1. The size of the tree to be transplanted
2. The Age of the tree
3. The overall health
4. What is the condition of the root system
5. Timing (when to transplant)
The size of the tree is most important, Not whether or not the tree will survive a transplant, but what size of a root ball must be dug, picked up, transported and replanted. In theory any size tree can be moved if enough of the root system remains undamaged during the transplanting. A root system of a mature 6-8 foot Crimson Queen Japanese Maple allowed to develop naturally without any restrictions can spread out over 12 feet wide and up to 3 feet deep. This is a huge root ball and probably not anything a home owner without heavy equipment would be able to tackle.
However a 3-4 year old tree 3 foot tall and wide is a size a home owner could move with some help. Generally speaking a tree with a trunk caliper of less than 1 inch (about the size of a broom handle) could be moved with a 12-18 inch root ball. That would be tall and wide and weighing about 50-80 lbs. A 1-2 inch caliper tree would need a 18-24 inch root ball weighing about 80-150 lbs, a 2-3 inch caliper tree a 24-30 inch root ball weighing about 150-300 lbs.
Age is also a factor, the older the tree the further away from the trunk the feeder roots are located. For the sake of not getting too complicated, the root tips are where the majority of water and nutrients are absorbed. The part between the root tip and the trunk of the tree is more for structural support and does little to keep the plant nourished. So the older and larger a tree is the larger the root ball must be to contain enough viable root tips to continue to supply the tree with water and nutrients.
A healthy tree will have a better root system and will be more likely to survive a transplant. Normally a tree that looks sick on top will also have a compromised root system. It is possible and likely that some of the only viable roots will be severed which will cause the tree to die as soon as it is stressed due to heat or drought.
Timing is also very important. It is best to transplant in late winter or very early spring just before the tree would naturally start breaking bud. (As opposed to the autumn, which is a primary time to plant a new Japanese Maple tree). I like this time because it gives the tree the shortest time with a compromised root system before the soil starts to warm up and allows new roots to grow. Also by cutting some roots when digging the root ball the tree will automatically be set back and will not push out new leaves as quickly. This will give the root system some extra time to become established before the tree has to support all the new leaves.
I also like to prune about 25% of the trees canopy back during the transplant. This will reduce the stress on the smaller root system.This is an important factor that most amateur gardeners do not follow and it can easily lead to a lack of success when transplanting. So prune, Prune, PRUNE!
Adding a low amount of a low nitrogen fertilizer and root stimulator during planting will help nourish the tree and aid in survival.
Keep the soil moist but not overly wet. Roots grow when looking for water. If you keep the soil overly wet the roots will have no need to grow and will remain weak. Once temperatures becomes hot the tree will quickly become stressed because of the weak root system, and will have a much lower rate of survival. It is important that this ratio of root to tree will exist regardless of the care taken, therefore you want to carefully follow the instructions of not over watering and at the same time not allowing to dry out. This can be tricky, and it takes baby-sitting the tree and checking it on a daily basis. Remember this is one of the most crucial times in the entire transplantation process. The same will apply to a newly planted tree. Don't let the roots dry out completely or you may lose the entire operation and your tree.
The tree watering bags that are sold at your nurseries come in very handy at this time. Put one in place, and keep it full of water. The soil will slowly absorb the water, remain moist but not wet, and will still not allow for the tree to dry out. But be careful that complacency doesn't set in and you ignore the signal that the tree is being over/under watered. Check with your fingers every day and make sure you are putting in the correct amount of water.
By following these tips and you should be able to successfully transplant your Japanese Maple Tree (or any other deciduous tree you may wish to move in your garden.)
