Acer palmatum Wolff’s Broom
Growing a potted Japanese maple in a hot climate raises problems over and above that caused by the potential stress of a hot summer. As I mentioned in leaf scorch on a Japanese maple, raising the temperature of the root ball can be potentially harmful if the sun is beating down on the pot but, as air temperatures are likely to be considerably lower, this tends to provide a mitigating effect.
Problems occur when the air temperature is too high to provide any cooling effect on the pot and consequently the root-ball is going to be overheating for much of the growing season. The normal method in a situation such as this is to grow it in the ground to ensure the roots are insulated from extremes of heat. Where this is not practicable or desirable we need to look at other ways of growing a Japanese maple in a pot so that it has the best possible chance of growing healthily or indeed, surviving.
The solution is to insulate the root ball, not with physical material, but with air. A wire cage an inch or so smaller all around than the internal size of the pot that’s going to be used needs to be constructed, either using rigid steel mesh or galvanized netting; the latter can be constrained from bulging over time by adding a few wire hoops to the outside of the cage. This is then lined with newspaper or similar to stop the potting mix from falling through the mesh as the plant is potted up. Course gravel should be placed at the bottom of the pot to preserve the air gap. The top of the cage should be mulched, and to blend the cage in with the pot, a lip can be formed at the top to close the gap and the topping, preferably coarse gravel as well, can be extended to the rim of the pot.
Two things will happen when this method is used. As the pot wall heats up, this causes upward air movement that keeps the inner cage cooler. If the gravel at the base of the pot is kept moist an evaporative cooling effect also takes place, further reducing the root-ball temperature. The second thing that will happen is the roots are eventually air pruned as they grow out and through the paper lining, thereby ensuring a more natural development of the root system and eliminating any circling or congestion.
If an open and well-drained potting mix is used there need not be any danger of over-potting and this method can be used to grow a Japanese maple for many years in the same pot as the roots will not become restricted due to the air pruning effect. They will also show more vigor as the roots can operate more efficiently as they spread evenly through the growing medium.