Bonsai Root Pruning

Part 19 of a series by Eugene Howell

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Bonsai DisplayBonsai root pruning is the one task, above all others, that can cause the death of your tree very quickly. However, avoiding the task may also readily kill your bonsai. Let’s discuss how to properly prune the roots of a bonsai to minimize the possibility of killing it.

Periodically every bonsai begins to outgrow its pot. Either the tree becomes too large for the pot it’s in or the tree’s roots become too crowded. Of these two conditions, overcrowded roots is the one you’ll most frequently encounter and it means you’ll need to remove the tree from its pot, wash all the dirt off it, untangled all the roots and then prune those same roots. If this task is not done carefully and correctly, your bonsai can die within a very few days.

To avoid killing the tree during root pruning, we have to first understand what it is about the process that’s potentially harmful. Stress.

Stress that is beyond the plant’s ability to withstand will kill a bonsai very quickly. Bonsai root pruning can cause this type of stress because afterwards, the tree’s leaves give off more water (transpiration) than its remaining roots can take in. As a result, the leaves wilt and die and the plant soon follows. This stress can come about by one of four mistakes. First, you’ve allowed the roots to dry out during root pruning; Second, you’ve pruned off the wrong roots; Third, after pruning, you placed the tree into the wrong environment; and Fourth, you performed the root pruning during the wrong time of the year.

Let’s cover each of these in more detail:

The tips of all roots have “root hairs”. It is this part of a plant that takes in water and nutrients from the soil. If the root hairs are allowed to dry out, the plant can no longer take in either water or nutrients. Within a few days the tree will wilt and die from dehydration. These root hairs are thinner than a human hair, so they dry out easily once they’re exposed to air and sunlight. When repotting a bonsai, spray the exposed roots with water immediately upon lifting the tree from the pot and repeat the process every minute or two until the tree is safely back in the pot and the soil has been thoroughly watered.

The second common cause of death as a result of bonsai root pruning is pruning off roots that were needed. The type of roots that you want to keep on the tree are the thin, short ones. These have many more root hairs than do thick, long roots. Roots that are the diameter of a drinking straw or larger and are long should be removed from the tree. After removing all the large roots, you should have a mass of very thin, short roots. They may be 8 or 10 inches long and can be reduced by 1 or 2 inches before being placed back into the soil, but again, don’t remove too much or you’ll stress the tree.

The third cause of death is placing a bonsai that’s had a recent root pruning out into the hot sun. The bonsai will very likely go into stress and die. The safe thing to do after bonsai root pruning is to place the well watered tree in a shady location, away from direct sunlight and strong winds. Let it stay there for two or three weeks. In this location there will be less transpiration from the leaves, giving the tree time for its circulatory system to reestablish itself using the reduced root system.

The fourth cause of death is pruning at the wrong time of year. You have to know your trees. By this I mean that you must know whether your tree is one that’s deciduous and goes dormant during the winter, is evergreen and goes dormant during the winter, or is evergreen and does not go dormant during winter. The first two categories must be root pruned only during the coldest part of winter after the plant is fully dormant. A few plants that fall into this category are Fla. Red Maple, Titi, Hornbeam, Black Amber, Elm, Juniper, and Pine. Some of the numerous ones (that we use here in central Florida) that fall into the last category (evergreen and never go fully dormant) are Ficus, Azalea, Bougainvillea, Neea buxifolia, Buttonwood, Brazilian Rain Tree, Portulacaria afra, Podacarpus, and Jaboticaba among others. Most of these trees can be worked on at any time between April and the end of October. Having said this, I must point out that Buttonwood and Brazilian Rain Tree should be worked on during the hottest part of the year (July and August).

Despite all that you have done, if the tree still appears to be in stress then emergency action is necessary. In this case the easiest thing to do is to place the tree and pot into a drycleaner’s bag (the thin clear plastic type that your cloths come back in from the cleaners) and use it like a make-shift greenhouse. Water the soil thoroughly and seal the tree into the bag using a twisty tie and place it in the shade for a few weeks. Assuming you didn’t commit one of the “killer” mistakes listed above, your bonsai should pull through. While the tree is in this mini-greenhouse you won’t need to water it very often, but you should check the soil moisture every couple of days to make sure it is still moist below the surface.

Repotting and root pruning a bonsai are not difficult. If you take care to prevent the four deadly mistakes listed above you shouldn’t have any problems with your trees.

Photo credit: flickr Creative Commons, Bonsai by Richard King

Part 18 of this series : Part 20 of this series