You will probably see the first growth in March when the new apple green young shoots burst through the Polar Cap. This growth will start to harden by May when the tree will be ready to put outside. Remember to aclimatise the tree gradually if you are moving it from a warm temperature. I would suggest the best time to put it out would be the middle to the end of May when the worst of the frosts have past. Protect the tree against the worst of the weather through its first winter outside but after that It should be mature enough to fend for itself.