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Weed control is a type of pest control, which attempts to stop or reduce growth of , especially , with the aim of reducing their competition with desired flora and fauna including 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 , smothering with mulch, lethal wilting with high heat, , and Benthiocarb with (weed killers).
Weeds compete with crops for space, , water and light. Smaller, slower growing seedlings are more susceptible than those that are larger and more vigorous. 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 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. Stellaria media ( 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 plant weeds with , such as lesser celandine and oxalis, or with persistent underground stems such as Elymus repens ( 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 chemicals that inhibit the growth of other nearby plants.
Weeds can also host pests and diseases that can spread to cultivated crops. Charlock and Shepherd's purse may carry clubroot, nematode can be harboured by chickweed, fat hen and shepherd's purse, while the cucumber mosaic virus, which can devastate the Cucurbitaceae, is carried by a range of different weeds including chickweed and groundsel.
Pests such as may first attack weeds but then move on to cultivated crops.
Some plants are considered weeds by some farmers and crops by others. Charlock, a common weed in the southeastern United States, are weeds according to row crop growers, but are valued by , who seek out places where it blooms all winter, thus providing pollen for and other . Its bloom resists all but a very hard freeze, and recovers once the freeze ends.
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 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 Woodchips mulch prevents some weeds from sprouting.
Gravel 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.
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 by Sea Spurge Remote Area Teams in Tasmania.
Mechanical tilling with various types of can remove weeds around crop plants at various points in the growing process.
An Aquamog, an aquatic weed harvester, can be used to remove weeds covering a body of water.
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.Research report of DLR Rheinlandpfalz, September 2010: Weed control in seed cultures, especially arugula , Author: Dr. Norbert Laun, Institute "Queckbrunnerhof", Schifferstadt (Germany). Viewed on 14. February 2011.
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.
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.
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 . Herbicides are generally classified as follows:
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.
Sulfur in some circumstances is accepted within British Soil Association standards.
Perennial plant weeds also propagate by seeding; the airborne seed of the dandelion and the epilobium parachute far and wide. Dandelion and dock also put down deep , which, although they do not spread underground, are able to regrow from any remaining piece left in the ground.
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 and long-chain fatty acid inhibitors, had become the last hope.
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.
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
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, Mucuna pruriens, 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, , use of grazing animals, and protection of natural predators. (2025). 9780135028148, Prentice Hall. ISBN 9780135028148 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, knapweed, and other toxic weeds have sprouted across the American West.
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 farming.
Organic approaches
Organic weed control involves anything other than applying manufactured chemicals. Typically a combination of methods are used to achieve satisfactory control.
Bradley method
The Bradley Method of Bush Regeneration uses ecological processes to do much of the work.
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 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'.Plant Production and Protection Division: What is Integrated Weed Management (fao.org)
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.
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. Weed Research - Wiley Online Library 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.
(1997). 9780642214010, publisher. ISBN 9780642214010
See also
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