The sites below aim at helping people identify plants within a certain category, such as weeds or wildflowers. Some present "keys" while others present photographs for visual matching. "Keys" are often dichotomous; that is, they offer the user two choices based on a plant's characteristics, and continue to offer choices until the choice is narrowed down to the plant's identity. For example, a tree key might ask are the leaves alternate or opposite, then are the leaves' edges smooth or serrated, etc. until a final decision about the plant's identity is arrived at.
Printed field guides are another aid to identifying plants. Field guides may also present keys, illustrations or both, and may be the handiest identification aides. A field guide generally covers a certain geographic area, such as the eastern U.S. or a particular state or region. See this list of field guides cataloged at the National Agricultural Library.
Recognizing plant families is a good first step in plant identification. Find books that help you learn the families.
