A weed is any plant that grows where it’s not wanted–a horticultural nuisance, a pasture pest, an orchard invader, or even a crop-destroying competitor. The term weed can be a derogatory pejorative, as in Ralph Waldo Emerson’s “Shall we not rejoice that the weeds have virtues not yet discovered?” But weeds are part of the farm ecosystem and provide food for birds, livestock, and insects, enrich the soil, and sometimes serve as the host for beneficial fungi that help to suppress disease in crops.
Many of the weeds that appear on state-by-state lists of noxious species are not native to the area where they are found; they have been intentionally introduced to the agricultural landscape and then spread into areas where they were not intended. Often, these invasive exotic species possess the ability to rapidly reproduce vegetatively and have adapted to their new environment by adapting to conditions such as low or excessive levels of certain nutrients; droughts or floods; temperatures extremes; or frequent mowing or grazing.
An excellent way to distinguish between similar weeds is to consult a good field guide or manual and use it’s illustrated glossary of terms. A good guide will also include photos of flowers, fruit, roots, stems, and other belowground parts to assist with identification. Keep in mind that some plants may be known by more than one common name, such as lambsquarters (Chenopodium album) or redroot pigweed (Amaranthus retroflexus). The UC IPM Weed Photo Gallery includes images of several hundred weeds from the Northern California region.