Probably the most misunderstood practice in bonsai is the art of Root Pruning. When I first started growing bonsai, root pruning was not a subject often breached by gardeners as a common practice for growing anything. It was kind of a magic bullet used in bonsai culture to keep a tree small. When someone’s association with growing things has been limited to the simple practice of putting plants in the ground or in planter boxes the concept of continually pruning the roots on a tree becomes as traumatic to the grower as it seems it would be for the tree. I was never told by anyone, neither was it written in the books that root pruning did not keep the tree small, it allowed you to keep the tree small. The small size was due to the confinement in the bonsai pot.
We have not been trained to think of the roots, as being something you should be fooling around with. Therefore most of us have come at this subject with fear and trepidation, fueled with a substantial quantity of misinformation and uncertainty about how and why we should proceed. The truth is, working on roots if done correctly, is not a big issue if you understand a few things and your timing is right.
To start with you have to have some sort of general idea what a root system is for and how it works. For the sake of brevity and attempting to keep things simple we will deal with the obvious and rudimentary though one could make a career out of understanding the function of roots. Keeping with our theme of obvious and rudimentary the function of a root system is two sided: One is to anchor the tree in the ground. Two is to move water and nutrients up and down the growing portions of the tree. It should be understood here that most of the activity that is crucial to the health and life of the tree takes place mostly at the very ends of the roots where the hair roots or feeder roots reside. This may make up in nature perhaps 30% of the tree’s root mass. The other 90% are anchorage, and probably 75% of that is below ground.
The goal in bonsai culture is to develop a root system where the 30% become the 90%. In following the above statements about the nature of a root system it is obvious that not everything that grows is important to the survival of the tree other than to keep it in the ground. In nature roots that have but one function to anchor the tree dominate a tree’s root system.
The problem in bonsai is learning how to encourage feeder roots in such abundance that the larger support roots become irrelevant. In nature as a tree grows it produces roots that more or less mirror the top of the tree, large stems, limbs and branches with leaves and feeder roots at the ends depending on which end of the system you are looking at. This kind of relationship being necessary to not only feed the tree but to produce an equal volume of mass below ground to anchor the tree against the weight of the top and the force of the elements on that weight.
A good portion of this underground mass is useless to the tree other than as anchorage. Understanding this is the key to developing a good root system for the purposes of bonsai. The goal in bonsai is to compel the tree to replace its large root anchorage with fine feeder roots that will, because of their abundance, anchor the tree as well as the large roots would in nature.
Most books have diagrams telling us to remove this and that leaving us with this wonderful and perfect soil mass, just like the drawings and photographs. In reality what really happens is far removed from what has been illustrated or described. In Japan professionals whose incomes are based on the quality of stock they produce have grown most stock used for bonsai. This quality is often judged by the treatment of the roots for the purpose of bonsai.
The Japanese tradition almost never encounters the horrible root systems Western growers are likely to encounter through the nursery trade, which unfortunately is where the majority of bonsai material is likely to come from today. Importation of really fine stock from Japan has become more difficult because of government regulations and not all, interested in bonsai, can or are able to journey into the mountains to harvest natural dwarfs. Though there are those who will argue the validity of nursery stock for bonsai we as interested growers are left with little in the way of alternatives, use nursery stock or grow from seed and cultivate your own material.
In looking at any bonsai pictured in any of hundreds of books and publications, it becomes obvious that something drastic must be done to get the kind of tree one finds in a nursery to fit into one of those ridiculously small and shallow bonsai pots. On this issue when it relates to beginners I have found two approaches. Those who are fearful to attempt much more than removing a little bit of soil, to those who will cut everything necessary to make the tree fit into a bonsai pot. The first group never get where they want to go, and the second group usually kill trees trying to get there too soon. Both suffer from lack of knowledge.
Most of these failures are due to the mainstream book’s paucity in explaining the process of developing a bonsai specific root system adequately, or to realistically describe the real world nightmares one is likely to encounter in the process. The fact of the matter is; the kind of root system needed in bonsai cannot be developed easily over night. A bonsai root system is a cultivated feature and does not naturally occur without human intervention.
It is necessary to take this process in steps over several years and several repottings where the right kinds of roots are cut for the right reasons and not just a by the book removal of “X” amount of soil and “Y” amount of roots. If you start with a root system developed by the books the X and Y approach works fine, but we are not talking about text book perfect bonsai stock we are discussing root bound nursery stock grown for landscape purposes.
For all intents and purposes nursery-grown stock and collected material have the same problem for exactly the opposite reason. The problem is the necessity, over time, to completely replace and re-grow an entire root system. The collected tree, because it did not have much of a root system to begin with, the nursery tree, because it had a massive root system totally tangled, over-grown, contorted and useless for the most part, as far as bonsai culture is concerned.
The sad truth is nursery stock useful for bonsai is not particularly useful for the landscape. Because of this it is more than possible the tree has been shopped over many times, maybe for years and as such has achieved the status of ignored material. It is most likely root bound beyond hope of root hook and chopstick. Attempts to approach this kind of material “by the book” is usually met with failure. The stress incurred by the ripping and tearing of the soil mass usually leaves the tree with few resources and a level of shock it is not able to recover from.
Through many years of dealing with this kind of material I have developed a method of dealing with stock in this condition. First and most important it is necessary for the grower to build or purchase some sort of box or planter that has screened in sides and bottom. This item greatly speeds up the process of fine feeder root development. There is a grower in Japan that uses colanders, the kind used in draining pasta. There are some that use pond baskets used in placing water plants in a pond setting, and there are those who purchase a product made for growing bonsai.
Which ever is chosen let it be known that simply over potting or planting in the ground, though useful for some purposes, only slows the process of developing a fine root system. It is too easy for the tree to continue to produce the kinds of roots we seek to replace. The perforated planters force the tree to produce feeder roots by disallowing the unchecked growth of lignified heavy roots. This happens because of two things, air pruning and light pruning. Air pruning, where as roots extend beyond the perimeter of the screens dry out causing the portion of the root under the surface to branch out. Light pruning, where the roots sensing sunlight withdraw because of the destruction of Auxins in the root tips causing the same thing as in air pruning. The result is the production of a predominantly fine root system. Large extending roots are prohibited by environment and not by intervention, like root pruning which induces shock. The idea is to continually work on the roots in the same way you would work on the vigorous portions of the top of the tree.
My method is to remove the tree from the nursery container, scrape off as much of the soil on the top of the soil mass necessary until surface roots are reached. Then with a sharp saw or knife remove one third to one half of what is left of the soil mass by cutting it off with the saw or knife. Next loosen the soil around the soil mass attached to the tree to free up what is left of encircling roots and using the root hook or chopstick loosen the surface that was cut with the knife or saw. The tree is then planted in the screened planter and left undisturbed for three seasons.
After the first three seasons the tree is removed from the planter, and three pie shaped segments are marked in what is left of the original soil mass and the soil is picked out of the three areas. The large roots are cut back as far as possible and the fine roots left alone. The tree is once more put into the planter and a three-season wait, then the process is repeated. Eventually the entire old soil mass is replaced; all of the large roots are cut back hard except those on the surface that have a visual impact on the bonsai. Two or three cycles with this type of planter will usually give the grower the kind of root system that will allow the tree to survive in any bonsai pot. Once the development of fine roots takes place it is possible to reduce down the vertical depth of the soil ball significantly, an additional fifty percent is common after the first cycle.