Thanks for the clarification Porkpal. I missed that detail.
I really don't think that it would survive in a container outside with the low temperatures mentioned. -25ºC is -13F and -30ºC is -22ºF. Having the pot on a balcony is more exposed as well.
I have had all my container roses die with -20ºC. from an unexpected cold snap in the fall. Even though the ground freezes thru the winter, the ground temperature will be much warmer than the air temperature. Especially if there is snow cover.
I've heard that you can overwinter container plants that are said to be hardy TWO zones colder than yours. The only plants that I had success with is an Ostrich Fern and a small evergreen tree.
