Weed
A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals.
[1][2][3][4] 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.
[6] 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.
[12] 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.
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.
[18] 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.
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.
[22] Pioneer species are specifically adapted to disturbed environments, where the existing plant and soil community has been disrupted or damaged in some way.
[10] Due to their ability to survive and thrive in conditions challenging or hostile to other plants, weeds have been considered extremophiles.
[27] 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.
[22] 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.
[28] Once established in an agricultural setting, weeds have been observed to undergo evolutionary changes to adapt to selective pressures imposed by human management.
Many weed species have moved out of their natural geographic ranges and spread around the world in tandem with human migrations and commerce.
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 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".
[37] 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.
[28] 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.
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.
Weeds may also act as a "living mulch", providing ground cover that reduces moisture loss and prevents erosion.
The dandelion is also one of several species which break up hardpan in overly-cultivated fields, helping crops grow deeper root systems.
[11] 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.
[63] 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.
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.
[70]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.
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.
