Funny -- I just got some Mexican petunia from a neighbor and didn't know it was "Ruellia" until I looked it up just now. Mine is not dwarf, though.

Apparently the non-dwarf species is native to Mexico and has escaped throughout Florida and other warm parts of SE U.S. Where it has escaped, it's become a threat to native plants in wetlands.

Read more about Ruellia at: http://www.floridata.com/ref/R/ruellia.cfm

I'll monitor mine carefully, and if it starts acting invasive, I'll pull it up. Luckily, I'm not near a wetland, and my winters are harsher than Florida's, for sure! As long as it behaves itself, I'll leave it for the bunnies.