Early spring, right after the ground has thawed, is the optimal time to plant all trees. While many trees may be successfully planted at other times of the year, spring is best.
Why very early spring? The act of transplanting a tree causes the removal of as many as 95% of the tree's root system. Late winter and early spring are times of rapid root growth in trees. Root growth slows to a crawl once the tree has produced leaves. By planting early, you provide trees with the optimal amount of time for root recovery prior to experiencing the stresses of summer heat and drought.