|Palo Verde tree, native to southern Arizona|
Many gardeners accept only indigenous plants as native. Gray's Anatomy of Botany says that to be indigenous there must be scientific evidence that the species has inhabited an area for a great length of time, that it's an integral part of local, evolutionary relationships with other plants and animals.
|Saguaro cactus, native to southern Arizona|
|Lantana, not native to southern Arizona|
And consider what we call invasive plants. They would not exist unless they were better adapted to current conditions than so-called native species. As Richard Darke says in the book The New American Landscape, " ...most natives are no more than earlier arrivals that established themselves because, at the time, they had a competitive advantage."
|gazania, not native to southern Arizona|
Non-native plants fulfill roles that can't be accomplished by natives. Many food plants grown in North American gardens do not have their origins here. Many flowers that gardeners grow are not indigenous to their area, but provide beauty, fragrance, color and pleasure, such as the lantana and gazania pictured above that I grow in my yard..
Most gardeners realize that no garden or landscape remains static, that local and global ecologies change. In the end, I think most gardeners strive for balance among all their plants.