Certified GKH Gardening Expert
Lemon balm can be pruned to harvest or to invigorate its growth. It can be invasive if left to reseed. Trimming off the flowers right away can prevent a lot of the reseeding. It will die back with a hard freeze, but should return in spring, in multiples if it reseeded. I looked up your average weather, and it doesn't traditionally freeze, so you could go ahead and plant it outside. It can be grown indoors as well. Since you have a greenhouse, you could keep it there for the winter.
Here is information on lemon balm and daphne. Daphne is hardy in your area, but since it's wintertime, I would leave it in the greenhouse, too, for the winter.