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Weed control is a type of pest control, which attempts to stop or reduce growth of weeds, especially
noxious weed A noxious weed, harmful weed or injurious weed is a weed that has been designated by an agricultural or other governing authority as a plant that is injurious to agricultural or horticultural crops, natural habitats or ecosystems, or humans or liv ...
s, with the aim of reducing their competition with desired flora and fauna including domesticated plants and livestock, and in natural settings preventing non native species competing with native species. Weed control is important in agriculture. Methods include hand cultivation with hoes, powered cultivation with cultivators, smothering with mulch, lethal wilting with high heat, burning, and
chemical control Chemical control may refer to: * Any method to eradicate or limit the growth of pathogens which transmit disease to animals and plants, especially: **Herbicides ** Insecticides ** Fungicides * In molecular biology, the use of biochemicals to contro ...
with
herbicide Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...
s (weed killers).


Need for control

Weeds compete with productive crops or pasture, they can be poisonous, distasteful, produce burrs, thorns or otherwise interfere with the use and management of desirable plants by contaminating harvests or interfering with livestock. Weeds compete with crops for space,
nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
s, water and light. Smaller, slower growing seedlings are more susceptible than those that are larger and more vigorous. Onions are one of the most vulnerable, because they are slow to germinate and produce slender, upright stems. By contrast broad beans produce large seedlings and suffer far fewer effects other than during periods of water shortage at the crucial time when the pods are filling out. Transplanted crops raised in sterile soil or potting
compost Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting m ...
gain a head start over germinating weeds. Weeds also vary in their competitive abilities according to conditions and season. Tall-growing vigorous weeds such as fat hen (''Chenopodium album'') can have the most pronounced effects on adjacent crops, although seedlings of fat hen that appear in late summer produce only small plants. Chickweed (''Stellaria media''), a low growing plant, can happily co-exist with a tall crop during the summer, but plants that have overwintered will grow rapidly in early spring and may swamp crops such as onions or spring greens. The presence of weeds does not necessarily mean that they are damaging a crop, especially during the early growth stages when both weeds and crops can grow without interference. However, as growth proceeds they each begin to require greater amounts of water and nutrients. Estimates suggest that weed and crop can co-exist harmoniously for around three weeks before competition becomes significant. One study found that after competition had started, the final yield of onion bulbs was reduced at almost 4% per day. Perennial weeds with bulbils, such as lesser celandine and oxalis, or with persistent underground stems such as couch grass (''Agropyron repens'') or creeping buttercup (''Ranunculus repens'') store reserves of food, and are thus able to persist in drought or through winter. Some perennials such as couch grass exude
allelopathic Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the germination, growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have ben ...
chemicals that inhibit the growth of other nearby plants. Weeds can also host pests and diseases that can spread to cultivated crops.
Charlock ''Sinapis arvensis'', the charlock mustard, field mustard, wild mustard, or charlock, is an annual or winter annual plant of the genus ''Sinapis'' in the family Brassicaceae. It is found in the fields of North Africa, Asia and Europe. ''Pieris r ...
and Shepherd's purse may carry clubroot, eelworm can be harboured by chickweed, fat hen and shepherd's purse, while the cucumber mosaic virus, which can devastate the cucurbit family, is carried by a range of different weeds including chickweed and groundsel. Pests such as cutworms may first attack weeds but then move on to cultivated crops. Some plants are considered weeds by some farmers and crops by others.
Charlock ''Sinapis arvensis'', the charlock mustard, field mustard, wild mustard, or charlock, is an annual or winter annual plant of the genus ''Sinapis'' in the family Brassicaceae. It is found in the fields of North Africa, Asia and Europe. ''Pieris r ...
, a common weed in the southeastern US, are weeds according to row crop growers, but are valued by beekeepers, who seek out places where it blooms all winter, thus providing pollen for honeybees and other
pollinator A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains. Insects are the maj ...
s. Its bloom resists all but a very hard freeze, and recovers once the freeze ends.


Weed propagation


Seeds

Annual and biennial weeds such as chickweed, annual meadow grass, shepherd's purse, groundsel, fat hen, cleaver, speedwell and
hairy bittercress ''Cardamine hirsuta'', commonly called hairy bittercress, is an annual or biennial species of plant in the family Brassicaceae, and is edible as a salad green. It is common in moist areas around the world. Description Depending on the climate ...
propagate Propagation can refer to: *Chain propagation in a chemical reaction mechanism *Crack propagation, the growth of a crack during the fracture of materials *Propaganda, non-objective information used to further an agenda *Reproduction, and other forms ...
themselves by seeding. Many produce huge numbers of seed several times a season, some all year round. Groundsel can produce 1000 seed, and can continue right through a mild winter, whilst
Scentless Mayweed ''Tripleurospermum inodorum'', common names scentless false mayweed, scentless mayweed, scentless chamomile, wild chamomile, mayweed, false chamomile, and Baldr's brow, is the type species of Tripleurospermum. This plant is native to Eurasia and ...
produces over 30,000 seeds per plant. Not all of these will germinate at once, but over several seasons, lying dormant in the soil sometimes for years until exposed to light.
Poppy seed Poppy seed is an oilseed obtained from the opium poppy (''Papaver somniferum''). The tiny, kidney-shaped seeds have been harvested from dried seed pods by various civilizations for thousands of years. It is still widely used in many countries, ...
can survive 80–100 years,
dock A dock (from Dutch language, Dutch ''dok'') is the area of water between or next to one or a group of human-made structures that are involved in the handling of boats or ships (usually on or near a shore) or such structures themselves. The ex ...
50 or more. There can be many thousands of seeds in a square foot or square metre of ground, thus any soil disturbance will produce a flush of fresh weed seedlings.


Subsurface/surface

The most persistent perennials spread by underground creeping
rhizome In botany and dendrology, a rhizome (; , ) is a modified subterranean plant stem that sends out roots and shoots from its nodes. Rhizomes are also called creeping rootstalks or just rootstalks. Rhizomes develop from axillary buds and grow hori ...
s that can regrow from a tiny fragment. These include couch grass, bindweed,
ground elder ''Aegopodium podagraria'', commonly called ground elder, is a species of flowering plant in the carrot family Apiaceae that grows in shady places. The name "ground elder" comes from the superficial similarity of its leaves and flowers to those o ...
, nettles, rosebay willow herb, Japanese knotweed, horsetail and bracken, as well as
creeping thistle ''Cirsium arvense'' is a perennial species of flowering plant in the family Asteraceae, native throughout Europe and western Asia, northern Africa and widely introduced elsewhere.Joint Nature Conservation Committee''Cirsium arvense'' The standa ...
, whose tap roots can put out lateral roots. Other perennials put out runners that spread along the soil surface. As they creep they set down roots, enabling them to colonise bare ground with great rapidity. These include creeping buttercup and ground ivy. Yet another group of perennials propagate by
stolon In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external s ...
s- stems that arch back into the ground to reroot. The most familiar of these is the bramble.


Methods

Weed control plans typically consist of many methods which are divided into biological, chemical, cultural, and physical/mechanical control.


Physical/mechanical methods


Coverings

In a domestic gardens, methods of weed control include covering an area of ground with a material that creates an unsuitable environment for weed growth, known as a ''weed mat''. For example, several layers of wet newspaper prevent light from reaching plants beneath, which kills them. In the case of black plastic, the greenhouse effect kills the plants. Although the black plastic sheet is effective at preventing weeds that it covers, it is difficult to achieve complete coverage. Eradicating persistent perennials may require the sheets to be left in place for at least two seasons. Some plants are said to produce root exudates that suppress
herb In general use, herbs are a widely distributed and widespread group of plants, excluding vegetables and other plants consumed for macronutrients, with savory or aromatic properties that are used for flavoring and garnishing food, for medicinal ...
aceous weeds. '' Tagetes minuta'' is claimed to be effective against couch and ground elder, whilst a border of comfrey is also said to act as a barrier against the invasion of some weeds including couch. A layer of wood chip mulch prevents some weeds from sprouting.
Gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...
can serve as an inorganic mulch. Irrigation is sometimes used as a weed control measure such as in the case of paddy fields to kill any plant other than the water-tolerant rice crop.


Manual removal

Many gardeners still remove weeds by manually pulling them out of the ground, making sure to include the roots that would otherwise allow some to re-sprout. Hoeing off weed leaves and stems as soon as they appear can eventually weaken and kill perennials, although this will require persistence in the case of plants such as bindweed. Nettle infestations can be tackled by cutting back at least three times a year, repeated over a three-year period. Bramble can be dealt with in a similar way. A highly successful, mostly manual, removal programme of weed control in natural bush land has been the control of
sea spurge ''Euphorbia paralias'', the sea spurge, is a species of flowering plant in the family Euphorbiaceae, native to Europe, northern Africa and western Asia. The species is widely naturalised in Australia. It invades coastal areas, displacing local ...
by Sea Spurge Remote Area Teams in Tasmania.


Tillage

Plough A plough or plow ( US; both ) is a farm tool for loosening or turning the soil before sowing seed or planting. Ploughs were traditionally drawn by oxen and horses, but in modern farms are drawn by tractors. A plough may have a wooden, iron or ...
ing includes tilling of soil, intercultural ploughing and summer ploughing. Ploughing uproots weeds, causing them to die. Summer ploughing also helps in killing pests. Mechanical tilling with various types of cultivators can remove weeds around crop plants at various points in the growing process. An Aquamog can be used to remove weeds covering a body of water.


Thermal

Several thermal methods can control weeds. uses a flame several centimetres/inches away from the weeds to singe them, giving them a sudden and severe heating. The goal of flame weeding is not necessarily burning the plant, but rather causing a lethal wilting by denaturing proteins in the weed. Similarly, hot air weeders can heat up the seeds to the point of destroying them. Flame weeders can be combined with techniques such as stale seedbeds (preparing and watering the seedbed early, then killing the nascent crop of weeds that springs up from it, then sowing the crop seeds) and pre-emergence flaming (doing a flame pass against weed seedlings after the sowing of the crop seeds but before those seedlings emerge from the soil—a span of time that can be days or weeks). Hot foam causes the cell walls to rupture, killing the plant. Weed burners heat up soil quickly and destroy superficial parts of the plants. Weed seeds are often heat resistant and even react with an increase of growth on dry heat. Since the 19th century soil steam sterilization has been used to clean weeds completely from soil. Several research results confirm the high effectiveness of humid heat against weeds and its seeds. Soil solarization in some circumstances is very effective at eliminating weeds while maintaining grass. Planted grass tends to have a higher heat/humidity tolerance than unwanted weeds.


Lasers

In precision agriculture, novel agricultural robots and machines can use lasers for weed control, called "laserweeding". Their benefits may include "healthier crops and soil, decreased herbicide use, and reduced chemical and labor costs".


Seed targeting

In 1998, the Australian Herbicide Resistance Initiative debuted. gathered fifteen scientists and technical staff members to conduct field surveys, collect seeds, test for resistance and study the biochemical and genetic mechanisms of resistance. A collaboration with
DuPont DuPont de Nemours, Inc., commonly shortened to DuPont, is an American multinational chemical company first formed in 1802 by French-American chemist and industrialist Éleuthère Irénée du Pont de Nemours. The company played a major role in ...
led to a mandatory herbicide labeling program, in which each mode of action is clearly identified by a letter of the alphabet. The key innovation of the Australian Herbicide Resistance Initiative has been to focus on weed seeds. Ryegrass seeds last only a few years in soil, so if farmers can prevent new seeds from arriving, the number of sprouts will shrink each year. Until the new approach farmers were unintentionally helping the seeds. Their combines loosen ryegrass seeds from their stalks and spread them over the fields. In the mid-1980s, a few farmers hitched covered trailers, called "chaff carts", behind their combines to catch the chaff and weed seeds. The collected material is then burned. An alternative is to concentrate the seeds into a half-meter-wide strip called a windrow and burn the windrows after the harvest, destroying the seeds. Since 2003, windrow burning has been adopted by about 70% of farmers in Western Australia. Yet another approach is the Harrington Seed Destructor, which is an adaptation of a coal pulverizing cage mill that uses steel bars whirling at up to 1500 rpm. It keeps all the organic material in the field and does not involve combustion, but kills 95% of seeds.


Cultural methods


Stale seed bed

Another manual technique is the ‘ stale seed bed’, which involves cultivating the soil, then leaving it fallow for a week or so. When the initial weeds sprout, the grower lightly hoes them away before planting the desired crop. However, even a freshly cleared bed is susceptible to airborne seed from elsewhere, as well as seed carried by passing animals on their fur, or from imported manure.


Buried drip irrigation

Buried drip irrigation involves burying drip tape in the subsurface near the planting bed, thereby limiting weeds access to water while also allowing crops to obtain moisture. It is most effective during dry periods.


Crop rotation

Rotating crops with ones that kill weeds by choking them out, such as
hemp Hemp, or industrial hemp, is a botanical class of ''Cannabis sativa'' cultivars grown specifically for industrial or medicinal use. It can be used to make a wide range of products. Along with bamboo, hemp is among the fastest growing plants o ...
, ''
Mucuna pruriens ''Mucuna pruriens'' is a tropical legume native to Africa and tropical Asia and widely naturalized and cultivated. Its English common names include monkey tamarind, velvet bean, Bengal velvet bean, Florida velvet bean, Mauritius velvet bean, Yo ...
'', and other crops, can be a very effective method of weed control. It is a way to avoid the use of herbicides, and to gain the benefits of crop rotation.


Biological methods

A biological weed control regiment can consist of biological control agents,
bioherbicide Bioherbicides are herbicides consisting of phytotoxins, pathogens, and other microbes used as biological weed control. Bioherbicides may be compounds and secondary metabolites derived from microbes such as fungi, bacteria or protozoa; or phytotoxi ...
s, use of grazing animals, and protection of natural predators. Post-dispersal, weed seed predators, like ground beetles and small vertebrates, can substantially contribute to the weed regulation by removing weed seeds from the soil surface and thus reduce seed bank size. Several studies provided evidence for the role of invertebrates to the biological control of weeds


Animal grazing

Companies using goats to control and eradicate
leafy spurge Leafy spurge may refer to several species of plant in the genus ''Euphorbia'', including: * '' Euphorbia esula'', native to central and southern Europe * ''Euphorbia virgata ''Euphorbia virgata'', commonly known as leafy spurge, wolf's milk leaf ...
, knapweed, and other toxic weeds have sprouted across the
American West The Western United States (also called the American West, the Far West, and the West) is the region comprising the westernmost states of the United States. As American settlement in the U.S. expanded westward, the meaning of the term ''the Wes ...
.


Chemical methods


Herbicides

The above described methods of weed control use no or very limited chemical inputs. They are preferred by organic gardeners or
organic farmers Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product o ...
. However weed control can also be achieved by the use of herbicides. Selective herbicides kill certain targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often based on plant hormones. Herbicides are generally classified as follows: * Contact herbicides destroy only plant tissue that contacts the herbicide. Generally, these are the fastest-acting herbicides. They are ineffective on perennial plants that can re-grow from roots or tubers. * Systemic herbicides are foliar-applied and move through the plant where they destroy a greater amount of tissue. Glyphosate is currently the most used systemic herbicide. * Soil-borne herbicides are applied to the soil and are taken up by the roots of the target plant. * Pre-emergent herbicides are applied to the soil and prevent germination or early growth of weed seeds. In agriculture large scale and systematic procedures are usually required, often by machines, such as large liquid herbicide 'floater' sprayers, or aerial application. These are thought to likely have several substantial detrimental impacts (e.g. on soils, health and insects) – which may partly explain the development of alternatives described here – and there are also systematic procedures using herbicides that have lower impacts such as robots and machines that apply low amounts with high precision.


Organic approaches

Organic weed control involves anything other than applying manufactured chemicals. Typically a combination of methods are used to achieve satisfactory control.
Sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
in some circumstances is accepted within British Soil Association standards.


Bradley method

The
Bradley Method of Bush Regeneration Bush regeneration, a form of natural area restoration, is the term used in Australia for the ecological restoration of remnant vegetation areas, such as through the minimisation of negative disturbances, both exogenous such as exotic weeds and e ...
uses ecological processes to do much of the work. Perennial weeds also propagate by seeding; the airborne seed of the dandelion and the rose-bay willow herb parachute far and wide. Dandelion and dock also put down deep tap roots, which, although they do not spread underground, are able to regrow from any remaining piece left in the ground.


Hybrid

One method of maintaining the effectiveness of individual strategies is to combine them with others that work in complete different ways. Thus seed targeting has been combined with herbicides. In Australia seed management has been effectively combined with
trifluralin Trifluralin is a commonly used pre-emergence herbicide. With about used in the United States in 2001, it is one of the most widely used herbicides. Trifluralin is generally applied to the soil to provide control of a variety of annual grass an ...
and clethodim.


Herbicide resistance

Resistance occurs when a target plant species does not respond to a chemical that previously used to control it. It has been argued that over-reliance on herbicides along with the absence of any preventive or other cultural practices resulted in the evolution and spread of herbicide-resistant weeds. Increasing number of herbicide resistance weeds around the world has led to warnings on reducing frequent use of herbicides with the same or similar modes of action and combining chemicals with other weed control methods; this is called 'Integrated Weed Management'.


Farming practices

Herbicide resistance recently became a critical problem as many Australian sheep farmers switched to exclusively growing wheat in their pastures in the 1970s. In wheat fields, introduced varieties of ryegrass, while good for grazing sheep, are intense competitors with wheat. Ryegrasses produce so many seeds that, if left unchecked, they can completely choke a field. Herbicides provided excellent control, while reducing soil disrupting because of less need to plough. Within little more than a decade, ryegrass and other weeds began to develop resistance. Australian farmers evolved again and began diversifying their techniques. In 1983, patches of ryegrass had become immune to Hoegrass, a family of herbicides that inhibit an enzyme called acetyl coenzyme A carboxylase. Ryegrass populations were large, and had substantial genetic diversity, because farmers had planted many varieties. Ryegrass is cross-pollinated by wind, so genes shuffle frequently. Farmers sprayed inexpensive Hoegrass year after year, creating selection pressure, but were diluting the herbicide in order to save money, increasing plants survival. Hoegrass was mostly replaced by a group of herbicides that block acetolactate synthase, again helped by poor application practices. Ryegrass evolved a kind of "cross-resistance" that allowed it to rapidly break down a variety of herbicides. Australian farmers lost four classes of herbicides in only a few years. As of 2013 only two herbicide classes, called
Photosystem II Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
and long-chain fatty acid inhibitors, had become the last hope.


Weed societies

Internationally, weed societies help collaboration in weed science and management. In North America the Weed Science Society of America (WSSA) was founded in 1956 and publishes three journals: ''Weed Science'', ''Weed Technology'', and ''Invasive Plant Science and Management.'' In Britain, European Weed Research Council was established in 1958 and later expanded their scope under the name European Weed Research Society The main journal of this society is Weed Research. Moreover, the Council of Australasian Weed Society (CAWS) serves as a centre for information on Australian weeds, while New Zealand Plant Protection Society (NZPPS) facilitates information sharing in New Zealand. Strategic weed management is a process of managing weeds at a district, regional or national scale. In Australia the first published weed management strategies were developed in Tasmania, New South Wales and South Australian 1999, followed by the National Weeds Strategy in 1999.


See also

* Control of Bramble *
Crop weed Crop weeds are weeds that grow amongst crops. Despite the potential for some crop weeds to be used as a food source, many can also prove harmful to crops, both directly and indirectly. Crop weeds can inhibit the growth of crops, contaminate harves ...
* Integrated pest management *
Invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
* Mechanical weed control * Mulch *
Noxious weed A noxious weed, harmful weed or injurious weed is a weed that has been designated by an agricultural or other governing authority as a plant that is injurious to agricultural or horticultural crops, natural habitats or ecosystems, or humans or liv ...
* Pesticide application


References

{{Authority control Agricultural pests Agronomy Horticultural techniques Weeds Pest control