A weed in an agricultural plot may be integral to a forest ecosystem and an invasive species in one country may be valuable in its native country.
Weeds and human civilization have a long history.
Ragweed growing in a field or vacant lot next to a residential area may cause problems with allergies from the pollen.
Perennial weeds often have underground stems that spread out under the soil surface or, like ground ivy (Glechoma hederacea), have creeping stems that root and spread out over the ground.
Weeds are those plants that are undesirable in their particular environment.
Plants commonly considered weeds generally share similar adaptations that give them advantages and allow them to proliferate in disturbed environments whose soil or natural vegetative cover has been damaged.
Some modern species of domesticated flower actually originated as weeds in cultivated fields and have been bred by people into garden plants for their flowers or foliage.
Dandelions, while being considered by some a lawn weed, are one of several species that break up hardpan in overly cultivated fields, helping crops grow deeper root systems.
The term weed in its general sense is a subjective one, without any classification value, since a weed is not a weed when growing where it belongs or is wanted.
A number of plants labeled by some as weeds, such as the dandelion Taraxacum, are edible, and their leaves and roots may be used for food or herbal medicine.
The term weed is not a biological term, since it is an arbitrary term.
One person's weed may be another person's flower or source of food or medicine.
Some so-called "weeds" may have other beneficial effects, such as drawing away the attacks of crop-destroying insects.
Weeds in pastures may be ones that are poisonous or inedible to the livestock.
Some have been classified as noxious weeds because if left unchecked, they often dominate the environment where crop plants are to be grown.
An example is the corncockle, Agrostemma, which was a common field weed exported from Europe along with wheat, but now sometimes grown as a garden plant.
Weed is a term for any plant growing in a setting where it is not wanted, such as undesirable plants in a garden, lawn, or agricultural field.
A number of weedy species have developed allelopathy—chemical means to prevent the germination or growth of neighboring plants.