Leaf scorch on Koshibori nishiki
Leaf scorch on a Japanese maple is caused by a number of factors, most of which are easily rectified. The signs usually start to appear in late spring or early summer as temperatures rise and, depending on the severity of the problem, manifests itself as burning of the tips all the way to leaves curling up and dropping in the middle of summer. In extreme cases this can be an annual occurrence and the plant is gradually weakened until it eventually dies.
The most common cause is excessive watering in summer, either on it’s own or coupled with a poorly draining soil. Not watering at all and the application of a mulch ensures there is always going to be an adequate supply of moisture at the roots. Poor drainage also compromises a Japanese maple in winter, causing roots to rot and the telltale signs of damage to appear the following spring.
Leaf scorch starting to appear on Mischa
For container grown Japanese maples, over-potting coupled with the above is guaranteed to produce leaf scorch and will often extend to die-back of shoots as well. Over-potting on it’s own is not going to cause problems providing the potting mix allows for perfect drainage. In practice, this means avoiding all bought in mixes – they are designed primarily for short term plantings and are not suitable for a Japanese maple, no matter what the salesman says! Commercial nurserymen wouldn’t dream of using one mix for all plant types and in fact have access to many dozens of different formulations, not only for a specific plant but also for a particular stage in it’s growth cycle. A mix with perfect drainage is easily achievable at home using readily available ingredients; more on this subject will be posted shortly.
A further complication with potted maples and something that is rarely taken into consideration is temperature: not the ambient shade temperature but the temperature of the pot wall caused by the sun beating down on it. This is transferred to the outer edge of the root-ball where the majority of the feeding roots are and they get gently steamed! Increasing the moisture content only compounds the problem; water is an excellent conductor of heat. Again, this is a situation that can be easily corrected and at the same time improve the root structure for long term container planting and I shall also come back to this subject shortly.
Last but not least, the condition of your Japanese maple when you purchase it has a big bearing on it’s subsequent well-being. It’s always desirable to choose a plant that is as perfect as possible; that way it’s going to be a lot easier to maintain it in good condition rather than having to nurse it back to health. When selecting a plant to buy, don’t attach too much importance to how the foliage or branches look; the defining point is the condition of the root-ball and a seller that has confidence in his plants should have no objection to you taking it out of it’s pot. By all means buy the scruffy little plant tucked away in the corner, but don’t pay through the nose for it and be prepared for a long convalescence!
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