Weeds

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. 

The notion of "wanted" is of course entirely in the eye of the beholder. A weed in one situation might be a wildflower in another. Some people love dandelions for their yellow buttons, like gold coins on the ground. Children enjoy blowing the puffball seed heads that form on the dandelion; and adults might utilize the dandelion root as a herbal medicine. In fact you can even find dandelion greens for sale in certain restaurants or grocery stores in the United States. Yet the caretaker of a lawn will generally regard the dandelion as a troublesome weed. 

Noxious weeds 

The term Noxious weed is now used for especially difficult to manage weeds. These typically are invasive plants, which may be difficult to control, or may be poisonous, or may be detrimental to the environment, including native wildlife. Typically they have been defined as noxious by legislation or government regulation. Government regulations usually make it an offence to cultivate them, transplant them, or disseminate their seeds. In some cases it may be an offence even to permit them to grow by inaction. 

Invasive species 

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 species (or exotic invasives). This term is applied when a plant is an introduced species that invades and disturbs natural ecosytems, displacing species native to the target ecoregion and causing harm. 

Weed control 

In order to reduce weed growth, many weed control strategies have been developed. The most basic is ploughing, which cuts the roots of annual weeds. In modern times, chemical weed killers known as herbicides have been widely used. However, to the extent that such chemicals leave a harmful residue in the soil, they can produce unanticipated adverse environmental effects, 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: 

Dandelion 
Giant salvinia 
Goutweed 
Japanese knotweed 
Kudzu 
Water hyacinth 
Johnson Grass 
Dock 
Plantain 
Purple dead nettle 
Henbit 
Shepherd's purse 
Cleavers 
Thistle 
Lambsquarters 
Violets 
Quackgrass 
Crabgrass 
Sunflower 
Hairy vetch 
Milkweed 
Queen Annes Lace 
Wild Onion 
and many more