The two worst scenarios you are likely to encounter in dealing with nursery trees are the mud ball core and layered concentric roots. Both conditions are caused by the way trees are handled in the nursery trade.
The first, the mud ball core, is the result of a tree being marketed as a balled in burlap item. These trees are grown in a heavy soil with high clay content. For the most part this is done to produce a soil mass that will hold together well in shipping and not be prone to a good deal of movement around the roots. Due to rough handling, which may include being dropped from the tail gate of a truck from three feet off the ground, the kind of soil we are accustomed to in bonsai would be impractical. Having worked the nursery trade years ago I have seen the way these trees are handled. Clay also holds more moisture than the more porous materials we would prefer to encounter; allowing the tree to set uncovered with minimal damage for hours in the hot sun. This may not be optimal treatment but it is none the less accurate in its content.
Very often in acquiring a tree from a nursery that is in a five gallon container or larger you are likely to encounter a mud ball core even though in your eyes this is not a balled in burlap acquisition. Most trees sought after as bonsai do not always have the features desirable as landscape trees. For this reason the tree may remain unsold for several seasons and the nursery having this tree may at some point be forced to place the tree in a large nursery container because the burlap is starting to rot.
The next condition is the concentric layered root system. This can be a vexing problem that is not so easily solved. When a tree is grown in a small container it will eventually become root bound. In the nursery trade when it becomes necessary to “pot up” a tree, seldom is the time taken to straighten out an overgrown root system. The tree is simply pulled from one container and placed in the next larger container, or what ever is available. If the tree remains unsold for a long period of time it is possible that it has gone through this treatment several times, advancing from one root bound condition to another, making roots that encircle the tree in layers like the inside of a base ball.
In both cases it is necessary to replace the majority of the original root structure with one more consistent to bonsai culture. The most difficult problem to solve is the concentric root mass, and that is the problem we will address here. Because it developed the way it did, in stages, I find it less risky to also deal with it in stages. The first stage is as described in the previous article, to find the actual surface where lateral roots have formed then remove one half to one third off the bottom of the soil mass by cutting it off with a knife or saw. This cut is made horizontally across the bottom of the soil ball. The tree is best planted in a screened container for two or three years and allowed to develop new fine roots that will allow the tree to survive what is to come.
At the beginning of the third or fourth season the tree is removed from the screened planter and the removal of the old soil that remained from the initial placement into the training planter is begun. This is a little tricky, your goal is to remove the old soil while attempting to leave the new roots growing in the new soil as unmolested as possible. This means you are going to have to remove the hole in the donut, to use a metaphor.
If you treated the tree properly at the beginning it should have a good deal of new roots at the perimeters in the new soil. This means that you still have a tough and tangled core at the center. I have found the best way to deal with this is with a high-pressure jet of water from a garden hose. I start near the base of the trunk and wash the soil out till I reach the point the new soil begins. Obviously you go around the trunk and work outward till the above point is reached.
From this viewpoint it is pretty easy to see the problem we have been discussing. The inside of the soil mass may have the appearance of a series of successively larger bird nests placed inside each other. Here we reach a crossroad of sorts. It has already been determined that we want to encourage a fine root system and abandon the larger, less productive roots. However, in bonsai culture large surface roots are desirable to the overall display of a mature tree in miniature. This is called in Japanese terms the Nebari. Traditionally these roots should radiate out ward like the spokes on a wheel.
This brings us back to the point where we have this impossible looking donut with a tree in the center surrounded by a series of over-lapping bird’s nests. It is at this point where you have to determine which of the large roots are necessary to the design goals of the tree, where there are gaps in the root spread that have to be dealt with and which of the remaining large roots can be eliminated.
The roots near the surface intended for design purposes should be carefully straightened out if they are part of the encircling configuration. This means that you may have to disturb the new soil to free them up. If it is determined that they are overly long but have secondary roots in closer to the trunk it is correct to shorten them. If you find voids in the surface root display with a large gap that needs to be filled it may be possible to do an approach graft with one of the roots you have decided to remove. Decide where the root should go, cut a slit in the base of the trunk down to the cambium (a bright green layer beneath the bark). Bring an acceptable root up over the top of the roots in an arch so that it approaches the slit area from above. Slice into the cambium on the root and tack it into place with a small nail or thumbtack. This then should be sealed with sealing paste.
This will give you a rather odd looking tree with these octopus like tentacles coming out of the soil and going into the soil at another location where they join the base of the tree. However once the grafts take they will be cut close to the trunk leaving a small stub. Once the stub starts to dry out it can be cut flush to the trunk. It is possible that this could take two years but if the root remains active one season’s growth is usually enough. Don’t try to do this with large roots, rather use the smaller more flexible roots that are not so likely to break during the process. I am still experimenting with this technique but I know of no reason it should not work provided the root bent over is not broken during the process. Next you remove, or cut back, half of the large roots remaining that are not necessary for design or feeding purposes.
The tree is then placed one more time in the training planter and new soil is added to the areas that were washed out with the hose technique. The tree will remain in this environment for two years where the process is repeated once more. At this time it may be possible to put the tree in a bonsai pot if the development of the tree is at a point where fine training, needle reduction, leaf reduction and ramification are called for. If not put it back in the training planter, after all what’s the rush?
I suppose by now you are wondering about the mud ball core. You treat it exactly like the above but it is likely you will not have the problems of the interwoven layers you had to deal with in the previous example, — with the exception of attempting to graft roots.