This site provides names, distributional data, species abstracts, characteristics, images, crop information (and more) about plants of the U.S. and its territories. View it here: plants.usda.gov
EBSCO's GreenFile - Find scholarly, government, and general interest articles covering all aspects of human impact to the environment. This is a library database, so NMC ID and password are required.
The Biodiversity Heritage Library is rich source of legacy literature on natural history and botany, ranging in date from 1450 to the present.