Weed

Weed is the generic word for a plant growing in a spot where it is not wanted. The most prominent use of the word is in connection with farming, where weeds may damage crops when growing in fields and poison domesticated animals when growing on pasture land. Many weeds are short-lived annual plants, that normally take advantage of temporarily bare soil to produce another generation of seeds before the soil is covered over again by slower growth; with the advent of agriculture, with extensive areas of ploughed soil exposed every year, the opportunities for such plants have been greatly expanded.

Other plants have become weeds by being transferred by human action to locations where they have no natural grazing predators; the classic case is the prickly pear (Opuntia stricta), which overran vast areas of Australia until a moth, Cactoblastis cactorum was introduced. This is frequently quoted as the classic example of successful biological pest control, eliminating >90% of the prickly pear infestation within 10 years.

In cases like the prickly pear in Australia, the weeds are termed invasive exotics (or exotic invasives). This term is applied when a plant is an introduced foreign plant that then invades and disturbs natural ecosytems, displacing species native to the target ecoregion.

In order to reduce weed growth many strategies have been developed. The most basic is ploughing, which cuts the roots of annual weeds. In modern times, chemical weed killers have caused environmental damage, and efforts are being made to reduce the use of such substances (see for example genetic engineering, organic gardening).

Plants that are often considered weeds include:

and many more

The 1881 Household Cyclopedia adds:

To clear the ground of weeds is an operation no less necessary in husbandry than the disposing it to produce vegetables of any kind in plenty.

Annual weeds, or such as spring from seed and die the same year, are most easily destroyed. For this purpose, it will be sufficient to let them spring up till near the time of ripening their seed, and then plough them down before it comes to maturity. It is also of service to destroy such weeds as grow in borders or neglected corners, and frequently scatter their seeds to a great distance, such as the thistle, dandelion, rag-weed, etc., for these propagate their species through a deal of ground, as their seeds are carried about with the wind to very considerable distances. A farmer ought also to take care that the small seeds of weeds, separated from corn in winnowing, be not sown again upon the ground; for this certainly happens when they are thrown upon a dunghill, because, being the natural offspring of the earth, they are not easily destroyed. The best method of preventing any mischief from this cause it to burn them.

Perennial Weeds are such as are propagated by the roots, and last for a number of years. They cannot be effectually destroyed but by removing the roots from the ground, which is often a matter of some difficulty. The only method that can be depended upon in this case is frequent ploughing to render the ground as tender as possible, and harrowing with a particular kind of harrow, in order to collect these pernicious roots. When collected, they ought to be dried and burnt, as the only effectual method of insuring their doing no farther mischief.

Besides those kinds of weeds which are of an herbaceous nature, there are others which are woody, and grow to a very considerable size; such as broom, furze and thorns. The first may be destroyed by frequent ploughing and harrowing, in the same manner as other perennial weeds are. Another method of destroying broom is by pasturing the field where it grows with sheep.

The best method of extirpating furze is to set fire to it in frosty weather, for frost has the effect of withering and making them burn readily. The stumps must then be cut over with a hatchet, and when the ground is well softened by rain it may be ploughed up, and the roots taken out by a harrow adapted to that purpose. If the field is soon laid down to grass they will again spring up; in this case, pasturing with sheep is an effectual remedy. The thorn, or bramble, can only be extirpated by ploughing up the ground and collecting the roots.

In the month of June weeds are in their most succulent state, and in this condition, after they have lain a few hours to wither, hungry cattle will eat greedily almost every species. There is scarcely a hedge, border, or a nook, but what at that season is valuable; and it certainly must be good management to embrace the transient opportunity, for in a few weeks they will become nuisances.

Weed is also common slang for marijuana.