You may do better asking on a forum with specialists on this plant such as growingontheedge.net or hardytropicals.co.uk. Rot sounds very bad. They are known to be difficult to grow. The trade in mature specimens got a poor reputation as many of them just slowly died because they didn't re-establish easily after being dug up. An important part of winter protection is to keep them (nearly) dry, so people who do keep them alive outside often use a rain roof or polytunnel as well as insulation. If you keep them dry enough in winter they can cope with about -6C or a bit lower even, I read.