After you pot it up to a larger pot, bury that pot in garden soil, in as protected a spot as you have. Take a larger black nursery pot, and cut the bottom out entirely. Invert this over the buried tree's pot, which will act as a wind break, and will maintain a slightly warmer interior temperature. If rain&snow doesn't do it, provide the Maple with a little water through the winter, when temperatures are at or above freezing. If you expect a real cold snap, fill the inverted pot with something like styrofoam peanuts, and cover with yet another pot for the duration of the cold snap.

Thank you Don. I'm so glad to know how to care for this tiny tree. I'll let you know if it lives come spring when it starts to put on leaves again.

Donn, Do you think this will work with physocarpus seedlings in zone 6? I have about ten seedlings from winter sowing that have survived this dry summer with minimal watering. Some are still tiny, about four inches tall, others are at least 12 inches high. I've noticed the ones with the darkest leaves are the smallest.

I probably should have fertilized them too. Oh well, it's undoubtedly too late now, ferts would make them put on a spurt of growth just as we're heading into fall and winter.

Or maybe I should just find a spot to plant them and then transplant them to more permanent spots in the spring?

Alison

Alison, Ninebark is much more cold hardy than Japanese Maple. It's good to at least zone 2, which means you could safely winter it in pots. Being overly cautious, I'd probably bury the pots in the soil anyway, but that's probably all I'd do.

It's not to late to fertilize them for root growth. Just don't use a high N fertilizer.