Introduction
What does re-potting mean?
Why bonsai trees need to be re-potted
The role of root pruning
How to tell if a bonsai tree needs re-potting
Choosing the best time to re-pot your bonsai trees
How to re-pot bonsai trees
How to secure your bonsai into its pot
Recommendations for re-potting deciduous bonsai
Recommendations for re-potting evergreen bonsai
Re-potting indoor bonsai trees
What soil to use for re-potting bonsai trees
How to select a bonsai pot
Aftercare of re-potted bonsai
How to deal with sickly & struggling bonsai
Bonsai tree roots and fungus
Soil additives for bonsai trees
Fertilizing recently re-potted bonsai trees
 

Introduction
Before I get into the meat of this subject I must apologize for it’s length and complexity. However if you study and learn to understand this subject, creating and maintaining beautiful bonsai will become very much easier. I PROMISE! Mastering the following information has been the most significant step in my success with bonsai. Even now after 20 years it seems that almost every month brings me another little wrinkle that just adds to my wonderment of trees.
Re-potting is probably the single most important and significant technique involved in the successful cultivation of bonsai trees. Tragically it is also the most misunderstood and poorly executed. More ignorance and bad information exists regarding re-potting than any other bonsai technique and this leads to massive problems and failures of bonsai trees everywhere. Re-potting is a simple, safe and very basic horticultural practice that has been used for generations in plant nurseries across the world. Mastering the understanding of why we need to re-pot and also how to complete the task successfully will transform your whole experience with bonsai trees.
I have spent more than twenty years cultivating bonsai at all levels and all stages of development. In that time I have learned that having a strong dynamic root system and healthy soil environment is THE single most important factor in cultivating bonsai trees. In all of those years I have discovered that each tree is unique and, as with all techniques available to the bonsai grower, it is important to decide what you want to achieve before deciding which technique to employ. Basically, one size does not fit all and in order to get the best from your bonsai tree a little knowledge of a tree’s needs is vital. Once you understand the basic science of the growth pattern of trees and plants the rest is VERY simple.
 

What does re-potting mean?
Put simply, because bonsai grow in a small volume of soil the tree will, in time, fill it’s pot with roots. After an extended period, which varies according to several factors, the plant will begin to suffer. In order to maintain the tree in a healthy condition re-potting will be required, this may or MAY NOT involve root pruning. Re-potting does not ‘stunt’ the tree in any way, in fact the opposite is true. A healthy tree that is re-potted properly will grow much more than in previous years if it was pot bound. The Chinese practice of binding the feet of young girls has been likened to growing trees in small pots. This is absolutely erroneous. A pot bound tree will in time stop growing and eventually die. Re-potting is done for the long term health and well-being of the bonsai.
Re-potting in a bonsai context should not be confused with ‘potting on’ of other container grown plants. The choice of pot for a bonsai tree is based on aesthetic principles, simply moving to a larger pot each time would soon mean your bonsai looked ridiculously over potted.
 

Why bonsai trees need to be re-potted
Permit me to indulge in a metaphor….. Imagine going on holiday. Suitcases packed and loaded into the car, kids settled into the back seat, mountain bikes strapped onto the roof, credit cards, hotel reservations, maps etc’. Doors are locked, milk cancelled, grab your keys, jump into the car, everything set and ready to go then you turn the key and…….NOTHING. You jump out lift the hood and shock horror NO ENGINE. What to do? You could push the car all the way but the smart move would be to get an engine.

A bonsai tree’s root system is akin to the car’s engine. Without one it is possible to make a progress, but NOT FOR LONG! In simple terms the more engine a car has the faster it will go and the more stuff it can carry. With bonsai the more root we have the faster the tree can grow and the better it will develop.
Sticking with the automotive theme, back in the 60s American car manufacturers persuaded us that bigger was better and the age of the muscle car was born. Huge engines ensued, great puffing wheezing, gas gulping monsters that sure did make power but at an enormous cost. Today technology has improved and it’s possible to get the same performance from an engine a quarter of the size and at a fraction of the running cost. And that’s how a bonsai tree manages to thrive in such a small amount of soil! Put simply it’s not the size that counts but the efficiency. Creating a small, dynamic and strong root system is the cornerstone of creating and keeping bonsai trees.

There are bonsai trees around today that have been growing in the same pots for 500 years and more. In order to understand how this can be achieved we need to understand exactly what a plant needs from it’s soil and how we can provide that year after year. It is also vital to understand how the plant grows within the soil and how the root zone changes in response to what is going on around and within it.

Getting down to brass tacks…. Unlike animals plants do not eat food, they create food from virtually nothing it would seem. In order to live any plant needs water, carbon dioxide and sunlight. These are the constituents of photosynthesis by which plants create carbohydrates (starch & sugars) in their green parts. Carbohydrates are burned in a process called respiration that drives cell division for growth. Respiration uses oxygen and creates, as by products, carbon dioxide and water. If insufficient oxygen is available the process yields ALCOHOL! Whilst oxygen is readily available above ground it’s harder to source within the soil around the roots. Here lies the whole basis of why re-potting is necessary and why it works so well.

If you were to fall over and hit your head on the ground you could be forgiven for thinking that soil is solid. However if we look at it on a smaller scale soil is in fact porous and almost sponge like in character. Looking at soil under a microscope reveals it to be a world of wonderment who’s basic structure comprises small individual particles with an endless maze of air filled voids between. These are the spaces that plants exploit and fill with roots. They are also the ‘lungs’ of the soil that bring oxygen to the roots to fuel respiration. We will come back to this subject later when considering what soil to use for bonsai but for now it’s useful to know that a soils capacity to hold air is known as it’s Air Filled Porosity (or AFP) and manipulating this is one of the most significant factors in how our bonsai grow. Basically the more densely packed the particles the lower the AFP and vice versa.

Because a bonsai grows in a relatively small amount of soil it can very quickly fill it’s soil pores with root and drastically reduce the AFP of the soil, this reduces the oxygen available to the roots which stifles their growth which in turn slows the growth of the top part of the tree, less leaves means less photosynthesis which means less carbohydrates available for respiration which means less cell division which means less growth and so we go. Once this situation reaches a critical level the trees health will begin to suffer and in order to arrest the situation we need to re-pot. The skill is in recognising the tell-tale signs a tree makes that re-potting will soon be necessary.

In summation, as the AFP of soil in a pot begins to fall roots find it harder to grow because of the reducing air flow. In time the soil will begin to retain more moisture further reducing the air available to the roots and this will begin to reduce the vigour of the tree. Left unchecked for long enough the tree will stop growing altogether and later still will die.
 

The role of root pruning
The role of plant 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 root 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 millimetre.

Roots grow by virtue of cell division (fuelled by respiration) and are pushed through the soil as a result. In order to prevent damage 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. So what this means is 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!

Recent root growth. The root cap is just visible. The arrow indicates the point at which the root has begun to lignify.	Rapidly extending root tip showing active root hairs.

Animals suffer a process called organismal senescence, that means a declining ability to respond to stress which 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 it’s own devices a tree will eventually crash into the law of limiting resources. Simply put, the tree will have exhausted it’s environment and reached the extent of its growth capability. For instance 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 that 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 motion and get progressively further from the tree itself. I have seen 12 feet of root 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 less root creates more growth? Think back to the analogy 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 it’s soil than a large inefficient one.
 

How to tell if a bonsai tree needs re-potting
This aspect of bonsai verges on the esoteric. I have heard people say that a tree will tell you when it needs re-potting which sounds like the ranting of a mad man. However there is an element of truth in the thought. Unless you are on illegal substances trees do not talk. However, by means of careful and thoughtful observation it’s absolutely 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.

Signs to look out for that MIGHT indicate re-potting is on the cards.
A slowing rate of growth over previous seasons.
Difficulty wetting the trees soil.
Reduced uptake of water in summer.
Rapidly reducing leaf size.
Early leaf drop in autumn.
Slight yellowing of foliage colour.
Reduced gloss on the foliage.
Die back of fine twigging in winter.
Liverwort and algal slime forming on the soils surface.
Root ball gradually rising in the pot.
Reduced leaf viability (discolouring and dropping after a few weeks in summer).
There are a great many other signs that you will learn to recognize in time but this list will get you started. Don’t think for a second that just because trees do any of the above it’s time for a re-pot and certainly one symptom alone is no sign of looming disaster. It is necessary to constantly observe your trees and that way you will begin to learn how to ‘read the tree’.

Most books seem to recommend re-potting every two or so years. This is not particularly good advice but it’s safe and a ‘one size fits all’ solution to a complex question. There are many variables to consider. For instance a fast growing tree like Chinese elm growing in a small pot may need re-potting every year. The same tree in a large pot may go two or even three years before requiring attention. A slow growing tree like pine or yew could manage 3-6 years in a pot and a very old mature slow growing specimen bonsai could quite happily go 10 years or more without attention. As a general rule the more open and free draining the soil the faster and coarser the trees growth will be. This explains the tendency to leave mature bonsai longer between re-pots.
In general my advice would be that if you see the growth rate of a tree declining and all other things remain equal (watering, feeding and weather conditions are not dramatically changed from previous seasons) re-pot the following spring. I am usually reluctant to re-pot a tree I have just obtained, preferring to wait a season and ascertain the trees state of health before jumping in.
 

Choosing the best time to re-pot your bonsai trees
Unusually this is a fairly simple question to answer. With a few exceptions it is best to re-pot just as the buds of deciduous trees begin to open. Evergreens should be worked just as the buds begin to unfurl in spring. I have found that, in the U.K, most folk re-pot too early (end of January through February). As a result the growth rate of the tree is seriously impaired and can take a considerable time to recover. Potting later can dramatically improve recovery time.
The main exception to this rule is concerning tropical species like ficus that should be re-potted at the height of summer. I have also found that Mediterranean species such as pistachio and bougainvillea are best tackled in summer. Most pine species re-pot very successfully in August once the annual top growth has finished.

As stated I find most people in the U.K begin to re-pot to early in the year. I guess we’re all eager to get busy after the long winter recess! Re-potting at the very first signs of life is Ok if you can then give the trees protection in a glasshouse or poly tunnel. Holding back until the tree is 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. U.K 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 post flowering. Personally I would recommend treating these species like any other and work as the buds (or flowers, whichever comes first) begin to open. There is a misunderstanding of the term “re-pot after flowering”. What this statement actually means 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.
 

How to re-pot 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 aluminium wire for securing both the mesh and the tree into the pot.

Having removed the tree from it’s 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 re-potted 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 pot 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 10-25mm 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 roots.

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 re-pots 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 re-pot again you can do the other side. However if you tree has a fairly loose root ball and, after raking out there are lots of roots in tact it should be safe to remove all of the soil. Try to remove as much soil as possible but keep the roots in tact.

Washing a trees 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 quercus) are fine as are junipers and taxus 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 was 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 be Ok losing about half the root mass. Sadly giving precise guidelines on this issue is impossible because every tree will be different. If in doubt remove 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. By careful observation during and after re-potting you will, in time, learn how to proceed.

Once you have a nice clean root mass, carefully pruned with sharp scissors, it’s time to return the tree to it’s pot. Before doing so secure plastic mesh over the drainage holes using aluminium wire. Next, using aluminium wire (copper corrodes in the soil), cut a length of wire sufficient to pass through the drainage holes with the loos ends upwards. This will be used to secure the newly potted tree into place. I prefer to use 2mm wire for small trees and 3 or 4mm for large trees. Securing a tree into it’s pot is very important, trees that rock around in their new soil will take much longer to re-establish. It will also prove invaluable with large and tall trees which can easily topple out of their pots when being moved.

Next put some of your chosen soil into the pot, sufficient to allow the tree to stand on it and remain at the desired planting height. Pass the loose ends of wire over some heavy roots and twist together until the tree stands on it’s own. Now add small amounts of soil, working it in between the roots with a chopstick using a circling motion, don’t ‘stab’ as this will damage the roots. Keep going until all the cavities are filled and the pot is settled at the desired height. Lastly tighten the securing wire by twisting it with pliers whilst pulling hard and pressing on the base of the tree.

The final step is to water the tree thoroughly. Using a hosepipe is preferable and allows you to water until the pot is flooded. When using products like akadama or pumice, water should be passed through the soil until it appears clear coming from the drainage holes.
 

How to secure your bonsai into its pot
As already mentioned I consider it vitally important to ensure bonsai are well secured into their pots, at all times. There are a great many ways to achieve this but over many years I have settled on using aluminium wire. I do not use copper, this will corrode in the soil and release copper oxide into the root zone of the tree. Galvanized steel will also corrode and release high levels of zinc into the soil. Plastic covered steel is Ok but it is very springy in smaller callipers and does not hold the tree so well, it also has a tendency to break. Thicker sizes are very stiff and hard to twist together.

Once a tree is secured after potting I leave the wires in place until next time. This does no harm to the tree if the wires were fitted properly in the first place and it ensures the tree is always secure. Also if in an absent minded moment you pick the tree up by the trunk it ensures the pot does not fall off and break.
To see some solutions for securing trees in pots see this slide show.
 

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 re-potting. 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 on the pot bound side is going to help.

If you have raw material that needs lots of growth then planting in deep flower pots and maintaining lots of free drainage will produce best results. If the plants are growing strongly re-potting can be done every one or two years. On the other hand if you are bringing good bonsai to final refinement then re-potting every 2-4 years may be more appropriate. With the exception of elm species I would recommend that recently collected deciduous yamadori be allowed 3 years before first re-potting. Always be guided by the tree. A plant growing rapidly from early spring until autumn will probably appreciate a re-pot. Conversely an old recently collected hawthorn that barely manages an inch or two’s growth all year should be left alone until it’s health returns.

The optimum time to re-pot all deciduous trees is in spring just at the leaves begin to emerge from the buds. In the U.K it can be two months from when the buds first begin to swell before we see them open. Waiting is always best. Trees can store a lot of energy in their roots over winter. Re-potting and root pruning too early can remove a lot of this. Waiting a few weeks ensures most of the plants energy reserves are active within the tree. It is also beneficial because the temperature will be slightly higher, this ensures rapid healing after the work.
Moving a deciduous tree to a larger pot without disturbing the roots can be done at any time, if for instance its pot was broken.
 

Recommendations for re-potting evergreen bonsai
Conifers tend to produce root much more slowly than other plants, it can also be much finer than broadleaf trees and has a much longer active life expectancy. Conifers are, for the most part, adapted to life in harsh conditions on poor soils and move at a much slower pace than deciduous trees. They are therefore very adept at getting the most out of their growing medium. Because of this it is not necessary to re-pot so often as their broadleaf cousins. As a general rule I have found re-potting of raw material achieves best results after 4-5 years. Vigorous juniper species can be done from two to four years, taxus 3-5 years and pines 2-7 years depending, of course, on the trees vigour. It is very important to allow recently collected conifers an extended period of time to regain their equilibrium. With very old trees this can take 10 years, less with young plants. This is one of the most painful and costly lessons I have had to learn in bonsai.
Considering established evergreen bonsai it is, as always, important to be guided by the tree. Season long vigour is a good sign that all is well, as is a good deep green colour and plenty of back-budding. Keeping pot bound evergreen bonsai is a simple enough task as long as the root ball is well soaked JUST BEFORE it dries out. It may not be necessary to water trees in this condition so often as others and the tree can exist quite happily for very many years without work. However as soon as the tree shows signs of stress, slowing growth, discolouration and excessive foliage drop in the second half of summer it will be time to make plans for re-potting.

My own preference for juniper bonsai is 4-6 years before re-potting. Pines can go 4-8 years and yew 2-5 years. These times are less for strong trees in small pots and obviously longer where very old trees in large pots are concerned. Just because the drainage of the soil has reduced does not mean it’s time to re-pot conifers, it may be that we have to change our watering regimen.

Japanese white pine bonsai suffer badly in U.K winters due to excessively wet soil. If you have one that has gone yellow over the winter and early spring it is important to re-pot as soon as the buds begin to open. I suggest 90% bare rooting the tree without root pruning or washing. Then using a very open soil mix of pumice and pine bark with just a little akadama. Within 2-6 weeks the tree should have it’s nice healthy blue colour back.
As with deciduous trees re-potting too early in the season is a bad idea. Always re-pot once growth begins. In the U.k this can be April, May or in cool years even early June. All pine species can be re-potted with great success in mid to late August. Once the seasons top growth is complete and the shoots begin to lignify and next seasons buds have formed, pines can be worked. I have had exceptional results working pines at this time of year. If you are unsure try working a small cheap pine to help you gain confidence in working at this time of year.
 

Re-potting indoor bonsai trees
The re-potting of species kept as indoor bonsai is fundamentally the same as other trees. One significant difference exists that does present a problem.
Because these species are kept in an artificial environment it’s quite possible that their natural rhythms are disturbed. This fact makes it very difficult to decide when to re-pot. Species from tropical zones, like ficus, will have a dormant period at the height of summer and should be re-potted then, this can also be applied to Mediterranean species like olive, pistachio and bougainvillea. Sub-tropical species like Chinese elm, pepper trees and sargretia should be done in spring as the new growth begins. If in doubt watch the tree carefully for a year and choose a time to re-pot when the growth has slowed or stopped.
 

What soil to use for re-potting bonsai trees
This is a vast subject and one of the most controversial within the bonsai growing community. I once published an article for a newsletter on this subject that ran to nearly 200,000 words! There are no absolutes here but if you understand a little of the science you will be well equipped to chose soil mixes for your bonsai. A lot of what has been published on this subject is based on some very spurious wisdom.
As we have already discovered in this article every bonsai tree is different, in order to get the best from any tree we need to try and tailor it’s growing conditions to best suit what we hope to achieve with the tree. Whilst the needs of plants are simple we can have a fundamental effect upon them by how we provide for those needs. I use the generic term ‘soil’ to mean the growing medium in which the trees roots grow. The rhizosphere (root zone) of plants is an incredibly complex world of delicately balanced relationships. A whole eco-system exists within a bonsai pot. Science is only just beginning to unravel the basic complexities of this fascinating environment. Because bonsai are grown in a very small amount of soil it is of absolutely paramount importance that this little pot of magic is in top condition. Even one missing component will mean your bonsai trees will struggle to meet their basic life needs. Just like the engine we considered earlier.

Within the context of this article I do not intend making a full exploration of the subject of growing media. However a little basic science will help us to understand exactly what it is we need to achieve to get the best from our bonsai. To begin it is necessary to empty our minds or whatever we think we know about soil in general, especially in relation to gardening. Soil in a bonsai pot behaves in a completely different manner to that in the garden.
So, here are the basics…

SOIL COMPONENTS
These are sourced form many and varied sources both natural and man made, some are organic, like bark and others are inert, like crushed stone. In almost every circumstance a mixture of more than one product will be required. The basis of bonsai soil mixes include …
Aggregates in the form of sands and grit.
Clay & silt in various forms.
Organic matter - When organic matter is fully decomposed within soil it forms humus. It is a black colloidal material that binds to the mineral particles, this gives good soil it’s dark colour. Humus is NOT peat or leaf mould or bark, that is what’s left when vegetable matter is completely decomposed by soil micro-organisms.

SOIL STRUCTURE.
Soil structure is the arrangement of particles in the soil. In order to provide a suitable root environment for plant cultivation the soil must be constructed in such a way as to allow good gaseous exchange whilst holding adequate reserves of water and nutrients. Good soil needs a high water infiltration rate, free drainage and an interconnected network of spaces to allow roots to find water and nutrients.
Besides the above it is important for bonsai soil to have a higher degree of physical integrity than soil in the ground due to the fact that it will experience much greater temperature fluctuation and more frequent watering.

WATER RETENTION.
This issue gets very complicated. It takes about 500Kg of water to produce 1Kg of plant growth. Obviously plants need a constant supply of water, this is taken up by the roots and lost by transpiration from the leaves.

AVAILABLE WATER.
This term refers to the water that the plant can actually use. Roots are able to remove water from soil held at tensions of up to 15 atmospheres. At that point the soil will reach it’s …

PERMANENT WILTING POINT.
This term refers to the point at which soil contains insufficient water for the plant to extract it. The soil particles hold the water so strongly the plant cannot pull it away. At the opposite end of the spectrum we have …

FIELD CAPACITY.
A soil that has been saturated and then allowed to drain freely without evaporation is said to be at field capacity. Soil that is at field capacity can be effected by …
CAPILLARY RISE.
This is one of the most fundamental issues regarding pot cultivation of plants and yet I have never seen it mentioned in a bonsai context. It fundamentally effects all of the above issues and is a significant reason why many bonsai are lost.
Capillary rise occurs only from saturated soils. Water is drawn upwards from the water table in a continuous network of pores. The effect is eventually overcome by gravity and will limit the height of the water’s rise in the soil. In the ground this effect is rarely of any consequence because the water table is deep within the ground. However in bonsai cultivation using shallow pots, for all practical purposes we can say that the bottom of the pot represents the water table. Whilst the drainage holes in the pot allow some water to escape, the effects of surface tension and capillary action will hold liquid water very strongly. Whilst the network of spaces within the soil are filled with liquid water no gaseous exchange can take place, this leads to a situation where the roots cannot breath properly. The roots respiration is incomplete (anaerobic respiration) which forms alcohol that poisons the roots. This is what we term root rot.
The situation outlined above is less noticeable in deep bonsai pots. Also it is of less consequence in summer due to the plants increased uptake of water and also more rapid evaporation due to higher ambient temperatures. In winter however it can sound the death knell of bonsai trees, especially those that are weak, planted in shallow pots or are adapted to dry conditions, olive for instance.
Capillary rise is dependent upon the size of pore spaces within a soil. In a coarse soil with large pore spaces (like akadama) the rise will be 20mm or less, however in fine soils (like John Innes No2 and sand) the effect can lift water 150mm, that’s deeper than most bonsai pots. Therefore if evaporation is not significant the tree can effectively be standing in a puddle.

DRAINAGE LAYERS.
Using a foundation of coarser soil in the bottom of a bonsai pot is popular. In theory this will work well. Increasing pore size reduces capillary rise. In practice the effects are of limited value. As already stated tree roots dive quickly to the bottom and outside of the pot and will fill the nice large pore spaces in the soil first and quickly. They will then progressively fill the spaces back towards the middle of the root ball. There is also a tendency for fine particles and silt within the soil to wash down into the bottom of the pot. Whilst it is personal preference I tend not to use a drainage layer. Having a well prepared soil of uniform consistency promotes more even rooting throughout the pot and, goes some way to preventing wet or dry pockets.

NUTRIENT RETENTION.
Plants require fifteen elements in order to grow including carbon, oxygen and hydrogen. The twelve essential minerals enter the plant tissue in the form of ions from the growing media or, to a lesser extent, through the leaves.
Foliar Feeding
Because a bonsai lacks the support of a natural eco-system like a forest the plants are dependant upon us for these needs. Even a very rich soil can be exhausted by a healthy bonsai tree in a matter of weeks. Most constituents of bonsai soil have no inherent nutrient. The important factor is to choose materials that can hold onto the nutrients we add so the plants can access them as required.
Minerals like iron and magnesium are known as Simple Cations which are positively charged ions(atoms). In order to hold these ions within our growing media we need an ingredient with a negative charge. This will hold the mineral particles like a magnet otherwise the nutrients we add will be washed straight through the pot. The plant can then grab these ions for it’s own use. This process is known as Cation Exchange. The higher the cation exchange capacity (CEC) of a soil particle the healthier your trees will be.
In simple terms clay has a high CEC, it’s tiny platelets are negatively charged. Aggregates, especially those from igneous rocks, like sand and crushed flint grit have almost no CEC. Plant matter such as peat has very little nutrient holding capacity of itself, however, once soil micro-organisms have broken it down to humus, which binds to hard soil particles, it has a high CEC.
Put simply clay is good and hard grit and stone is bad. However I did say at the beginning of this section that things were going to get involved so bear with me….

STRUCTURAL INTEGRITY.
This aspect of soil is more significant for the bonsai grower than to other horticulturalists. Because bonsai look good in small pots we have to water often, this action alone can break down soil structure. Constant watering and large temperature fluctuations cause expansion and contraction of soil particles which then conspire to break them down. Broken soil particles lead to increasingly smaller pore spaces within the soil which in turn leads to less gaseous exchange……and so we go. Using very hard soil particles like crushed stone retains pore space and a high AFP but does not hold nutrients. In this case grit is good and clay is bad.

PRACTICAL CONSIDERATIONS FOR CHOOSING SOIL.
So having made it this far the only thing that is clear is, choosing bonsai soil is confusing, it’s also going to involve some compromise. Put simply, if you can provide the corresponding care a bonsai will grow in just about any soil you choose, but, before you rush off and buy something it’s a good idea to consider the practicalities of how you care for your bonsai trees. It’s not much use choosing a soil that has excessive drainage to encourage fast growth if you are not available to water often enough, the tree will suffer. If, like me, you love watering bonsai then using a fine grained soil may not be ideal because the tree will always be too wet and will suffer. If you keep moisture loving trees like maples but have a garden exposed to lots of wind then you will have to choose a more moisture retentive soil than if your trees are in a protected shady spot. If you have been happy with the growth rate and watering frequency of a tree in the past then using a similar soil mix is the answer. However if you would like to see more growth or water less often then change the mix.

THE GOLDEN RULE.
Over the last 20 years I have used every conceivable soil product and every combination of those products. Keeping upwards of 1200 trees gives me plenty of scope for experimentation. Every year I re-pot in the region of 450 trees of all types and at every level of development. I have studied horticulture, both as an amateur and a professional, fanatically in all those years and the only absolute and firm rule that I have discovered is that as long as all of a bonsai’s soil ingredients are absorbent then the soil will work. Using grit, gravel, shingle or crushed stone makes soil heavy and cold, it holds NO nutrient and is not even particularly effective at improving drainage. I have kept trees in percentages of grit and stone from single figures up to 100% and have not found one single species that does well in it. There are a great many reasons for this but now is not the time to discuss these.

THE GENERAL RULE.
The faster a species grows the more moisture and nutrients it will need, therefore the finer the particles of soil and the more clay the better. For instance Chinese elm can be grown in akadama dust but this will cause root rot with junipers. Most deciduous trees are happy in a fairly compact soil with a large proportion of organic matter. Most conifers like a high AFP but are quite happy with a medium or low CEC soil.
Using deeper pots allows trees to do well in soil that has finer particles, very shallow pots are a challenge because they need high drainage rates but will also dry out very quickly.
Bonsai that are not re-potted frequently require soil consisting of harder particles for longevity.

SOIL INGREDIENTS.
Good quality bonsai soil costs money. Products that have suitable properties for bonsai growing are rare and many are transported great distances and are often exhaustively graded, this all adds to the price. However, in view of the cost of good bonsai trees the cost of quality soil is very low.
The mainstay ingredients for most of the soil I have used successfully include …
Akadama
Pumice
Pine Bark
Moler
Other useful ingredients include sieved composted bark, pine needle litter and some forms of leaf mould. There are also various forms of crushed lava that can be very good.

How to select a bonsai pot
This is a much simpler process than choosing soil. I have heard that a bonsai’s pot is akin to a picture’s frame. There are some basic ‘rules’or guidelines for choosing bonsai pots but first and foremost if it looks right then it should be okay. If as you approach a bonsai the first thing you notice is the pot then something is awry. In bonsai everything is about the tree. No matter how beautiful and expensive a pot may be if it does not compliment the tree, it’s all wrong.
The size and shape of a pot are the most significant factors. A simple rule of thumb I use is that, looking in a strictly two dimensional view. The pots mass or surface area should be one third of the trees. For upright trees the width of the pot should be about two thirds the height of the tree. This is obviously a very broad generalization but it will get you started.

I have noticed that as beginners most folk choose pots that are much too large. That’s perhaps no bad thing because keeping trees in the appropriately small pot is not easy to begin with. However once a tree is ready for the show bench the correct selection of pot is very important, getting it wrong will cause you some embarrassment.

Choosing bonsai pots is an area of great debate and many people like to experiment with the artistic relationships between tree and pot. I have no intention of getting into this just now. My best recommendation for helping you choose a pot is to get a hold of one of the many commemorative exhibition books available and have a long, careful look at what others have done.

These are my basic recommendations for choosing pots and can be applied IN GENERAL to the majority of trees. Once you have mastered the basics you can begin to experiment.

The width of the pot should be approx’ two thirds the height of the tree.
The depth of the pot should be one or two times the diameter of the trunk, depending on the height of the tree. IE tall slender trees may need smaller deeper pots to look right.

Use glazed pots for deciduous trees and unglazed for conifers.

When choosing colours try to find a link to the colouration of the tree. Alternatively a pleasing contrast can work wonders. For instance red foliage maples look good in blue pots. Most people use brown pots for junipers to highlight the red/brown bark. However brown next to brown is often too much. Try an unglazed grey pot, this often highlights the foliage beautifully. Satsuki azaleas, with their amazing show of flowers can look good in unglazed brown pots that make the flowers even more vibrant. Privets have a tinge of green to them and using a copper green pot works very well. Deciduous trees that make great autumn displays look good in cream glazed pots. The variety of colour combinations is infinite and great fun can be had experimenting.

Heavy trunk trees go best with deep pots and narrower than those used for tall slender trees. But, once the tree starts to become a literati style the pot becomes much smaller and deeper, maybe only a quarter of the height and four to six times the width of the trunk to the depth.

A bonsai pot formalizes the look of a bonsai tree. Planting that same tree on a slab or some form of informal pot can completely change the look and feel of the bonsai. In general I prefer not to use informal pots for very informal looking trees. Unless this is carried out with great dexterity and skill the result can look very undisciplined and messy. To see great use of informal planting schemes see Marc Noelanders book.

DO NOT BE TOO KEEN TO GET YOUR TREES INTO BONSAI POTS!

Bonsai go into bonsai pots. I have seen lots of raw material crammed into bonsai pots years ahead of time. A bonsai pot does not make bonsai! If you do this you will end up with a lot of redundant pots in the future. But, worst of all it will take twice as long to develop your trees into bonsai. The whole point of putting bonsai into small pots is to slow their growth rate. If you are developing raw material into bonsai use deep flower pots of either black plastic or terracotta. Plants will grow very well in those. Never try to develop raw material in seed flats of shallow wooden boxes, remember our discussion of drainage and root growth!

 

Aftercare of re-potted bonsai
If you re-pot a bonsai tree at the correct time of year very little special aftercare will be required. If you are not working at the best time a few simple precautions will ensure success.

After re-potting water the tree thoroughly until the pot is flooded. Watering will not then be required until the soil becomes dry again.
Regular misting of the tree is widely recommended. However this is a practice that does not stand up to close scrutiny. The idea probably comes from the old horticultural practice of raising cuttings under mist. In this situation cuttings are placed in a heated propagating bed and are automatically misted, the operative word being ‘misted’. This method is used for softwood cuttings in spring and summer. In no way could bonsai be called softwood cuttings! Many times I have seen recommendations to regularly spray recently re-potted bonsai with water. This does not constitute misting, commercial misting equipment creates a super fine mist that raises atmospheric humidity. Slopping water over a bonsai with a hosepipe or watering can is not the same thing.

The reasons I do not recommend regular spraying or ‘misting’ with water are many. First off keeping a plant constantly wet is just asking for fungal problems and spraying to prevent these causes the plant some degree of stress. Secondly unless you are using an incredibly fine mist, that’s almost invisible to the eye, a lot of water is going to end up in the soil. Keeping recently potted bonsai too wet in their soil is, again, asking for fungal problems. Thirdly unless the tree starts to grow very quickly, constant damping down will leach nutrients from the foliage at a time when the plant needs them most. If you want to provide humidity for your trees the best solution is to surround them with other plants, in this way they all support each other.
If you have to re-pot very early in the year (winter) it will be beneficial to keep the tree in a cool greenhouse or under cover to avoid over wetting the roots with rainfall. With the exception of tropical and sub tropical species cold and frost will not be a problem. Hardy species can take quite low temperatures with no ill effects.

Another widely disseminated myth is that re-potted bonsai be put in the shade. If you live in Spain or Sicily this is good advice but in the U.K even the height of summer is not really warm as far as plants are concerned. In order to recover from the ordeal of re-potting a plant needs to grow new roots by cell division. Cell division is fuelled by the energy produced in the foliage by photosynthesis. In order for this to take place a tree needs good, bright light. Although direct sunlight is not required a re-potted tree, especially evergreens, should be placed in a very bright position. I think the word shade was probably misunderstood for shelter somewhere along the line. Keeping recently potted bonsai out of the wind and in a bright warm spot is the ideal.

After re-potting choose a good, sheltered (from wind) but bright spot for your trees, preferably one where there are lots of other plants close by. Leave the tree there, only watering when dry, until it has resumed normal growth. Keep a close eye out for pest infestation, this can happen quickly to stressed trees in recovery. Constantly moving a tree in and out of a greenhouse or shed for overnight protection will just add to the stress on the plant. Indoor bonsai should be returned to their normal position immediately after potting.
 

How to deal with sickly & struggling bonsai
This heading could make a library of books all on it’s own. The reasons why bonsai have problems are as numerous as the bonsai trees they affect. However I have included a small mention here because some common problems bonsai suffer are related to re-potting issues and on rare occasions re-potting can be the solution to a problem.

Before deciding to dive in and re-pot your tree it’s important to consider all possibilities and try to pinpoint the cause of a problem. Re-potting a stressed or weak tree is adding insult to injury and may just push the tree over the edge. Some re-potting problems can be overcome by a change of care regimen and this can give the tree time to gain strength before we come to re-pot.

Common situations that cause bonsai health problems are either too much or too little water in the soil. Too much is easy to spot. Look for a dark black colour soil, slimy algae and liverwort growth and lots of small flies constantly buzzing around the pot. In this case, if it’s re-potting season and the tree has a fair vigour level go ahead and re-pot. If it’s not a good time to re-pot the tree can be helped by reducing watering frequency. Also scrape the algal growth off the soil surface and loosen the soil, this will help air ingress when watering.

The other very common problem is overly dry soil. This is not always so easy to spot. Often the soil surface looks damp and probably has nice green moss growth on it. But if a tree is badly pot bound it will often be raised up in the pot and the domed soil surface appears to drain well. However what happens is that the water runs quickly towards the outside of the pot and disappears down between the pot and roots. This will in time lead to a core of soil around the trunk that is bone dry, leaving the tree struggling to access a thin band of moisture around the outside of the root ball. In this case virtually no growth is possible. A simple temporary solution is to open up some holes in the middle of the root ball using a chop stick and loosening the soil surface with a rake.
If you are convinced that your bonsai is suffering because of a root ball problem and it’s not a good time to re-pot I would suggest the following action. Remove the tree from it’s pot. Using fingers only try to loosen the root ball a little by squeezing and pulling but without ripping it apart. Then put the tree into a large, deep plastic or terracotta plant pot with good soil all around. Secure firmly in place with wire, water well and put in a bright sheltered spot. Do not water until dry and do not feed until you see strong growth. Then, depending upon the vigour of the tree I would recommend leaving the tree in the large pot for one, two or even more years until the vigour is fully returned. Once the tree is strong it can be re-potted in the usual way and put back in it’s bonsai pot.
 

Bonsai tree roots and fungus
Not all fungi are bad! The rhizoshpere is a world of teeming activity. If a bonsai tree is to be healthy it’s root system needs to be surrounded by a healthy well balanced mini eco-system of organisms, micro-organisms, bacteria and fungi all pulling together to do their part.
Bacteria and fungi are responsible for breaking down organic compounds ( such as fertilizers) in the soil to release nutrients for their own growth and also the plants.

Most plants have a symbiotic relationship with a group of soil born fungi called mycorrhiza. This is a very large group of fungi and often the associations with plants are very specific between different species. Many times the fungi are invisible to the naked eye but, they are there all the same. The vegetative parts of mycorrhizal fungi are called mycelium and can often be seen as a network of fine filaments permeating the soil. It is beneficial to a bonsai tree to add these fungi to the soil at re-potting time and there are several products available to do this.
Rootgrow +

Fungi and bacteria feed, for the most part on dead organic matter. So, unless you have used a completely inert soil ingredient like akadama, there will be plenty of activity in your soil mix. Organic fertilizers like Green Dream bonsai fertilizer rely upon bacteria and fungi to release their nutrients. Leaf mould and many other organic substances bring fungi and bacteria to the soil. From time to time, especially in warm humid conditions, don’t be surprised if you suddenly see a white bloom of fungi on the soil surface or some tiny mushrooms or an array of other fruiting bodies.

There are a great number of fungal diseases of plant roots. Most of which are impossible to identify by the amateur horticulturalist. Even if you could identify a problem our nanny state would not allow you to have a product to control it. In 20 years of cultivating bonsai in the U.K I have never had fungal root problems that could not be eradicated by good nursery practice and careful choice of soil mixes.

Soil additives for bonsai trees
There are a great many soil additives available for use in horticulture. In bonsai we use a lot of inert and inorganic soil products like Akadama and Pumice. In this case soil additives can be very beneficial.
As a general rule soil additives are either humate products or mycorrhizal seeding products. I would not recommend adding NPK fertilizers or organic fertilizer products to the soil mix at re-potting time.
Humic acid (contained in humate soil additives) is one of the major components of humic substances which are dark brown and major constituents of soil organic matter, humus, that contributes to chemical and physical quality. Humic acid is a good chelator of positively charged ions. This facilitates the uptake of these ions by plants and also prevents their leaching from the soil. It also has a positive influence on nutrients bioavailability.
Mycorrhizal seeding products are widely available. Personally I consider these products very useful but try to avoid the powdered clay versions. I use a product called Rootgrow + and have had very good results.
Many folk highly recommend seaweed products for use post re-potting. Seaweed products
 

Fertilizing recently re-potted bonsai trees
Again this subject could run to a small library of titles. Because of very poor nomenclature and misunderstood science many people are under the impression that fertilizer is ‘plant food’. Because of this there prevails a notion that if a plant is ailing or under stress giving it a good ‘feed’ will help. If like me your mum or gran’ crammed you with food every time you coughed or sneezed as a kid then the misunderstanding is forgivable. Whilst some politicians may have cabbage for brains people are, by and large, not plants. Plants do not eat food, they make their own food from chemicals, water and sunlight and explains why they are the foundation of the food chain.

The only time you should ever fertilize a bonsai tree is when it is actively growing. If your re-potting exercise went well you can expect to see the tree growing again within 2-6 weeks depending upon species and conditions. Once a tree is making good extension growth it’s okay to begin application of fertilizers. If a bonsai is dormant, growing badly or is very stressed due to some external circumstance applying fertilizer is going to add MORE stress to the tree.
 

I hope this article has been of value in helping you understand the importance of creating a vibrant healthy Rhizosphere for your bonsai trees well being. To this end I would very much value your comments. info@kaizenbonsai.com

Graham Potter
June 2009

The information given in this article is provided in good faith based upon our experiences. We accept no responsibility for actions or consequences arising from it’s practical application.

Copy right Kaizen Bonsai Ltd 05/2009