BONAP's North American Plant Atlas (NAPA)
US County-Level Species Maps: List by Genus
BONAP (Biota of North America Program) maintains relatively complete phytogeographic and related botanical databases for all free-living vascular plants found in North America (north of Mexico). For over four decades, we have worked collaboratively with many federal and state government agencies, private groups and individuals across the continent to produce and edit our databases.
last updated 2014.
Seems to have more county data than the USDA Plants Database. Also the maps are more user friendly than those in the USDA database.