for their next novel or to perfect the brilliant repartee with which they should have encountered a relative's latest example of unreasonableness.<ref>Christopher Lloyd, The Well-Tempered Garden, 1973</ref>}}

===Under climate change===
As anthropogenic climate change increases temperatures and atmospheric carbon dioxide, many weeds are expected to become harder to control and to expand their ranges, at the expense of less "weedy" species. For example, ], the infamous invasive vine found throughout the Southeastern United States, is expected to spread northward due to climate change. Increased competitive strength of agricultural weeds in future climate conditions threaten future ability to grow crops. Existing weed management practices will likely fail under future changes in climate conditions, meaning new agricultural techniques will be needed for global food security. Suggested techniques are holistic, transitioning away from reliance on herbicide, and include aggressive adaptation of ] and use of ] crop residues to suppress weeds.<ref>{{cite journal |last1=Anwar |first1=Parvez |last2=Islam |first2=A.K.M. Mominul |last3=Yeasmin |first3=Sabina |last4=Rashid |first4=Harun |last5=Juraimi |first5=Abdul Shukor |last6=Ahmed |first6=Sharif |last7=Shrestha |first7=Anil |title=Weeds and Their Responses to Management Efforts in A Changing Climate |journal=Agronomy |date=2021 |volume=11 |issue=10|page=1921 |doi=10.3390/agronomy11101921 |doi-access=free }}</ref>


==See also== ==See also==
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==References== ==References==
{{reflist|2}} {{reflist}}


==External links== ==External links==
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* {{Webarchive|url=https://web.archive.org/web/20101105230041/http://www.issg.org/database/welcome/ |date=2010-11-05 }}
* {{Webarchive|url=https://web.archive.org/web/20200810075705/https://definedterm.com/a/definition/200717 |date=2020-08-10 }}
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* Queensland Government. (1021 species, 55 characters)
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Latest revision as of 03:48, 13 October 2024

Plant considered undesirable in a particular place or situation This article is about plants specifically called "weeds". For the psychoactive plant commonly called "weed", see Cannabis (drug). For other uses, see Weed (disambiguation).

Weeds growing in the cracks of a concrete staircase (Epilobium roseum, Chelidonium majus, Oxalis corniculata, Plantago major)

A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals. Plants with characteristics that make them hazardous, aesthetically unappealing, difficult to control in managed environments, or otherwise unwanted in farm land, orchards, gardens, lawns, parks, recreational spaces, residential and industrial areas, may all be considered weeds. The concept of weeds is particularly significant in agriculture, where the presence of weeds in fields used to grow crops may cause major losses in yields. Invasive species, plants introduced to an environment where their presence negatively impacts the overall functioning and biodiversity of the ecosystem, may also sometimes be considered weeds.

Taxonomically, the term "weed" has no botanical significance, because a plant that is a weed in one context, is not a weed when growing in a situation where it is wanted. Some plants that are widely regarded as weeds are intentionally grown in gardens and other cultivated settings. For this reason, some plants are sometimes called beneficial weeds. Similarly, volunteer plants from a previous crop are regarded as weeds when growing in a subsequent crop. Thus, alternative nomenclature for the same plants might be hardy pioneers, cosmopolitan species, volunteers, "spontaneous urban vegetation," etc.

Although whether a plant is a weed depends on context, plants commonly defined as weeds broadly share biological characteristics that allow them to thrive in disturbed environments and to be particularly difficult to destroy or eradicate. In particular, weeds are adapted to thrive under human management in the same way as intentionally grown plants. Since the origins of agriculture on Earth, agricultural weeds have co-evolved with human crops and agricultural systems, and some have been domesticated into crops themselves after their fitness in agricultural settings became apparent.

More broadly, the term "weed" is occasionally applied pejoratively to species outside the plant kingdom, species that can survive in diverse environments and reproduce quickly; in this sense it has even been applied to humans.

Weed control is important in agriculture and horticulture. Methods include hand cultivation with hoes, powered cultivation with cultivators, smothering with mulch or soil solarization, lethal wilting with high heat, burning, or chemical attack with herbicides and cultural methods such as crop rotation and fallowing land to reduce the weed population.

History

It has long been assumed that weeds, in the sense of rapidly-evolving plants taking advantage of human-disturbed environments, evolved in response to the Neolithic agricultural revolution approximately 12,000 years ago. However, researchers have found evidence of "proto-weeds" behaving in similar ways at Ohalo II, a 23,000-year-old archeological site in Israel.

The idea of "weeds" as a category of undesirable plant has not been universal throughout history. Before 1200 A.D., little evidence exists of concern with weed control or of agricultural practices solely intended to control weeds. A possible reason for this is that for much of human history, women and children were an abundant source of cheap labor to control weeds, and not directly acknowledged. Weeds are assumed to have existed since the beginning of agriculture, and accepted as an "inevitable nuisance."

Though the plants are not named using a specific term denoting a "weed" in the contemporary sense, plants that may be interpreted as "weeds" are referenced in the Bible:

Cursed is the ground because of you; through painful toil you will eat of it all the days of your life. It will produce thorns and thistles for you, and you will eat the plants of the field. By the sweat of your brow you will eat your food until you return to the ground.

Some early Roman writers referenced weeding activities in agricultural fields, but weed control in the pre-modern era was probably an incidental effect of plowing. Ancient Egyptians, Assyrians, and Sumerians had no specific word for "weeds," seeing all plants as having some use. The English word "weed" can be traced back to the Old English weod, which refers to woad, rather than a category of plant as in the modern usage; in early medieval European herbals, each plant is regarded as having its own "virtues".

By the sixteenth century, the concept of a "weed" was better defined as a "noxious" or undesirable type of plant, as referenced metaphorically in William Shakespeare's works. An example of a Shakespearean reference to weeds is found in Sonnet 69:

To thy fair flower add the rank smell of weeds: / But why thy odour matcheth not thy show, / The soil is this, that thou dost common grow.

In London during this period, poor women were paid low wages to weed gardens and courtyards.

After the Reformation, Christian theology that emphasized the degradation of nature after the Fall of Man, and humankind's role and duty to dominate and subdue nature, became more developed and widespread. Various European writers designated certain plants as "vermin" and "filth," though many plants identified as such were valued by gardeners or by herbalists and apothecaries, and some questioned the idea that any plant could be without purpose or value. Laws mandating the control of weeds emerged as early as the seventeenth century; in 1691 a law in New York required the removal of "poysonous and Stincking Weeds" in front of houses.

In the nineteenth century, manual labor was used to control weeds in European towns and cities, and chemical methods of weed control emerged. For example, a French journal in 1831 documented a mixture of sulfur, lime and water boiled in an iron cauldron as an effective herbicide to prevent grass from growing among cobblestones.

The cultural association between weeds and moral or spiritual degradation persisted into the last nineteenth century in American cities. Urban expansion and development created ideal habitats for weeds in nineteenth-century America. Reformers consequently saw weeds as a part of the larger problem of filth, disease, and moral corruption that plagued the urban environments, and weeds were seen as refuge for "tramps" and other criminal or undesirable people. The St. Louis Post-Dispatch credited weeds as causing diphtheria, scarlet fever, and typhoid. In St. Louis between the years of 1905-1910, weeds became viewed as a major public health hazard, believed to cause typhoid and malaria, and legal precedents were set in order to control weeds that would help facilitate the adoption of weed control laws throughout the country.

Ecological significance

A dandelion is a common plant all over the world, especially in Europe, Asia, and the Americas. It is considered a weed in some contexts (such as lawns) but not others (such as when it is grown as a vegetable or herbal medicine).

"Weed" as a category of plant overlaps with the closely related concepts of ruderal and pioneer species. Pioneer species are specifically adapted to disturbed environments, where the existing plant and soil community has been disrupted or damaged in some way. Adaptation to disturbance can give weeds advantages over desirable crops, pastures, or ornamental plants. The nature of the habitat and its disturbances will affect or even determine which types of weed communities become dominant. In weed ecology some authorities speak of the relationship between "the three Ps": plant, place, perception. These have been very variously defined, but the weed traits listed by H.G. Baker are widely cited.

Examples of such ruderal or pioneer species include plants that are adapted to naturally-occurring disturbed environments such as dunes and other windswept areas with shifting soils, alluvial flood plains, river banks and deltas, and areas that are burned repeatedly. Since human agricultural and horticultural practices often mimic these natural disturbances that weedy species have adapted for, some weeds are effectively preadapted to grow and proliferate in human-disturbed areas such as agricultural fields, lawns, gardens, roadsides, and construction sites. As agricultural practices continue and develop, weeds evolve further, with humans exerting evolutionary pressure upon weeds through manipulating their habitat and attempting to control weed populations.

Due to their ability to survive and thrive in conditions challenging or hostile to other plants, weeds have been considered extremophiles.

Adaptability

Due to their evolutionary heritage as disturbance-adapted pioneers, most weeds exhibit incredibly high phenotype plasticity, meaning that individual plants hold the potential to adapt their morphology, growth, and appearance in response to their conditions. The potential within a single individual to adapt to a wide variety of conditions is sometimes referred to as an "all-purpose genotype." Disturbance-adapted plants typically grow rapidly and reproduce quickly, with some annual weeds having multiple generations in a single growing season. They commonly have seeds that persist in the soil seed bank for many years. Perennial weeds often have underground stems that spread under the soil surface or, like ground ivy (Glechoma hederacea), have creeping stems that root and spread out over the ground. These traits make many disturbance-adapted plants highly successful as weeds.

On top of the ability of individual plants to adapt to their conditions, weed populations also evolve much more quickly than older models of evolution account for. Once established in an agricultural setting, weeds have been observed to undergo evolutionary changes to adapt to selective pressures imposed by human management. Some examples include changes in seed dormancy, changes in seasonal life cycles, changes in plant morphology, and the evolution of resistance to herbicides. Rapid life cycles, large populations, and ability to spread large numbers of seeds long distances also allow weed species with these general characteristics to evolve quickly.

Dispersal

The concept of weeds also overlaps with the concept of invasive species, both in the sense that human activities tend to introduce weeds outside their native range, and that an introduced species may be considered a weed. Many weed species have moved out of their natural geographic ranges and spread around the world in tandem with human migrations and commerce. Weed seeds are often collected and transported with crops after the harvesting of grains, so humans are a vector of transport as well as a producer of the disturbed environments to which weed species are well adapted, resulting in many weeds having a close association with human activities.

Some plants become dominant when introduced into new environments because the animals and plants in their original environment that compete with them or feed on them are absent; in what is sometimes called the "natural enemies hypothesis", plants freed from these specialist consumers may become dominant. An example is Klamath weed, which threatened millions of hectares of prime grain and grazing land in North America after it was accidentally introduced. The Klamathweed Beetle, a species that specializes in consuming the plant, was imported during World War II. Within several years Klamath weed was reduced to a rare roadside weed. In locations where predation and mutually competitive relationships are absent, weeds have increased resources available for growth and reproduction. The weediness of some species that are introduced into new environments may be caused by their production of allelopathic chemicals which indigenous plants are not yet adapted to, a scenario sometimes called the "novel weapons hypothesis". These chemicals may limit the growth of established plants or the germination and growth of seeds and seedlings. Weed growth can also inhibit the growth of later-successional species in ecological succession.

Introduced species have been observed to undergo rapid evolutionary change to adapt to their new environments, with changes in plant height, size, leaf shape, dispersal ability, reproductive output, vegetative reproduction ability, level of dependence on the mycorrhizal network, and level of phenotype plasticity appearing on timescales of decades to centuries. Invasive species can be more adaptable in their new environments than in their native environments, occupying broader ranges in areas where they are invasive than in areas where they are native. Hybridization between similar species can produce novel invasive plants that are better adapted to their surroundings. Polyploidy is also observed to be strongly selected for among some invasive populations, such as Solidago canadensis in China. Many weed species are now found almost worldwide, with novel adaptations that suit regional populations to their environments.

Negative impacts

Invasive Canada Goldenrod as a roadside weed in Poland
Australia, 1907: Cattlemen survey 700 carcasses of cattle that were killed overnight by a poisonous plant.

Some negative impacts of weeds are functional: they interfere with food and fiber production in agriculture, wherein they must be controlled to prevent lost or diminished crop yields. In other settings, they interfere with other cosmetic, decorative, or recreational goals, such as in lawns, landscape architecture, playing fields, and golf courses. In the case of invasive species, they can be of concern for environmental reasons, when introduced species outcompete native plants and cause broader damage to ecosystem health and functioning. Some weed species have been classified as noxious weeds by government authorities because, if left unchecked, they often compete with native or crop plants or cause harm to livestock. They are often foreign species accidentally or imprudently imported into a region where there are few natural controls to limit their population and spread.

In a range of contexts, weeds can have negative impacts by:

  • competing with the desired plants for the resources that a plant typically needs, namely, direct sunlight, soil nutrients, water, and (to a lesser extent) space for growth,
  • providing hosts and vectors for plant pathogens, giving them greater opportunity to infect and degrade the quality of the desired plants;
  • providing food or shelter for animal pests such as seed-eating birds and Tephritid fruit flies that otherwise could hardly survive seasonal shortages;
  • offering irritation to the skin or digestive tracts of people or animals, either physical irritation via thorns, prickles, or burs, or chemical irritation via natural poisons or irritants in the weed (for example, the poisons found in Nerium species);
  • causing root damage to engineering works such as drains, road surfaces, and foundations,
  • in the case of aquatic plants, obstructing or clogging streams and waterways, which interferes with boating, irrigation systems, fishing, and hydroelectric power.

Positive impacts

See also: companion plant, beneficial weed, List of beneficial weeds, and list of edible flowers

"What would the world be, once bereft,
of wet and wildness? Let them be left.
O let them be left; wildness and wet;
Long live the weeds and the wilderness yet."

— Gerard Manley Hopkins' poem Inversnaid

While the term "weed" generally has a negative connotation, many plants known as weeds can have beneficial properties. A number of weeds, such as the dandelion (Taraxacum) and lamb's quarter, are edible, and their leaves or roots may be used for food or herbal medicine. Burdock is common over much of the world, and is sometimes used to make soup and medicine in East Asia. Some weeds attract beneficial insects, which in turn can protect crops from harmful pests. Weeds can also prevent pest insects from finding a crop, because their presence disrupts the incidence of positive cues which pests use to locate their food. Weeds may also act as a "living mulch", providing ground cover that reduces moisture loss and prevents erosion. Weeds may also improve soil fertility; dandelions, for example, bring up nutrients like calcium and nitrogen from deep in the soil with their tap root, and clover hosts nitrogen-fixing bacteria in its roots, fertilizing the soil directly. The dandelion is also one of several species which break up hardpan in overly-cultivated fields, helping crops grow deeper root systems. Some garden flowers originated as weeds in cultivated fields and have been selectively bred for their garden-worthy flowers or foliage. An example of a crop weed that is grown in gardens is the corncockle, (Agrostemma githago), which was a common weed in European wheat fields, but is now sometimes grown as a garden plant.

Ecological role

As pioneer species, weeds begin the process of ecological succession after a disturbance event. The rapid, aggressive growth of weeds rapidly prevents erosion in newly exposed bare soil, and has substantially slowed topsoil loss due to anthropogenic disturbances.

In climate change adaptation

It has been suggested that weeds, with their aggressive ability to adapt, could provide humans with vital tools and knowledge for climate change adaptation. Some researchers argue that researching weed species could offer valuable insights for crop breeding, or that weeds themselves hold potential as hardy, climate-change-resistant crops. Adaptable weeds could also be a source of transgenic genes which could confer useful traits upon crops.

Weed species have been used in the restoration of land in Australia using a method called natural sequence farming. This method allows non-native weeds to stabilize and restore degraded areas where native species are not yet capable of regenerating themselves.

Weeds as adaptable species

"We've got to be one of the most bomb-proof species on the planet."

— Paleontologist David Jablonsky

An alternate definition often used by biologists is any species, not just plants, that can quickly adapt to any environment. Some traits of weedy species are the ability to reproduce quickly, disperse widely, live in a variety of habitats, establish a population in strange places, succeed in disturbed ecosystems and resist eradication once established. Such species often do well in human-dominated environments as other species are not able to adapt. Common examples include the common pigeon, brown rat and the raccoon. Other weedy species have been able to expand their range without actually living in human environments, as human activity has damaged the ecosystems of other species. These include the coyote, the white-tailed deer and the brown headed cowbird.

In response to the idea that humans may face extinction due to environmental degradation, paleontologist David Jablonsky counters by arguing that humans are a weed species. Like other weedy species, humans are widely dispersed in a wide variety of environments, and are highly unlikely to go extinct no matter how much damage the environment faces.

Plants often considered to be weeds

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White clover

White clover is considered by some to be a weed in lawns, but in many other situations is a desirable source of fodder, honey and soil nitrogen.

A short list of some plants that often are considered to be weeds follows:

Many invasive weeds were introduced deliberately in the first place, and may have not been considered nuisances at the time, but rather beneficial.

Weed control

A field of beets being weeded in Colorado, United States, in 1972

Weed control encompasses a range of methods used by humans to stop, reduce or prevent the growth and reproduction of weeds within agricultural or other managed environments. Some weed control is preventative, implementing protocols to stop weeds from invading new areas. Cultural weed control involves shaping the managed environment to make it less favorable for weeds. Once weeds are present in an area, a wide variety of means to destroy the weeds and their seeds can be employed. Since weeds are highly adaptable, relying on a single method to control weeds soon results in the invasion or adaptation of weeds that are not susceptible. Integrated pest management as it applies to weeds refers to a plan of controlling weeds that integrates multiple methods of weed control and prevention.

Methods of preventative weed control include cleaning equipment, stopping existing weeds in nearby areas from producing seed, and avoiding seed or manure that could be contaminated with weeds. A wide variety of cultural weed control methods are used, including cover cropping, crop rotation, selecting the most competitive cultivars of crops, mulching, planting with optimal density, and intercropping.

Mechanical methods of weed control involve physically cutting, uprooting, or otherwise destroying weeds. On small farms, hand weeding is the dominant means of weed control, but as larger farms dominate agriculture, this method becomes less feasible. On many operations, however, some hand-weeding may be an unavoidable component of weed control. Tillage, mowing, and burning are common examples of mechanical weed control on larger scales. New technology increases the range of mechanical weed control options. One newly emerging form of mechanical weed control uses electricity to kill weeds.

Mechanical weed control has been increasingly replaced by the use of herbicides. The reliance on herbicides has resulted in the rapid evolution of herbicide resistance in weeds, making previously effective herbicide treatments useless for the control of weeds. In particular, glyphosate, which was once considered a revolutionary breakthrough in weed control, was relied upon heavily when it was first introduced to agriculture, resulting in rapid emergence of resistance. As of 2023, 58 weed species have developed resistance to glyphosate.

Herbicide resistance in weeds has rapidly developed into new, increasingly challenging forms as the plants continually evolve. Non-target site resistance, or NTSR, is particularly difficult to counteract, since it may confer resistance to multiple herbicides at once, including herbicides the plants' ancestors were never exposed to. Various methods of adjusting herbicide application to avoid resistance, such as rotating herbicides used and tank mixing herbicides, have all been questioned in terms of their efficacy for preventing resistance from arising.

Understanding the habit of weeds is important for non-chemical methods of weed control, such as plowing, surface scuffling, promotion of more beneficial cover crops, and prevention of seed accumulation in fields. For example, amaranth is an edible plant that is considered a weed by mainstream modern agriculture. It produces copious seeds (up to 1 million per plant) that last many years, and is an early-emergent fast grower. Those seeking to control amaranth quote the mantra "This year’s seeds become next year’s weeds!". However, another view of amaranth values the plant as a resilient food source.

Some people have appreciated weeds for their tenacity, their wildness and even the work and connection to nature they provide. As Christopher Lloyd wrote in The Well-Tempered Garden:

Many gardeners will agree that hand-weeding is not the terrible drudgery that it is often made out to be. Some people find in it a kind of soothing monotony. It leaves their minds free to develop the plot for their next novel or to perfect the brilliant repartee with which they should have encountered a relative's latest example of unreasonableness.

Under climate change

As anthropogenic climate change increases temperatures and atmospheric carbon dioxide, many weeds are expected to become harder to control and to expand their ranges, at the expense of less "weedy" species. For example, kudzu, the infamous invasive vine found throughout the Southeastern United States, is expected to spread northward due to climate change. Increased competitive strength of agricultural weeds in future climate conditions threaten future ability to grow crops. Existing weed management practices will likely fail under future changes in climate conditions, meaning new agricultural techniques will be needed for global food security. Suggested techniques are holistic, transitioning away from reliance on herbicide, and include aggressive adaptation of agroforestry and use of allelopathic crop residues to suppress weeds.

See also

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