What is a "Native" Plant?
Walter Muma

The term "Native Plant" is used with quite a bit of latitude these days. Different people have differing ways of determining what a "Native Plant" really is. Here's a brief rundown of some of them. Please note that this discussion is based on Ontario, Canada. However, the general principles apply anywhere.

Geographical distribution: 

Continental: For some people, a Native Plant is one that occurs anywhere in North America. That's probably an overly wide definition, as some plants that are truly native to western North America can wreak havoc when grown in the eastern part of the continent.
Political: Some people actually define a Native Plant as one that grows in Canada, or in the United States.  This is rather nearsighted, as nature does not respect political boundaries.
State/Province: Same story for these political divisions
Regional: Some people define a plant's "native area" as being within a certain radius, say 25km, or 100km or whatever. The reasoning here is that the genetic make-up of the plant will vary from region to region. So, you should not transplant any plant further than the defined radius from it's "home" area, nor plant seeds from it beyond that area.
Historical ranges: This can get uncertain without accurate historical data.
Ecosystem: This is one of the best determinants of "Native". The problem is that determining whether a particular ecosystem is one, or two, or more separate ecosystems. There are a great many variables: soil, vegetation, climate & micro-climate (rainfall, temperature, humidity, cloud cover, wind, etc), human influence, animal & birdlife, ground moisture, geographical, proximity of other ecosystems and what type they are, and on and on and on!