Unhealthy Acer palmatum

Japanese maple question: What is killing my Japanese Maple? The leaves started dying this summer, half were withered up and the other half remained healthy. I thought maybe the problem was abating, but now most if not all are withered up, but new leaves are growing in spots. This maple has a twin, in the same size pot, both receive the exact amount of water via an automatic watering system, what else can I say? What can I do to either save the unhealthy one, or prevent the healthy one from becoming ill?

Healthy Acer palmatum

Japanese maple answer: Looking at the healthy and unhealthy plants side by side,  there appears to be a considerable difference in trunk diameter. In all probability, this would have been the position when the plants were purchased, so setting the automatic watering to supply the same amount to both plants has the potential to cause problems. You don’t say what type of potting mix they are growing in, when you bought them or how long they have been in those pots, but the pictures would suggest a general multi-purpose mix that is not going to have sufficient long-term drainage for the size you are using. The presence of only annual weeds on the surface would also suggest that they have been potted up recently.

The essence of the problem is that they are getting over-watered. The healthy tree is coping at present but there are signs of leaf loss on a few of the top-most branches, whilst the unhealthy tree is exhibiting considerable die-back, particularly the upper branch on the right hand side as shown below. Excessive moisture, coupled with poor drainage, will suffocate the roots and cause them to die off, and the fine feeding roots that sustain the leaf canopy are highly susceptible to water-logging. The die-back is, in effect, the plant trying to balance the demands for moisture above ground with what it is able to obtain through a reduced root system. The new growth would indicate you still have some healthy roots so the plant is not completely lost.

Branch die-back

The remedy, to ensure the long term survival of both Japanese maples, is to immediately reduce the amount of water they are getting. If the automatic watering system has no facility for adjustment then it should be removed and watering continued by hand, preferably ensuring the surface is kept only just moist. If the maples are going to continue living in the same pots then the potting mix needs to be changed to improve the drainage for both as the pots are overly large, especially for the smaller one. The existing growing medium can be re-used but will need to be opened up and some suggestions can be found in the post peat based potting mixes for a Japanese maple. Try to ensure as much of the old potting mix as possible is removed from the roots and avoid using any fertiliser until next summer, although a growth stimulant such as Superthrive can be applied now.

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