Weed control

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.

Onions are one of the most vulnerable, because they are slow to germinate and produce slender, upright stems[citation needed].

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].

Transplanted crops raised in sterile soil or potting compost gain a head start over germinating weeds.

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.

One study found that after competition had started, the final yield of onion bulbs was reduced at almost 4% per day.

Some perennials such as couch grass exude allelopathic chemicals that inhibit the growth of other nearby plants.

Charlock 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.

For example, several layers of wet newspaper prevent light from reaching plants beneath, which kills them.

Although the black plastic sheet is effective at preventing weeds that it covers, it is difficult to achieve complete coverage.

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.

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.

Mechanical tilling with various types of cultivators can remove weeds around crop plants at various points in the growing process.

Several research results confirm the high effectiveness of humid heat against weeds and its seeds.

[10] Their benefits may include "healthier crops and soil, decreased herbicide use, and reduced chemical and labor costs".

A collaboration with DuPont led to a mandatory herbicide labeling program, in which each mode of action is clearly identified by a letter of the alphabet.

[11] 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.

[11] Another manual technique is the ‘stale seed bed’, which involves cultivating the soil, then leaving it fallow for a week or so.

These are thought to likely have several substantial detrimental impacts (e.g. on soils, health and insects)[18][19][20] – 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.

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.

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.

[25] Herbicide resistance recently became a critical problem as many Australian sheep farmers switched to exclusively growing wheat in their pastures in the 1970s.

[11] In 1983, patches of ryegrass had become immune to Hoegrass, a family of herbicides that inhibit an enzyme called acetyl coenzyme A carboxylase.

Hoegrass was mostly replaced by a group of herbicides that block acetolactate synthase, again helped by poor application practices.

As of 2013 only two herbicide classes, called Photosystem II and long-chain fatty acid inhibitors, had become the last hope.