How to Identify a Weed
A weed is any plant that grows where and when it shouldn't, in competition with crops or other desirable plants. The term often applies to plants that compete with and choke out agricultural crop species, but can also refer to other plants that cause harm to people or wildlife, or interfere with some aspect of land use. Serious weeds that disrupt ecosystems and degrade or displace other species may be designated “noxious” or “invasive,” and subject to regional or nationwide coordinated eradication efforts (think kudzu or purple loosestrife).
Some weeds are native and therefore not considered harmful, while others are introduced and able to thrive in human-modified environments where they're allowed to expand unchecked. Some weeds are considered beneficial because they help stabilize soil, add organic matter, provide valuable food or fodder, or provide habitat for wildlife.
A well-written field guide or manual of agricultural weeds, or an online or computerized taxonomic key to weeds in your area, will help you identify a weed by its easily observable characteristics. A good key starts you with a series of “either-or” pairs of characteristics, and gradually narrows down your options until you reach a final list of possibilities that match the specimen in question.
Once you've narrowed down your choices, examine the plant in detail to learn its characteristics (see A Basic Illustrated Glossary of Plant Identification Jargon). If you can collect a sample for further study (recommended), take it home and study it under various light conditions.weed