40s and 50s will slow but not stop growth. To stop growth you'd need low 30s. After it leafs out it will be pretty freeze sensitive.

You could just bring it into the house near a south facing window for them winter. Its not going to fruit for several years so the period of dormantcy isn't important yet anyways.

One a side note, I've found that to be a tough tree to grow in NC. The deer go for its leaves before anything else. Mine are down to their roots and if I can get them started again, I plan to leave them in pots on the screened porch until I can train them to a single stem about 5ft tall. Once they clear the deer's reach I'll put them back in ground.