Too much of a good thing

Andrew Edge

Late Winter and aftercare post repotting are critical times for trees. The temperature is cold and the trees are only just awakening. If you have root pruned, the fresh cuts are susceptible to disease. Movement of water from the roots to the canopy is impaired until new fibrous roots are grown and the temperature warms. Watering of the tree is now crucial, especially for deciduous trees that are just at bud swell and have not developed foliage to ‘pull’ water from the substrate through transpiration.

The substrate must be allowed to partially dry out between watering to allow new roots to develop and start to ‘search’ for moisture. If watered too much the roots will struggle to grow and can develop disease if not remedied. This was becoming evident on a Japanese maple that had just been repotted into 100% akadama. The new leaves that grew were distorted as if the cells were overfull of water. I immediately stopped watering and put the pot on a tilt to extract more water from the soil column. Each subsequent watering was only after the substrate was visibly dry and then was given a full drenching ensuring it was fully soaked.

This better exchange of oxygen and water allowed the tree to stave off the disease, Anthracnose, a fungal disease which causes distorted leaves with blotches and ‘burnt’ edges. It is more prevalent in trident maples but can affect any of the maple species.

Maple leaf with ‘bloated’ cells. Leaf looks wrinkly

Anthracnose on a trident maple

Leaf size comparison. The small leaf is the average for the subsequent growth after watering correctly

The leaves on the maple returned to a normal look after a week, however they were more than twice the size of any of the subsequent foliage with the internode length also unusable and therefore this part of the growing season has been lost for that tree as a bonsai. Luckily for me this tree has only started it’s bonsai journey. I had planned to defoliate any large leaves to allow better sunlight and air circulation into the inner branches anyway, but this instance has shown just how careful watering needs to be to continue the evolution of our trees as they become more ramified.

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