Root Pruning and Repotting

Anne Spencer
Pacific NW Bonsai Convention, September 18, 2002

Roots live in a world of darkness, use oxygen and give off carbon dioxide. They provide an anchor for the tree, search for and make available moisture and nutriment for the tree, and grow constantly. The root tips push the way, casting off old cells that act as lubricant. They give off small amounts of acid which dissolves necessary elements. Behind the root tips are root hairs that collect those nutrients in solution to feed the tree.

The goal in root pruning is to increase the number of root tips through which the plant gets food. Thus the plant becomes more vigorous and healthy. At the same time the root system stays a compact size. Drainage is improved if the plant is root pruned before it gets too pot bound, and proper root pruning prevents roots from getting too big within the root ball. Root pruning also gives you the opportunity to examine the root system for any damage, or the existence of diseases or pests.

With regular pruning the root system is kept perpetually young and active. More space is made for fresh soil and nutrients so the roots don't have to work too hard to get what they need. The growth speed slows down and the plant starts the process of becoming a bonsai.

At the same time that is happening we are guiding the roots to grow outward from the trunk. We want them to stay shallow so when the plant becomes a bonsai it will fit it a bonsai pot.  To do this we cut off most large root growth under the trunk area. One of the effects of guiding the root growth outward is that the trunk base starts to widen.

This is the first stage in developing a superior bonsai: Work on the compactness of the roots while the trunk is getting larger, especially at the base. In this stage the top of the plant is cut back in the spring when the roots are pruned, and allowed to grow in the summer to provide strength so the roots system and trunk develop. Some guidance of the top growth is wise in this stage and will become more important in later stages.

Once you put a bonsai in a bonsai pot of the correct size, trunk size development slows way down, so in the first stage you must pot your plants in larger training sized pots. Also, a new-to-bonsai plant usually doesn't have a good enough root system to survive in a bonsai pot at first. The root system development will tell you when the plant will be able to survive in a correctly sized bonsai pot.

There are lots of techniques to improving faulty or ugly surface roots. These can be done during the repotting time. Crooked roots can be straightened, new roots can be added, and overly large roots can be shortened.

The timing of repotting is usually the end of the dormant season in the spring, just before the main growth period. The buds are swelling but not open. The roots need to make new growth and heal after being pruned, so late summer to early autumn, at the end of the summer dormancy is the second best time for repotting some species. The optimum temperature for root growth is 60 to 75 degrees. Growth slows at 86 degrees & above. When the temperature starts cooling growth starts again for a short time before winter and the cut roots can heal.

Frequency and extent of soil changing and root pruning varies according to age, species, and health of the plant, and the stage of development as a bonsai. The best indicator of repotting need is the condition of the tree. Since soil changes and cuttings can bring on new problems they shouldn't be done more often than necessary. If a plant is suddenly weak no matter what season and you can't figure out what has gone wrong, bare the roots and replant in coarse soil.

There are other things to consider before root pruning besides the age and condition of the plant. You have lots of choices to make about what style and size it is to be, where the front is, which branches to keep, which to remove, what angle it is to be planted, what pot to use, and where it is to be planted in the pot. If you plan to wire, do it before taking the original soil off.

Lastly, are your repotting supplies ready? They include, clean pots from which to choose, screen for the pot holes, soil mix, sharp clean tools, a turntable, a spray bottle, Superthrive (if you use it), a good place to work, a hand lens that magnifies those little white spots so you can see if they are pieces of potting soil or little bugs, a paper and pencil to write a record of what you do to which plant, and a camera to record the development of the plant.

You're ready. Work fast! (Dry roots are dead roots!) And enjoy this process. You never know, before you take the soil off the roots, whether you have a poor, average, or great potential bonsai. It's like unwrapping a present each time. And sometimes, if you do things right, after six or seven or twelve years, an average plant can surprise you by becoming a great bonsai.