Nativity opens a lot of problems.
One problem is how long the list can be as Evan pointed out. Symphyotrichum lanceolatum is shown to be native to all of the Lower 48 states as well a Canada. Listing them all would create a long list.
Another problem is, going to be how you define Native. One common definition for a US native is a plant that was growing in a location before Europeans arrived. But others have argued that Native Americans were using plants and moved them to different location long before Europeans arrive so there is no way to know if a particular plant is truly native to that location.
There is also the problem of what source do you use to determine nativity? Kim you mentioned USDA, but while researching Hibiscus I found that the USDA listed Hibiscus bifurcatus as native to Florida, but gave no county level info. Biotia of North America (another source I use) does not even show H. bifurcatus as even growing in the US, much less native. I contacted someone in Florida that works with natives of that state and he told me he knows of no records of it ever being recorded in Florida.
Then there is the problem of cultivars. Cultivars are created by people so, I do not see how they can be considered native. I have a Lonicera sempervirens 'Major Wheeler'. While L. sempervirens is a native, I do not consider 'Major Wheeler' a native, but nurseries jumping on the native bandwagon will sell it and other cultivars as native. I guarantee that someone will buy on these plants and list it in the database as a native.
