If you let them put their effort into root development instead of flowers they are much more likely to overwinter. Winter wetness and poor drainage do them in. I also do not plant after July. I have had much better luck with them since reading this article and following what it says. I now have many plants for over 5 years.
http://www.ballpublishing.com/...
http://plantsnouveau.com/2009/...
