A dragon was rescued from a freezing roadside here in Iowa last month. Shows how tough they can be and how well they can tolerate cold temperatures if needed, granted this was a VERY extreme case.
http://www.kcrg.com/content/news/Dubuque-County-Deputies-Save-Frozen-Bearded-Dragon-369178371.html
http://www.kcrg.com/content/news/Dubuque-County-Deputies-Save-Frozen-Bearded-Dragon-369178371.html