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Root pruning is a very important aspect of bonsai cultivation. As with any plant, if a tree stays in a container for any length of timw, eventually it will become root bound and the health of the plant will be affected. Plants are alwaysin need of moist, nutrient rich material in which to to grow their roots.
This is not an issue in nature, as the roots can travel deep into the ground to find it. The situation in a container is totally different though. Bonsai roots tend to saturate the container with roots. Sometimes, they even will push the plant upward from within the container due to the sheer volume of new roots. When this happens the plant is known as being root bound.
If root bound (bonsai) plants are not top pruned then they will quickly dry and will need more than one watering per day on a hot day. Top pruning the plant will help with the water storage problem by reducing transpiration, however there will be additional problems too if the plant is not also root pruned to balance the top.
In order to adequately absorb major and minor elements from the soil solution, plants seem to need fresh new root growth. One the first symptoms of a root bound plant is a general chlorosis even thoughthe plant is adequately fertilized. Root pruning bonsai and repotting a root bound plant will invigorate it and cause a flush of new growth.
Another symptom of a bonsai plant being root bound is a loss of vigor, despite having ideal conditions and adequate fertilization. The leaves will tend to be very small and the internodes very short. Whilst this is exactly the type of growth that we look for in bonsai, and we do achieve it by keeping roots constrained in these tiny pots - BUT - growth is not a static condition and your plants will begin to decline in health if the roots aren't given a freshen up from time to time.
The object in growing bonsai plants is to keep trees dwarf and about the same size once they are finished. What this means is that periodically they need to be root pruned and then repotted back into the same pot with fresh new soil.
The frequency of pruning is determined by a number of factors:
a) the species,
b) the container size, and
c) the environment.
Some species are slow growing and will therefore require root pruning and repotting less often. Other species grow at a a very fast rate and may require root pruning and repotting two or three times a year. Many flowering species tend to be quite vigorous and will require yearly repotting in order to maintain vigor with quality flower and fruit production. Plants of this nature include Malus and Prunus, apples and plum and cherry.
Usually in small containers most plants will actually colonize the pot with in a single growing season. This is especially true of very small bonsai. Very small bonsai should be repotted every year. Bonsais under ten inches should be repotted every year to two - depending on the species and the growing conditions.
Large potted specimen plants however, may be root pruned and repotted every other year. Sometimes even as long as ten years depending on the actual species. Many large Pines, Cedars and Spruce are quite comfortable with five to ten year root pruning programs.
Plants grown under less than ideal conditions will grow more slowly and therfore require root pruning and repotting equally less often.
The pruning damages a plant, and restricts its ability to take up needed water and nutrients, and so must be done at times of the year that least stresses the plant. For temperate climate bonsais the best times are late autumn and early spring. Tropical plants can usually be root pruned and repotted during periods of slower growth.
In late autumn, deciduous plants have ceased supplying leaves with moisture. Evergreen trees are slipping quietly into winter dormancy. At this time temperatures are cooling and heat stresses no longer an issue. Despite all this slwo down in surface activity, the roots are becoming alive with activity. They have stored an whole season's supply of food and the upper tissues of the plant are still moving food down to the roots. The growth of roots is not dependent on light, but rather only depedent on the supply of food and soil temperatures. Despite air temperatures falling in Autumn, soil temperatures will remain higher since the days are still realitively warm and also because of the radient earth heat. This is especially true for plants that are planted in the ground, or are in contact with the earth.
So long as daytime temperatures are above about 12.5C - 13C (55F) during the day, then the roots are in a hive of activity. By root pruning during this period, you should find that there will be a new flush of root growth before the temperatures fall into the winter range.
Likewise in the spring the temperature stresses are low which means that root pruning will also not cause excessive transpirational losses. In deciduous trees, leaves have not yet formed and so there are no losses there either. In evergreens the foliage is still intact however the temperatures are low enough which means there is no heat stress. Of course spring is a time of rampant plant activity. The dormancy needs have been satisfied and the increasing air and soil temperatures will ensure that both top and root growth will be soon to follow. It is at this time that the roots reverse the Autumn process of storing food and will begin to pump food and water back up to the buds and stems.
Plants can also be root pruned and repotted during periods other than the foliage dormant periods. But be careful.as any root loss results in consequential loss of moisture to the upper parts of the plant. Your bonsai will wilt. Pruning during the growing season must therefore be accompanied by a equal amount of top growth pruning to balance the water equation. If you remove too many roots and not enough top growth, then you can kill your plant. A general rule of thumb is to always remove the same percentage of top growth as you do root growth.
Whilst you can root prune and repot at any time of the year under the right conditions by manipulating the environment, beginners should stick always to the safe periods of Autumn and spring.
Bonsai root pruning can also be a very valuable tool in the bonsai training process. Dormant pruning without any top pruning will result in food storage losses and therefore will resultin weaker top growth with small leaves and close internodes. Conversly, dormant top pruning without any root pruning gives the opposite effect. This will reduce the number of buds and also sites for growth and will therefore result in larger leaves, long intenodes and rampant coarse growth. | 
