A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs or goals. It can be a serious problem in agricultural and recreational areas, but it also provides valuable food, fuel and medicine.
If you’re not familiar with a specific weed, the easiest way to identify it is to compare it with photos or diagrams in a good weed identification manual. A strong likeness and close match of defining characteristics suggests a correct identification. A discrepancy in one or more defining characteristics indicates that the identification is incorrect and that you need to try again.
The University of Illinois and Virginia Tech Cooperative Extension have excellent weed identification resources, including a searchable database that allows you to narrow down possibilities by flower color or other taxonomic keys. Clemson University has a similar tool.
Another option is to visit your county Extension office or a university weed science department for help with identification. You can also bring in samples of suspect weeds at different stages of growth and at various times of the year for evaluation.
Some weeds cause problems in crop fields by competing with crops for water and nutrients, or by blocking access to sunlight. Others, such as wild mint and poppy, can poison livestock, and certain weeds, such as ground ivy and creeping Charlie, may interfere with the mycorrhiza symbiosis between roots of plants and soil fungi, weakening or killing them.