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Keeping Your Bonsai Healthy With Root Pruning

Every plant that lives in a container will need some sort of root work in its lifetime if it is to stay in that container.  Plants are always in search of water and nutrient rich material to grow in, and when in their natural habitat, roots may travel far from the plant stem in search of these nutrients.  In a container, the situation is very different.  Because space in a container is limited, roots will colonize an area of the container, and can, if not trimmed, saturate the entire container with roots, leading to the situation called being “root bound”.   Root bound plants are susceptible to drying out, and though top pruning the plant will reduce transpiration and some water storage problems, the plant will ,at some point, need to be root pruned or transplanted into a larger pot if health of the plant is to be maintained.

In the art of bonsai, the object is to keep trees dwarf and maintain their small size once they are “finished”.  This means that they have to be root pruned and then repotted into the same pot with new soil.  In addition to caring for a finished bonsai, when in training, roots need to be reduced as part of the process that allows them to get into the final desired bonsai pot of choice.

The frequency of root pruning is related to the environment, container size and the species of the plant.  All bonsai will benefit from root pruning when showing symptoms of decline, pushing out of the pot due to abundant root growth, or chlorosis.  Some species are slow growing, and will not require frequent repotting, while others may need to be repotted more than once a year in order to maintain vigor.   In very small containers, most plants will colonize the pot in a single growing season.  This is especially true of mame bonsai, which should be repotted every year.  Larger specimen plants may be root pruned every other year up to as long as ten years, it all depends on the species in question and the conditions in which it is grown. 

It is inevitable that root pruning will stress out any plant by restricting its ability to take up water and nutrients.  This is why it must be done at times of the year that offer the least stress for your bonsai.  Tropical Plants can be root pruned during slow growth, and temperate climate plants will do better in late fall or early spring.  Deciduous plants stop supplying leaves with moisture in late fall which concentrates its vital life energy into the root zone which ensures rapid healing from root pruning during this period of time.  In the spring you are also assured that stresses to your plant will be lower as root pruning during this period will not cause excessive transpiration loss.

Root pruning a container plant is a relatively simple process. Simply un-pot the plant and proceed to comb out the roots in a radial pattern with a root hook or chopsticks to ensure minimal damage. You will want to sacrifice larger thick roots at the expense of the smaller fine and hair roots. These smaller roots are much more efficient at absorbing water, the immediate necessary factor following root pruning. When severe root pruning is required to get a nursery plant into a bonsai pot, do not remove more than two thirds of the roots of a container plant if root pruning during the dormant season. Severe root pruning should only be done during dormancy.

The roots should be trimmed all around so that the root ball will fit into its new container without having to fold or tuck long roots into it. Place the root ball on a shallow layer of fresh soil in the pot and proceed to add additional soil into the spaces between the roots with a chopstick. Keep poking holes until there is a slight resistance to ensure adequate filling and absence of air pockets.  Lightly press the soil surface and water thoroughly in order to saturate the roots, and ensure that the soil has settled.

While it may seem intimidating to new bonsai artists to attempt root pruning, taking small steps to gain confidence will ensure that you are well on your way to having the craft mastered.  Be brave… and good luck!

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