Japanese Maple (Acer palmatum) are one of the classic bonsai species and make great trees for bonsai, especially for beginners. They grow well in containers and are generally easy to care for. [I particularly love their spring foliage and delightful autumn colors.]
Japanese Maples need direct sunlight every day to maintain health. We recommend 4-6 hours in the morning, and then indirect light the rest of the day. In hotter and drier climates some filtered sun may be better. Early spring work on mature Japanese Maple bonsai includes pinching shoots and thinning out leaves. Without this work, interior growth will become weak and potentially die off from lack of sunlight.
Before you pinch or prune your maple in spring, think about what your goal is for pruning. If you have a young tree and you are developing branches, or want the trunk to become larger, then pinching or removing shoots will be counter-productive. If you have a tree that has a good size trunk and you just want to make branches, then pinching and thinning might be a good idea.
Pinching shoots stops branch elongation. In early spring work, focus on pinching back the stronger shoots that have two or more sets of leaves. Usually the strongest shoots are at the top of the tree. Allow the smaller, weaker buds and shoots to grow. Up to half the shoots may need pinching - that’s ok. This helps to balance growth throughout the tree.
Later in the spring, another round of pinching the tips of new shoots may be needed. Sometimes in the first round of pinching you might miss a shoot that needed to be pinched. In that case, it may grow longer and begin to form wood, so it will require cutback with scissors instead of pinching.
Thinning Leaves is key for the long term maintenance of maples because they can create relatively dense canopies of leaves. If the interior leaves are shaded too much they may end up dying and the buds that they were supporting may also die. Thinning the exterior foliage allows more light into the interior.
Japanese Maples grow leaves in pairs from each node. To begin thinning, remove one leaf from each pair at each node, on the outside of the canopy. If the twig structure of your tree is really dense, you may choose to thin leaves from the interior as well. Thinning work is done once you can see enough light through the tree to know that the light will reach the interior leaves.
Defoliating Mature Japanese Maples
Later in the spring, come back to your Japanese Maple to decide if it’s ready for defoliation. Removing all of the leaves will force the tree to grow new leaves, leading to a reduction in the size of the leaves and an increase in ramification (production of fine twigs). In warmer climates it's kind of a no-brainer to defoliate Japanese Maples because they grow but in cooler climates it’s important to check that your tree is healthy. In San Francisco, we usually defoliate in the first half of May to ensure the tree issues new leaves before the summer fog really starts in July.
You can watch Eric work on a mature Japanese Maple bonsai tree on our YouTube channel:
Managing Early Spring Growth on a Mature Japanese Maple https//www.youtube.com/watch?v=KVLoGaqFh
Spring Maple Care Thinning Leaves on Mature Bonsai https//www.youtube.com/watch?v=qEUHXxuJCQ