USDA plants database

The USDA has an online plants database that you might find useful. You can search the database by common or scientific name and by state.

"The PLANTS Database provides standardized information about the vascular plants, mosses, liverworts, hornworts, and lichens of the U.S. and its territories."

With 30,000 plant photos and plenty of information about each, it certainly makes it easier for the average client to check out the things they hear from their landscape architect or the local nursery. (Hat tip to MetaFilter)