A plant that is growing out of place or causing crop or garden problems is often called a weed. Some weeds are so troublesome that they are listed in state-by-state “noxious weed” lists that require them to be controlled.
Using a good weed identification guide, it is usually possible to identify most common weeds by their leaves, flowers or seeds. Flowering specimens are usually the easiest to identify to species, since many defining characteristics are clearly visible. If you have a hard time identifying a weed at its flowering stage, try it again later or collect several specimens at different growth stages to compare with photos and descriptions in an identification manual.
Some plants that grow in places where they are not wanted, such as poison ivy and thistle, are also classified as weed Others, like clovers and tall fescues, are grown as lawn grasses but sometimes become troublesome in gardens because they compete with desired crops. They are able to propgate easily by seed or runners and can be difficult to control with chemical herbicides.
Some invasive plants from other regions, countries or continents are also considered weeds because they can choke out natural ecosystems and invade pastures and crop fields. They are typically designated as invasive exotic plant species or WEEDS and may become the focus of regional or national coordinated eradication programs. On the other hand, some weedy plants are valued as cover crops or forage in agricultural fields. They are adapted to a wide range of environmental conditions and provide food for animals, soil health benefits and other functions.