Agricola   (U. S. Department of Agriculture -- National Agricultural Library)
American Phytopathological Society (plant diseases)

Burpee Seed Company    This commercial site has several useful references for gardeners including: a plant nutrition guide, a growing zone finder, and a library of articles about various garden vegetables and flowers.

Cal Photo: Plants A searchable database of 58,833 images of plants that can be browsed by either scientific or common names.

Cornell University Poisonous Plants Informational Database

Internet Directory for Botany

Plant Diseases Diagnostics

Plant Disease Control (Oregon State U.)  

Gallery of Plants -- from the USDA   and  USDA Plants National Database

Plants for a Future   A searchable database of edible and medicinal plants

Plants In Motion  Watch plants do amazing things and learn some interesting stuff while you are at it!

Poisonous Plants Reference Chart from the Texas State Department of Health and the National Safety Council

Trees -- Fact Sheets

What Tree Is It?

National Arbor Day Organization Tree Identification Guide

U. S. National Arboretum   The National Arboretum offers the best tools for identifying trees including a definitive U.S. zonal map showing where various trees grow.

Weeds Gone Wild: Invasive Plants

Yahooligans Botany