Dav4

I've never kept boxwoods as bonsai, so take this for what its worth. Boxwoods are hardy broadleaf evergreens which are commonly planted as landscape trees in regions where the temps routinely fall to 0F or lower. Unless you have a less hardy variety, as Mcspeed mentioned, your tree should be able to tolerate temps much lower then -4c provided some winter protection is offered. Broadleaf evergreens are very suseptible to dessication from the winter wind and sun. Up here in zone 6 MA, lots of folks just finished wrapping their landscape boxwoods and hollies with burlap to prevent foliage winter burn...keeping your tree in a protected area out of the wind and sun does the same thing. Mulching the pot will also protect the roots from extreme winter cold, as well. Hope this helps,


Dave

Edit- I just read your last reply. That situation may work, with some caveats. Generally, the goal with overwintering trees is to keep them dormant until springtime. Dormancy requires temps that don't exceed 40F consistently. You're going to have to watch the temps closely to make sure it doesn't get too warm, which would allow the tree to break dormancy and start to grow, and I doubt there is enough light there to allow for normal growth. You will have to closely monitor you tree for watering needs, as well.

I've learned that the most effective overwintering scheme is usually the most simple one. I'm a big fan of putting my trees away for the winter and essentially forgetting about them until spring. Placing your tree in a protected area outside and mulching the pot will allow you to do this, more or less.

Dave