from my experience if you plant you seeds towards the end of winter and let them come up naturally when it starts to warm up they seem better "acclimated" and can handle a frost much better than plants that havent been out in the cold and then get hit with a frost. I usually start my seeds in the ground in feb when its still snowing and ive never had one be damaged by frost even though they usually see a lot of it the first few weeks after they pop up. I was given this advice from an old timer and while i think half the shit he told me was purely made up or myth that advice actually seems to work...at least for me. Also putting a box up in a tree doesnt seem like it would work much better than using pots. The ground helps to insulate the plants and the box and pots would have about the same insulating properties which is probably pretty minimal. the most important thing for you babies in colder weather is making sure they gets lots and lots of direct sunlight.