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Plants are constantly in search of new water rich, nutrient rich material to grow their roots. In the earth this is no problem, roots can travel many tens of feet from the plant stem in search of nutrients and water. In a container the situation is totally different. Roots tend to 'colonize' an area or container. That is, they saturate the container with roots. In some cases they even will push the plant upward from the container by the sheer volume of new roots. When this happens the plant is said to be root bound.
If root bound plants are not top pruned they will quickly dry out on a hot day, requiring more than one watering per day. Top pruning the plant will help with the water storage problem by reducing transpiration, but there will be additional problems if the plant is not also root pruned.
Plants seem to need fresh new root growth in order to adequately absorb major and minor elements from the soil solution. In fact one the first symptoms of a root bound plant is a general chlorosis despite the fact that it is adequately fertilized. Root pruning and repotting a root bound plant will invigorate it and cause a flush of new growth.
Another symptom of a root bound condition is a loss of vigor, even under ideal conditions and adequate fertilization. The leaves will be very small and the internodes very short. Of course this is exactly the type of growth that we desire in bonsai, and we achieve it by keeping roots constrained in these tiny pots. However, growth is not a static condition, and if plants begin to generally decline it is probably time to root prune and repot.
The object in bonsai is to keep trees dwarf and approximately the same size once they are finished. This means that they must be root pruned and repotted back into the same pot with new soil periodically. Of course another important aspect of bonsai is creating them from collected material or nursery stock. Here, the roots must be substantially reduced as part of the training process to get them into the final desired bonsai pot.
The frequency of root pruning is related to a) the species, b) the container size, and c) the environment. The bottom line is that a bonsai should be root pruned and repotted when it shows symptoms of decline and/or chlorosis as described above, or when it begins to push out of the pot.
Some species are simply just slow growing and will require root pruning and repotting less often. Other species grow at a phenomenal rate and may require root pruning and repotting more than once a year, such Salix (Willow). Many flowering species are quite vigorous and require yearly repotting to maintain vigor with profuse flower and fruit production. These include Malus and Prunus, apples and plum, cherry.
In small containers most plants will colonize the pot within a single growing season. This is especially true of mame (very small bonsai). Nearly all mame should be repotted every year. Shohin (under ten inch bonsai) should be repotted every year to two years depending on the species and the growing conditions.
Large potted specimen plants may be root pruned and repotted every other year to as long as ten years depending on the species. Many large pines are comfortable with five to ten year root pruning programs. Cedars and Spruce may be similarly treated.
Plants grown under less than ideal conditions will of course grow more slowly and require root pruning and repotting less often.
Root pruning damages a plant, restricting its ability to take up water and nutrients. Therefore it must be done at times of the year that the stresses on the plant are minimal. For temperate climate plants there are two times of the year that these conditions are optimal, late fall and early spring. Tropical plants can usually be root pruned and repotted during periods of 'quiescence' or slow growth.
In late fall, deciduous plants have stopped supplying leaves with moisture, evergreens are slipping into dormancy, ambient temperatures are cooling, heat stresses are gone. Contrary to this cessation in activity to the top portions of the plant, the roots are once again becoming alive with activity. They have stored an entire season's supply of food and the upper tissues are still moving food down to the roots. Growth of roots is not dependent on light, only on the supply of food and soil temperatures. Eventhough air temperatures are slipping in the fall, soil temperatures remain higher since the days are still warm and the warm earth is still radiating heat. This is especially true for plants that are in the ground, or in contact with the earth.
As long as the daytime temperatures are above about 55F during the day the roots are in a flurry of activity. If you root prune during this period, there will be a new flush of root growth before temperatures fall into the winter range. This is especially true of mild climate areas of the West Coast and the South. This means that cut ends will heal over quickly and that new root growth will proceed until the temperatures dip.
In the spring the temperature stresses are also low so that root pruning will not cause excessive transpirational losses. In deciduous trees the leaves have not yet formed so there are no losses there. In evergreens the foliage is intact but the temperatures are low enough that there is no heat stress. Spring is a time of rampant plant activity. The dormancy requirements have been met and the increasing air and soil temperatures insure that both top and root growth are soon to follow. At this time the roots reverse the Fall process of storing food and begin to pump food and water up to the buds and stems.
The buds already contain enough food to expand into leaves if the roots can pump up enough water to open them. This phenomenon allows for a unique period of root manipulation. If a plant is heavily root pruned during this period the buds will open but new shoots will not expand. This will limit transpiration and the following photosynthesis from the opened leaves will restore the food lost from the root pruning. I have used this principle to root prune bare rooted field grown seedlings (see Root Pruning Bare Root Seedlings). This removes the tap root and starts the shallow root system so desired in bonsai.
Plants can also be root pruned and repotted at times of the year other than the foliage dormant periods. In these cases particular attention must be paid to the transpiration/ root capacity equation. That is, the roots are responsible for supplying the plant with moisture and minerals, not carbohydrates during the growing season. Any root loss results in consequential loss of moisture to the upper parts of the plant. Put simply, they will wilt. Pruning during the growing season must also be accompanied by a commensurate amount of top growth to balance the water equation. If you remove too many roots and not enough top growth, wilt and death can result. A good general rule of thumb is to remove the same percentage of top growth as root growth.
I have used this principle to collect California native oaks out of season (June) and apples as well. With experience one may root prune and repot at any time of the year under the right conditions by manipulating the environment. But beginners should stick to the safe periods of fall and spring.
Root pruning a container plant is a relatively simple process. Simply unpot the plant and proceed to comb out the roots in a radial pattern. There is one superior tool for this process, the root hook. This is a single tined tool similar to a hay or meat hook. It will untangle and straighten roots with a minimum of damage. They can be purchased or easily made at home. Small plants may only require chopsticks to do the job.
The larger thick roots should be sacrificed 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. In the case of pruning nursery container plants in order to move them into bonsai pots, a substantial amount of roots must be removed. Bonsai plants require a substantially shallower root system than nursery plants. This means that the lower portions of the one gallon or five gallon can must be totally removed.
Very often it will not be possible to move from the nursery container to the bonsai pot in one step. In this case remove as much of the root ball as safely possible and repot to a smaller, shallower training pot. After a year or two it should be safe to further root prune and shift to the final bonsai pot.
When severe root pruning is required to get a nursery plant into a bonsai pot, I use tools such as an axe or sometimes even a chainsaw to remove the lower portion of the roots. To be on the safe side 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. There is no substitute for experience in guiding you here. Start root pruning conservatively and only progress to more severe pruning as you gain confidence.
If the plant was in a fairly good nursery mix with coarse materials, it is not necessary to completely remove all of the old soil, although most of it should be removed. When plants are collected from native earth during the dormant season it is probably best to remove as much of the heavier material as possible to make the transition to container mixes. This heavy clay material will hold far too much water in container culture ( see Soils for Containers and Bonsai) . It is tempting to blast the root ball with water to remove the old soil, and for some species this is perfectly acceptable, however it does cause more damage to the fine root hairs than combing out the soil.
Remove the circling roots, the thick tap root remnants under the crown and shorten larger storage roots. Try to keep as many of the fine roots as possible. The roots should be trimmed all around so that the root ball will fit into its new container with having to fold or tuck long roots into it. This is also a good time to start looking at the nebari and deciding if any of the surface roots need to be removed for aesthetic reasons. Place the root ball on a shallow pad of new soil in the pot and proceed to dibble more soil into all the spaces between the roots with a chopstick. Keep poking holes until there is a slight resistance, this means the voids have been filled with soil. Lightly tamp the soil surface and water thoroughly. This first watering should saturate and flush the soil. Water two or three times longer than you normally would to settle the soil and wash out the fine material.
You can finish by placing a layer of heavier gravel or decomposed granite on top, applying moss, or both. Then water again.
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