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DarrenI have always found them perfectly cold hardy - they even survive the worst winter sitting un-plunged in seed pots.I think as with so many plants it is the moisture levels at the time of the cold that is critical. If they are relatively dry they come through but sitting wet the suffer.I have lost a fish box full of plants in the past during a wet period but then they succumbed to wet rot.I was very active in cross pollinating them during the warm weather of last week but the 20degreeC fall in temperature that we have is not exactly conducive to fertilisation.
I remember a forumist expressing surprise at my losing Rhododendron keiskei 'Yaku Fairy' to the cold last year, but another forumist nearby backed me up - it seems the cause was a freeze following an extremely wet period.