Answer: Often the ground freezes and thaws numerous times during fall and spring and can cause heaving of shallowly rooted plants, and this is actually more dangerous than solidly frozen ground or ground covered with snow. So I think the main concern is to try to plant them early enough that they can root well before the season starts into hard freezes (as opposed to mere frosts). Unfortunately I don't have a date to give you, so I would have to say the sooner you plant them out the better. You should also use several inches of mulch around the crowns to help insulate the ground. Good luck with your seedlings!
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