We have a terrace with a Japanese maple growing in a pot. The tree is about 5 years old. We would like to re-pot it in a larger container. The leaf buds are almost opening. Is it safe to do it now? What is the best soil to use. We have triple mix and compost with river stones for drainage. Any help would be greatly appreciated. We live in downtown Toronto and the tree gets a mix of sun and shade.
Experts in growing Japanese maples have a slight preference for the fall for transplanting, whether into another container or to a new spot in the garden. This is because new root growth continues to take place during the fall and early winter when temperatures are still not below freezing, and the tree can direct its carbohydrate stores to root growth. Transplanting in early spring is fine as well, as long as you are careful to avoid damaging any newly emerging soft growth.
It is important to choose a container that is not too big for your tree. Your container should have drainage holes. The general rule is to use a container that is no larger than double the volume of the rootball. A mixture of half container soil and half aged bark or compost is a good growing medium.
If your tree has been in its container for five years, when you remove it from its pot you should check to see whether it is rootbound: i.e., how crowded its roots are. It may benefit from root pruning. It is often recommended that this is best done when the tree is dormant, another reason that transplanting in fall is preferred. But it can be done in spring with care. Remove the tree carefully from its container, and gently prune or cut off the outer layer of the roots, especially any long and tangled roots that curl around the edge of the pot. Gently loosen other roots, being careful not to disturb the centre of the rootball. This may seem daunting, so for a thorough video presentation of this process, you may be interested in “Root pruning Japanese maple container gardens”: https://www.youtube.com/watch?v=q-M3bJHJcfM
Finally, here is a website that gives some step-by-step directions for the repotting process – don’t forget the final step: water well. https://homeguides.sfgate.com/repot-rootbound-japanese-maple-21059.html
Your Japanese maple is a lovely, shapely specimen, obviously in a location that suits it very well.