Cold hardiest Cordyline australis I've personally witnessed is about -9c but generally -7 or -8 celcius without trouble. I have some seedlings started from a parent plant in the Northern part of Holland that is reported to have shaken -10c without any ill effects. This, from a grower and reputable nurseryman there. Whether or not they hold up to that reputation here, remains to be seen. Cheers, LPN.