The Role of Root Pruning:

The role of the plants roots is to take up water and mineral nutrients from the growing medium and to anchor the plant in that medium. The primary function of the roots is to make contact with ‘soil water’ and move that water into the plant. This is achieved by way of microscopic root hairs which can number 200-400 per square millimeter.

Roots grow by cell division (fueled by respiration) and push through the soil as a result. In order to prevent damage to the fine roots by the hard soil particles, the pointed tip of the root is covered by a hard cap, a little like having a fingernail. Behind this cap is a small section of soft, usually white tissue that supports the root hairs. Normally after a couple of weeks this white root tip lignifies (becomes woody) and the hairs are lost. However, because the root is continuously extending there are always plenty of new root hairs to keep the plant going. This means that roots feeding the tree are rarely more than a couple of weeks old. This also applies to trees growing in the ground. Even ancient trees are supported by root hairs that are only a few days old!

Animals including us humans suffer a process called organismal senescence that means a declining ability to respond to stress this ultimately ends in death. This is programmed into our DNA. Trees show negligible senescence and can in theory live forever given ideal conditions. Left to its own devices a tree will eventually crash into the law of limiting resources. Simply put, the tree will have exhausted its environment and reached the extent of its growth capability. If it takes us more effort to find and consume food than the energy that food returns we will eventually die of hunger.

One aspect of root growth will eventually slow the growth of a tree is that by constantly moving outwards feeder roots get further and further from the tree. In bonsai cultivation once the root tips hit the inside of the pot they begin to grow in a circular form and get progressively further from the tree itself. Some 12 feet of root can be uncoiled from a 10” wide bonsai pot. It takes a long time to move water and nutrients that far, much better a couple of inches!

Root pruning combats the problem of ever extending roots. Once pruned properly and returned to the soil a tree develops new white feeder roots from the cut ends of the old lignified roots and as a result the tree is much stronger than before. Thinking logically this would seem to be back to front, having fewer roots creates more growth? An analogy would be of the big muscle car engines. Today it’s possible to get the same power from an engine a fraction of the size because of improvements in efficiency. A small healthy and dynamic root system will get more out of its soil than a large inefficient one.

 

How To Tell If A Bonsai Tree Needs Repotting:

This aspect of bonsai verges on the esoteric. You may hear people say that a tree will tell you when it needs re-potting this sounds like vaguely crazy, how can a tree tell you? However, there is an element of truth in the thought. By means of careful and thoughtful observation it’s possible to discern the changes in the growth patterns of a bonsai tree.

Bonsai trees are TOTALLY reliant upon the people that keep them. Try going away on holiday for two weeks in summer without making provision for your trees; the results will be disappointing! Bonsai trees need constant care, the obvious being watering. Beyond that we determine how much soil and what type the tree grows in, how much nutrient is within that soil, how much sunlight the tree gets, it’s exposure to wind and rain etc. By manipulating these elements it is possible to get the best from our bonsai OR the worst!

Plants have a limited range of responses to outside stimuli. For instance yellowing leaves can mean a great many things. Interpreting the trees responses accurately is the single most significant skill the bonsai enthusiast can develop. Not being able to do this will ultimately result in poor quality bonsai that are never at their best.

Re-potting should be carried out once the soil becomes very hard and compact. This does not mean that bonsai soil should always be loose and granular, there is a case for keeping a tree in a slightly pot bound condition for an extended period of time. This can help mature the bonsai and slow the growth, thus reducing internode length, increasing flower production and increasing the aged look of a bonsai. However, once it has become very difficult to get adequate water into the soil some action will be required. Never simply re-pot bonsai by a calendar! Always be guided by the growth patterns of the tree.

Most people in the UK begin to re-pot too early in the year. I suspect we are all eager to get busy after the long winter. Re-potting at the very first signs of life is fine if you can then give the trees protection in a greenhouse or poly-tunnel. Waiting until the tree is a little further advanced ensures more rapid healing and much better growth results. However, with care and experience it will be found that, in emergencies, it is possible to re-pot almost any species at almost any time of year.

Re-potting in the autumn is possible; indeed most fruiting species are best done at this time of year but timing and aftercare is critical to ensure the best results. UK winters are long, cool and wet; this will make it very difficult for a tree, re-potted in autumn, to recover quickly. Try to re-pot before the weather gets too cool and where possible give the tree winter protection from rain. If the soil is kept too wet after re-potting this can cause problems.

Flowering trees are often re-potted after flowering. What this statement should actually state is AS THE FIRST FLOWERS BEGIN TO FADE. For best re-potting results it is necessary to remove all fading flowers AS WELL AS open flowers and flower buds. If you wait until the LAST flowers fade the vegetative growth will have begun in earnest and your best chance for re-potting will have passed. Who wants to repot when a tree is still in flower?

 

How to Repot Bonsai Trees:

Before re-potting try to dry the root ball just a little by withholding water for a few days. Working with very wet soil is a horrible experience both for us and the tree. Make sure that everything you need is on hand. Choose a suitable pot and enough soil to fill it. You will also need plastic mesh to cover the drainage holes as well as aluminum wire for securing both the mesh and the tree into the pot.

Having removed the tree from its pot, it is necessary to begin to rake out the old soil from the roots. This can be done with a chopstick but the work will be much quicker using a small three pronged rake. If the tree has not been repotted for a long time you may find a solid root mass and very little soil. As stated earlier most of the root growth will be around the outside of the root ball and if the mass is too solid it will not be possible to rake out the soil. If this is the case you should cut away the outer 10-25 mm of root ball using stout root pruning shears or an old serrated edge knife. Once the outer mass of roots is removed it should be possible to begin raking out the soil from among the inner root mass.

How much root ball to clean out is a difficult question to answer and really depends upon the species of tree. Over the course of several repots it is necessary to remove all of the old soil, otherwise the core of the root ball will become solid and the trees health will suffer. However, with many species it is not recommended to remove all of the soil at one time. If you are working with elm species it is possible, indeed preferable, to remove all of the soil every time you re-pot. My advice would be that if your tree has a solid core of soil around the base of the trunk and, if it is growing well and is healthy, it is safe to clean one side of the root ball completely. When you repot again you can do the other side. However, if your tree has a fairly loose root ball and after raking out there are lots of roots intact, it should be safe to remove all of the soil. Try to remove as much soil as possible but keep the roots intact.

Washing a tree’s roots with a jet of water can be useful especially if the root mass is very dense. However, some species do not respond well to this, particularly pines. Most deciduous trees (except oaks) are fine as are junipers and Taxus (yew) varieties.

Once your trees roots are raked out it’s time to consider how much root to prune away

• A deciduous tree with masses of fine root will tolerate having 75% pruned away.
• If the root ball is in bad condition and the tree weak, do not prune any roots; simply return everything back to the pot.
• If the tree has very long roots that circled the pot, prune these back by half or more, preferably to a junction with some small side roots.
• Most healthy evergreens will tolerate losing about half the root mass.

If in doubt remove much less root. You can always try a little more next time, but being over enthusiastic this time could cost you the tree. Let experience be your teacher. Observe during and after repotting and you will, in time, learn how to proceed.

** The risks of taking off too much root can be highlighted by the outcome of a demonstration witnessed at a convention where, on repotting a group planting, the demonstrator informed the audience that there would be no problem in removing some 80% of the roots and then replanting the group in a shallower tray. This was carried out during mid-summer; the result looked impressive. As we later learned however, within a matter of a few months, the group planting died.

 

Specific Recommendations for Re-Potting Deciduous Bonsai

Deciduous trees do, by and large, produce roots much faster than most evergreens. Deciduous trees are also hungry feeders that make great demands upon the growing medium, both in terms of nutrient uptake but also gaseous exchange.

For the most part, deciduous trees will need more frequent repotting. The rapid growth activity in spring and summer takes a lot of energy to achieve and a healthy root system and growing medium is absolutely vital if this process is not to be hindered. However, if your bonsai is very mature and your aim is to achieve fine ramification with short intermodal spaces then keeping the tree a little pot-bound will help.