by Dave DeGroot
The best suggestion I can make regarding repotting is to consult a local instructor or successful club member for advice, because timing related to climate, available soil materials, and prudent aftercare are all affectedly local conditions. Here are a few generalities:
TIMING – Temperate climate plants usually initiate new roots before bud expansion, so using change of bud size or color as a clue to repot is useful. Somewhat anticipating expansion is best, if you have sufficient experience. Some northern conifers extend existing roots throughout the winter if the soil is not frozen, so they can be done earlier; however, aftercare should include protection from freezing. Tropicals, of course, respond best when overnight temperatures are near or above 70 degrees.
CONDITION – Most Japanese professionals prefer to repot trees when the soil is in a dry condition, because new soil can be more easily worked into a dry root ball than a wet one. Root washing may be necessary if decayed roots are present. In that case, it is important to remove as much darkened soil and dead roots as possible.
BALANCE – Root removal depends on the health of the tree. Trees in stressed condition should have few or no live roots removed. Healthy trees can usually tolerate up to one third removals without negative effects, and sometimes much more, but better a healthy tree than a dead one. Be careful. If the tree has more foliage than you think the pruned root system can support, you can remove some, but be aware that new root generation is stimulated by auxins produced in terminal buds. Therefore, it is better to thin by removing old foliage than by shortening or removing branches.
SOIL REPLACEMENT – Old soil can be most safely removed near the trunk, around large roots, as they serve support and storage functions, rather than absorption functions. Fine roots require more protection, and I always prefer to leave at least a little old soil around them. For unmanaged, severely root bound plants, or those with old, compacted soil, I prefer to remove old soil in multiple thin wedges around the trunk, rather than a large amount of soil or roots in any one place. When replacing soil, remember that very fine soil holds water much longer than coarser soil. Many trees have been injured by watering after repotting for the new soil condition, and allowing the old, unseen soil to remain perpetually wet. If a large amount of old soil remains, the new soil for the first re-potting should be somewhat similar in texture, so that drainage characteristics between old and new soils will not be too different. Also, a relatively deep, roomy “training pot” should be used in transitioning trees from a nursery pot to bonsai pot. Old, neglected bonsai undergoing significant root thinning and/or soil change should also be moved temporarily to a training pot. A complete soil change might take 2 or 3 re-pottings, or more. Be patient. Be observant.
DRAINAGE – There is an enormous difference in drainage characteristics with the same soil in pots of different depths. In general, the lowest layer of soil is the last to drain, so apart from influences such as water absorption by the plant, or evaporation caused by ambient heat, temperature or wind conditions, a greater proportion of soil (not depth of soil) in a shallow pot will remain wet than the proportion of soil in a deep pot. It might be necessary to adjust soil texture or components to pot depth, but so many variables affect need to water that local advice should be sought on this issue.
FERTILITY – Those who regularly use organic or timed release fertilizers do not have to be overly concerned with nutrient holding capacity of their soil mix, as nutrients are released gradually over time. Those who use liquid fertilizers on a periodic basis should ensure that their bonsai are not exposed to “feast or famine” nutrient availability – particularly regarding nitrogen – by incorporating materials that have nutrient holding ability into their soil mix. Sand, lava, pumice, slag, hadite and fired clays have little or no such abilities, but natural clays, organic matter and charcoal do. Organic matter tends to break down into sludge over time, and clays de-flocculate into a muddy consistency that can impair drainage. Charcoal is effective and remains structurally intact over many years, but a significant proportion – up to 12% or more – might be needed to provide meaningful nutrient-holding results. This is another issue where research is being done and results are affected by so many variables that for the time being trial and error might be the only way to find what is right for you.
AFTERCARE – It will probably take a minimum of several weeks of new root growth before good root/soil connection can be re-established after repotting. During that time, soil and roots should be kept moist – meaning an absorbable film of water on the surface of soil particles and roots – without being kept wet, as too little air space in the soil. Light shade protection and spraying of the foliage one to several times a day is helpful for a couple of weeks, especially in areas of low to moderate humidity. In cases of trees under high stress, an “intensive care unit” can be created by making a simple 1X2 wood frame and stapling sheet plastic on all sides, with one side that can be opened. In areas where ambient temperatures are below 50 degrees, a small propagating mat can be placed on the bottom. This ICU “tent” should be placed in the brightest area that does not include direct sunlight. After the plant is placed inside, an occasional watering, but misting often enough to keep some condensation visible will ensure that the foliage is sustained in an environment of 100% humidity until the roots are up to the task. This could take several weeks or even a couple of months, but when new growth emerges and begins to mature, the tent can be gradually opened until the tree is withdrawn about 2 weeks after the first opening.
Despite all of the above, there is no substitute for local experience; and if your present re-potting process works for you, there is no reason to change to a different process that might not. There are always multiple ways to achieve success, and outcome is more important than technique.
I hope this helps. Good Luck!