Mature Japanese maple dropping leaves each year earlier and earlier. The tree is around 35 years old and is 15 feet high. For the past few years, some leaves start shrivelling and dropping in August. The leaves do not appear to be diseased. Not all leaves drop like this, but it seems that more do so each year. I have tried using deep watering techniques although I understand the tree has superficial roots. I don’t fertilize the tree. Should I deep water or not, other suggestions?
Japanese maples do not like wet feet, so it is important not to over-water. The water that you provide should drain from the soil within an hour.
Sometimes if the tree is in full sun, leaves may get a bit sunburned/stressed (esp on the southwest side of the tree). This will happen every year when the weather is hot and dry, if the tree is in full sun. You may have to put up with this, as you don’t have the option of moving the tree to an area where it gets morning sun/afternoon shade or all-day dappled light (which it prefers), or planting other plants to shade the tree from direct sun!
As well, some experts say that it is not unusual for these trees to lose entire branches, for no known reason. They recommend pruning away the affected branch and the tree should fill in.
The leaf drop does not sound like a disease or insect. For example, the following may result in leaf drop:
- Verticillium wilt is a disease that can cause leaves on a branch to discolour and die, in late summer or early fall, but would cause yellowing/shriveling of lower leaves and would be much more extensive than what you describe.
- Aphids can damage leaves such that they curl up and can drop from the tree. You can get rid of them with an insecticidal soap or a blast of water from the hose. I think you’d notice them, though as infestations are usually visible – they might colonize the underside of younger leaves.
- Japanese maple scale is an insect that has a white-coloured “armor” and sucks sap from the plant and you could see leaf drop and/or branch dieback, and they can even kill the plant. You should see these beasties on the branches/stems. Scrubbing the branches with soapy water might help, because these insects typically only attach to the tree stem (not the leaves). It’s hard to control them as they have a protective armour. Again, this does not sound like what your tree is suffering from.
Keep an eye on how the tree does this coming summer (it’s now near the end of April). Which leaves are dropping, the ones in full sun, or other ones? Is there evidence of pests or disease (e.g., can you see critters on the stems or undersides of leaves, do the leaves just curl up or do they discolour, get spotty, or become sticky?) If more leaves drop, bag a couple and take them to your local nursery to see if they have any suggestions.
The Toronto Master Gardeners have a guide that you may wish to refer to:
Growing Japanese Maples: A Toronto Master Gardeners Guide https://www.torontomastergardeners.ca/gardeningguides/growing-japanese-maples-a-toronto-master-gardeners-guide/