Editing Weed control (section)

== 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]]s, water and light. Smaller, slower growing seedlings are more susceptible than those that are larger and more vigorous. [[Onion]]s are one of the most vulnerable, because they are slow to germinate and produce slender, upright stems{{Citation needed|date=August 2019}}. 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{{Citation needed|date=August 2019}}. 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 [[Chenopodium album|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|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.{{Citation needed|date=August 2019}}

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.<ref name="BleasdaleJohn)1991">{{cite book|first1=J. K. A. |last1=Bleasdale|last2=Salter |first2=Peter John | title=The Complete Know and Grow Vegetables|url=https://books.google.com/books?id=Sk0sAAAACAAJ|date=1 January 1991|publisher=Oxford University Press|isbn=978-0-19-286114-6}}</ref>

[[Perennial plant|Perennial]] weeds with [[bulbil]]s, such as [[lesser celandine]] and [[oxalis]], or with persistent underground stems such as [[Elymus repens|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]] chemicals that inhibit the growth of other nearby plants.<ref>{{Cite journal |last1=Ringselle |first1=Björn |last2=De Cauwer |first2=Benny |last3=Salonen |first3=Jukka |last4=Soukup |first4=Josef |date=2020-08-12 |title=A Review of Non-Chemical Management of Couch Grass (Elymus repens) |journal=Agronomy |volume=10 |issue=8 |pages=1178 |doi=10.3390/agronomy10081178 |issn=2073-4395|doi-access=free |hdl=11250/2682534 |hdl-access=free }}</ref>

Weeds can also host pests and diseases that can spread to cultivated crops. [[Charlock]] and [[Shepherd's purse]] may carry [[clubroot]], [[nematode|eelworm]] can be harboured by chickweed, fat hen and shepherd's purse, while the [[cucumber mosaic virus]], which can devastate the [[Cucurbitaceae|cucurbit family]], is carried by a range of different weeds including chickweed and groundsel.

Pests such as [[cutworm]]s 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|US]], are weeds according to row crop growers, but are valued by [[beekeeper]]s, who seek out places where it blooms all winter, thus providing pollen for [[honeybee]]s and other [[pollinator]]s. Its bloom resists all but a very hard freeze, and recovers once the freeze ends.