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Root pruning is really quite a simple task. Just unpot your plant and comb out the roots in a radial pattern. The root hook is a great tool for this. It will untangle and straighten roots generally with a minimum of damage if you are careful. Small plants may only require some chopsticks to do the job just as effectively.
The larger thick roots should always be sacrificed at the expense of the smaller fine and hair roots as these smaller roots are more efficient at absorbing water. This water absorbtion is crucial following root pruning. When pruning nursery container plants to move them into bonsai pots, a large amount of roots generally must be removed. Bonsai plants need a substantially shallower root system than nursery plants and this means that the lower portions of the root mass must be totally removed.
Often it will not be possible to move straight from the nursery container nto the bonsai pot in just a single step. In these cases, remove as much of the root ball as is safely possible and then repot to a smaller, shallower training pot. After a period of time (say a year or two) it should be quite safe again to further root prune and shift the plant into the final bonsai pot.
When severe root pruning is needed , to be on the safe side do not remove more than two thirds of the roots of a container plant if you are root pruning during the dormant season. This kind of severe root pruning should only ever be done during dormancy. One should begin root pruning conservatively and only ever progress to more severe pruning as you gain in confidence and experience.
It is not necessary to completely remove all of the old soil if the plant was in a fairly good nursery mix with coarse materials, though most of it should be removed. It is probably best to remove as much of the heavier material as possible to make the transition to container mixes when plants are collected from native earth during the dormant season . This heavy clay material will hold much too much water in container culture. You should comb out the soil to avoid damage to the fine root hairs.
How to prune the Roots
- Remove the circling roots, the remnants of the thick tap root under the crown and also shorten larger storage roots.
- You should try to keep as many of the fine roots as you can.
- You should trim the roots all around so that the root ball will fit into its new container not having to fold or tuck any long roots in.
- This is also a good time to decide if any of the surface roots need to be removed for purely aesthetic reasons.
- Place the freshly trimmed root ball in the pot on a shallow pad of fresh soiland proceed to gently pack more soil into all the spaces between the roots with a chopstick.
- Keep poking holes until there is a slight resistance as this means the holes have now been filled with soil.
- Lightly tamp the soil surface and thoroulghy water.
- The first watering should saturate and also flush the soil.
- Water for two or three times longer than you would normally to settle the soil and also wash out any of the fine material.
- Finish by placing a layer of some heavier gravel or some decomposed granite on top, and / or apply moss, or both.
- Then water again.
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