Pruning a Japanese maple – don't try this at home

How not to prune a Japanese maple would be a more accurate title of the video above that I recently came across on Youtube. The biggest mistake is pruning in the summer, the second worst time of the year to do any remedial work or shaping to a Japanese maple. To understand why, a basic appreciation of the annual growth cycle that a maple goes through is helpful.

As the plant comes in to growth in the spring there is very little root activity and growth is fueled by the food reserves in the roots. As the new shoots develop a small amount of root activity starts and this is limited to the fine feeding roots that will take up nutrients to supplement the existing reserves that are rapidly being used up.

Once all the spring shoot growth has finished, the leaves start the job of building up the food reserves for next spring and this is when development of the primary roots start, i.e., those roots that are going to eventually become part of the basic root structure of the plant. These are sometimes known as water roots and their activity becomes more pronounced as the plant goes into late summer and early autumn.

The picture below shows the development of the root system in early July, clearly showing the primary roots that will store the food reserves for next year’s growth. The finer, feeding roots can also be seen but these take a secondary role in the long term health of the root system.


Acer palmatum root system

The development of these roots and their subsequent importance to next year’s growth is entirely dependent on the leaves the plant has at the above time of year and any wholesale removal as shown in this video is going to curtail their development and lead to reduced growth next spring. Although the plant will eventually recover in a few years, the net effect is going to be a drastic reduction in the amount of growth next spring.

The rationale for the pruning was to give the shrubs next to it room to grow. A far better option would have been to remove the shrubs and give them their own bed elsewhere; the existing bed being barely large enough for no more than a few year’s growth before it starts impinging on the lawn next!

As to the decision to show the trunk, the whole point of planting a dissectum form is for it to weep to the ground. This particular plant is, in all probability, a top-worked graft and the trunk is actually the understock, but grafted higher than normal to give it some height to cascade down from. The comment ‘we want to see the trunk and give it a more tree-like look’ is incompatible with this particular form and a far better option would have been to plant Seiryu instead.

This video generated a number of comments that questioned the aesthetics of what was done to this Japanese maple and you can read them here.

One subject that the presenter didn’t touch on is the matter of the ground that was originally shaded by the maple now being exposed to the sun. Previously, the canopy would have kept the soil cool and allow roots to come up to the surface as the moisture level would be fairly constant. With the soil exposed, fine surface roots are going to dry out and the moisture level of the soil is going to drop rapidly, to the detriment of the plant. This is compounded by the pruning being done in the middle of summer. No mention either of giving supplementary irrigation until the plant has recovered.

The obvious solution is to immediately water the plant thoroughly and then mulch the surface with an impermeable barrier so there is no interruption or variation in the moisture available to the plant; the alternative is going to be badly scorched leaves.

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