I have had a 2 ft. potted Blue Spruce inside since Christmas, in a cool room. It started to grow new branches at the top, but now the bottom branches are losing their needles. I would like to transplant it outside this spring, and wondered how long I need to wait (in Toronto). It will be in partial shade.
Once the soil in your garden is completely thawed, dried out and workable, you can go ahead and move the tree out into your garden. Find the sunniest location you can away from your house and any other structure as Blue Spruce trees can reach 60 feet in height and 20 feet in width at maturity. Their growth rate is moderate which means about 12 inches each year.
After identifying a planting location, dig a hole that is about twice the width of the current container. This will allow you to maneuver and straighten the tree in the hole with less damage. Remove your tree from its container and examine the roots. If they are tightly wound in circles or if it is totally pot bound, use your hands to gently tease the roots apart. The larger hole will also enable you to spread out the roots before backfilling. Make sure the root flare is above or at soil level – not below. Try to fill in all the space around the roots with the soil that was removed from the hole. Do not add amendments to the hole.
Water the plant in thoroughly and continue to water daily for the first couple of weeks. After that you can water every second day throughout the summer, continuing until freeze up in the late fall. When you water give a deep soaking, soaker hoses work well for this. A 2-3 inch layer of mulch around the tree will help maintain soil moisture and keep down weeds but be careful to keep the mulch from touching the tree trunk bark.
The Garden Guide Planting A Tree for Life gives step by step instructions for the successful planting of a tree.