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You want to put it where it will stay coldest the longest (below 40F). The problem isn't so much the minimum cold temperatures as it is late spring freezes. Mume wants to come out of dormancy very early, which is what we treasure about them: the early flowering. But once they flower, they also lose much of their freeze resistence. This is the reason I am having so much trouble growing them here even though I am in zone 8. The longer you can delay flowering, the better. If in the ground covered with snow and in the shade will keep them dormant until freeze danger passes, then that's fine. I wouldn't trust leaving them outside if they are in the sun, that's just asking for trouble. I worry that your shed may heat up too fast in the spring, but you would know better than I about that.
Brent
EvergreenGardenworks.com
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