Repotting bonsai is one of the most important maintenance techniques and is very important for keeping bonsai trees healthy in the long term. Unfortunately, it is often neglected or incorrectly executed. Repotting the trees is not complicated, but a simple and basic technique in horticulture.
Essentially, you have to repot at the right time, choose a suitable bonsai pot and use the right bonsai soil. On this page we have tried to summarize the points that should be considered when repotting bonsai. Before we go into detail, it should first be explained why bonsai have to be repotted at all.
Why repotting ?
Bonsai are living things. They are constantly changing, both above and below ground. In order to grow and thrive, they need energy (obtained through photosynthesis in the leaves and stored in sugars) and nutrients, which they take from the soil. In order to be able to absorb the nutrients, they need many roots. These are constantly re-forming in the soil.
Since there is only a limited space available in the bonsai pot, the entire bonsai substrate in the pot will be rooted through after a certain time. This creates a problem: there is no more space for new roots.
And there is an even bigger problem: all the pores in the soil are filled with roots. But the air in these pores is very important for the roots. Like all cells, root cells need oxygen to live. While oxygen is readily available in the air above ground, oxygen supply in the ground can be difficult. Without air in the soil, there is no oxygen. Then even the existing roots die and the bonsai can be damaged or can even die.
The problem is compounded by the disintegration of the bonsai soil over time. When freshly filled into the pot a reasonable bonsai soil has a coarse structure. Due to the influence of organisms in the soil and freezing in winter, the structure of the earth becomes finer and finer over the course of 2-3 years. The finer the soil, however, the smaller the pore space in the substrate, which is then compacted by the newly formed roots. In the end, the necessary oxygen is always missing in the soil. A big problem for the bonsai trees.
Another problem arises with compacted bonsai soil: When the root cells breathe, oxygen is consumed and carbon dioxide is produced. If the soil in the pot is dense and moist, the carbon dioxide dissolves in the water and carbonic acid is produced. As a result, the soil slowly and steadily acidifies, which most trees do not like.
We can only solve all of these problems by repotting the bonsai regularly. The disintegrated, fine soil is replaced by new, coarser substrate. And in many cases part of the roots is removed by a root cut, thus creating space for new growth.
If a bonsai is not repotted regularly, it will grow weaker and weaker over the years and sooner or later it will die. It shouldn't get that far. In addition - the longer you wait before repotting, the more difficult the intervention will be for the tree and also for the bonsai friend. It is then often extremely difficult and sweaty to remove the old bonsai soil.
But - no rule without exception: without repotting, the entire growth slows down over the years. The distances between the leaf nodes (internodes) on a shoot are shortened. Often the trees also bloom better. This can add to the aged look of more mature trees. This means that with mature trees it can sometimes be beneficial to slow down their growth a little by postponing repotting such bonsai for 1 year.
When should bonsai be repotted ?
Above all, a bonsai has to be repotted when the soil becomes too compact. The compaction of the soil increases mainly due to the growth of the roots and the slow decay of the bonsai soil. Accordingly, depending on the speed of the root growth and the type of bonsai soil, a bonsai must be repotted after approx. 3-5 years.
Young plants from fast-growing tree species (maple bonsai, elm bonsai, apple bonsai) often have to be repotted after 1-2 years. Older bonsai (shape is largely complete) of tree species with slow root growth (pine bonsai, juniper bonsai) can often stand 1-2 years longer in the same substrate.
Bonsai that have been potted in a fine or rapidly disintegrating substrate (e.g. peat, potting soil, reused Akadama) must be repotted earlier than bonsai that are in a structurally stable substrate (e.g. expanded slate, high-quality Akadama, Kiryuzuna).
Sometimes a salinization of the bonsai soil (e.g. after over-fertilization with mineral nutrient salts) or the excessive infestation of the soil with pests can be a reason for repotting. But both are very rare.
How can you determine if the bonsai soil is compacted ?
The best way to do this is to carefully lift the bonsai out of the pot. If only roots can be seen on the outside of the root ball, it is usually high time to repot.
If the bonsai was properly fixed in the pot with bonsai wire during the last potting, lifting it out is difficult. But there are other ways of assessing the need for repotting.
- If a deciduous tree has not been repotted for 3 years and a conifer for 5 years, it is usually high time to change the substrate.
- If you have bought a freshly imported tree, it usually stands in the same soil for at least 2 years. As long as most of the imported bonsai are in an export nursery, they are checked regularly and usually never repotted during this time.
- If the tree grows more slowly than is typical for the tree species or variety, repotting the bonsai may be necessary.
- If the soil hardly absorbs any water when the bonsai is poured, the bonsai soil is often highly compacted.
- If the root ball slowly pushes up out of the pot, the root ball of the bonsai was not properly attached with wire and too many roots have usually already formed. Repotting is usually needed.
- If, despite good fertilization and watering, the bonsai has yellowish leaves in an optimal location, the substrate is usually compacted.
However, all the arguments listed above the right time should not be implemented too strict. In other words, do not repot just because 1 point has been fulfilled. Observe your bonsai carefully, analyze their growth and only after weighing all the important points should you make a decision. Repotting too often can also have negative consequences. All of the above-mentioned symptoms are basic pointers to give the beginner facts for a decision. Just because 3 years have passed, a deciduous tree bonsai does not necessarily have to be repotted.
The question of when a bonsai should be repotted can also be understood as a question of the right season.
When is the best season ?
For almost all bonsai, early spring is the best time to repot. In Germany we understand early spring to be around the end of February-beginning of March. In southern european countries the best time can be the end of January, in northern european countries it may not be until April. The decisive factor is: The winter with permanent frosts should be over, the tree should be about to sprout and it should not be too warm yet.
In individual cases it can also be repotted at the end of summer. Some evergreen tree species such as pines then form new roots. The important thing here is: the heat of midsummer should be over.
Some tree species such as azalea bonsai are often repotted in Asia after flowering (i.e. around May-June). The humidity there is very high and the plants grow well even during this time. In Europe, however, you should only repot in summer if the bonsai can be put in a greenhouse for a few weeks to grow after potting.
Why is early spring the best time to repot ?
The plants are then still in hibernation and hardly evaporate any water without leaves. As a result, they survive the repotting phase with a partial loss of their roots better. The wounds on the roots that occur when repotting can heal better. And above all - new root hairs can form until the leaves have sprouted out, which ensure the supply of the tree in summer. If you repot too early, the healing of the cutting interfaces slows down. If you repot too late, the water and nutrient supply of the tree can suffer.
When should bonsai not be repotted ?
The most important note of all is: Do not repot a bonsai until you have informed yourself and weighed all the arguments for and against. There is always 1 week to read, to inquire at a bonsai dealer or to take the tree to a meeting of a bonsai working group to analyze it together. A bonsai tree will stand in a bonsai pot for many years. Except in the case of massive over-fertilization with nutrient salts, we do not know any arguments why a bonsai has to be repotted immediately. There is always time for 1-2 weeks to clarify everything.
Bonsai should not be repotted under the following circumstances:
- Don't repot at the wrong time of year. Midsummer is usually not a good time to repot, as is winter.
- Bonsai taken from nature (Yamadori) should only be repotted when they have shown significant growth in the previous year.
- Do not repot several times a year. Even with very fast-growing tree species, this is by no means necessary.
- Don't repot just because 3 years have passed. If the bonsai soil is not well-rooted, but otherwise still ok, you can do without repotting.
- A newly acquired bonsai does not necessarily have to be repotted immediately. It is better to first observe the tree for a period of growth and then make a decision.
Step by step
1. Selection of the bonsai pot
When we have decided that a bonsai tree should be repotted, the selection of the right bonsai pot is on the plan. In many cases you take the same pot again because you have been selected it to match the tree the last time you repotted. That means repotting a bonsai does not necessarily mean taking a new bonsai pot for the tree. If the bonsai pot looks good with the bonsai, it is cleaned and reused.
We are often asked how much a new bonsai pot should be larger than the old one. Our answer is mostly astonishing: in most cases it does not need to be larger. A larger bonsai pot only needs to be chosen if your tree has become significantly larger and is supposed to stay that way. Otherwise a pot of the same size is selected.
Why ? Bonsai pots are selected according to aesthetic principles. If you choose a larger bonsai pot for each potting without changing the size of the bonsai, it would look strange at some point. A simple basic rule for pot size is: The length of the bonsai pot should be about 2/3 the height of the tree.
If the root ball is too big for an otherwise suitable pot, it will be reduced. This means that the root ball of the bonsai is reduced to match the pot and not the bonsai pot is selected for the root ball. If the root ball cannot be reduced in size all at once as required, then the bonsai does not actually belong in the final bonsai pot.
2. Selection of bonsai soil
When you hear the term soil you often think of what you find in the garden: very fine, humus-containing, often loamy, with rotting components. This soil is unsuitable for bonsai. That is why experienced bonsai friends often do not use the term bonsai soil, but mostly speak of bonsai substrates. These are usually not fine and contain hardly any humus and only few rotting components.
A bonsai tree often stands in a bonsai pot for many years without changing the substrate. Over this long period of time, the bonsai soil must ensure an optimal supply of nutrients, oxygen and water. Commercially available garden soil or even potting soil is not suitable for this. If you use such fine potting soil, your bonsai will in most cases be damaged.
A good bonsai substrate should meet the following requirements: Good crumb structure (ventilation) and dimensional stability, good drainage with high water retention capacity and sufficient pH buffering capacity.
The bonsai substrates we offer meet these criteria. The Akadama can be used for almost all bonsai. Kanuma with its low pH value is more suitable for azalea bonsai. Kiryu is structurally stable for longer than Akadama and is therefore preferred for conifer bonsai that have longer repotting intervals. Expanded slate and pumice gravel can be used to further increase the structural stability by adding them.
On the Internet you can find many recipes for bonsai soil according to the following scheme: Soil mixture for deciduous trees: 25% pumice gravel, 50% Akadama, 25% lava granulate and for conifers: 33% pumice gravel, 33% Akadama, 33% lava granulate. Sometimes I ask myself why there are not 2 decimal places. Anyone who passes on such detailed recipes should have scientifically examined the whole thing on many thousands of trees using statistical methods. However, the writers often only have a few bonsai trees at home.
If you look at good soil for plants (e.g. by looking at it with a magnifying glass) you can see a porous, spongy structure. There are many air-filled cavities teeming with life. Every naturally occurring soil is unique and highly complex. But all soils on which plants can grow well have similar properties. One of them is the large pore space. And that's exactly what we need in the bonsai pot for several years.
That means whether 10% expanded slate and 13% pumice gravel is not so important.
Much more important is: No matter what you take, the substrate should be well permeable and structurally stable.
Both pumice and expanded slate can increase the structural stability and permeability of e.g. Akadama. But this is only needed if the tree is to stand in the pot for a very long time. All in all, one can say in general: Take established bonsai soils such as Akadama or Kiryu and sift out the dust before potting. If the bonsai is to stay in the pot for more than 3 years, substrates such as expanded slate and pumice gravel can be added as desired. Specifying exact percentages requires detailed studies on many thousands of comparable bonsai under standardized conditions. We don't know of any bonsai nursery in Europe that have investigated this.
But it is always important: Fine, dusty components do not belong in a good bonsai soil. Rotting components are also problematic as they break down quickly. Akadama, Kanuma or Kiryu pure and without dust is suitable for almost all cases. The whole thing combined with an organic bonsai fertilizer such as Biogold or Hanagokoro - and the bonsai is happy. In our bonsai nursery we have been using a substrate with a lot of expanded slate for almost all bonsai (except azaleas). And we are very satisfied with the growth.
In most cases, trees live in symbiosis with mycorrhizal fungi, i.e. in a partnership for the benefit of both partners. Many trees are absolutely dependent on this symbiosis and cannot survive without the help of the fungus.
If we remove a large part of the old substrate, part of the mycorrhiza is lost. It's not a big problem. The mycorrhiza will form again over the next few months. We can help the tree to regenerate the mycorrhiza by adding mycorrhiza to the substrate before potting.
One possibility is to add some of the old bonsai soil to the new substrate (5-10% should be sufficient). The problem with this is that we mix in old, decayed substrate and immediately clog the important pores again.
Another possibility is to add commercially available mycorrhiza. Since it is much more concentrated, you need to add much less volume. Usually about 1% is sufficient here (e.g. 500ml for 50l bonsai substrate).
You can find detailed information on the subject of mycorrhiza at: Mycorrhiza and bonsai
3. Put together the accessories for repotting
When repotting a bonsai you should work quickly so that the roots with the fine hairs do not dry out too much. It is therefore very useful to put all accessories together before starting work so that you do not have to interrupt work later.
What accessories do you need for repotting bonsai ?
- In any case, you need a bonsai pot. In many cases, the current pot will be reused
- Bonsai soils and substrates and possibly mycorrhizal fungi are required in sufficient quantities
- Sometimes a sieve is needed to sift the dust from the new substrates
- Cover grids for the drainage holes of the bonsai pot should be ready. The old covering nets can often be reused
- The tree is often fixed in the pots with wire. Bonsai wire cutters are well suited for cutting
- Root hooks and root claws are very important and indispensable for removing the old bonsai soil
- A hand brush to clean the roots of the bonsai is very helpful
- Separate bonsai scissors and bonsai cutters are needed to remove roots
- Aluminum bonsai wire is used to fix the tree in the pot
- Soil shovels for filling in the substrate are helpful. If you fill in new substrate with your hands, you should have rubber gloves ready
- A small wooden stick is useful for working the bonsai soil between the roots
- So that the roots do not dry out when repotting, they can be moistened with a spray bottle. In a pinch, a ball shower for bonsai trees is also possible
- Last but not least, you often need a watering can to water after potting
4. Remove the tree out of the pot
Removing the bonsai tree out of the pot is sometimes not that easy. Especially if the bonsai soil is very compact.
Often times, the fixation wire that holds the bonsai in the pots has to be cut first. To do this, tilt the bonsai a little to one side and cut the bonsai wire under the bottom of the bonsai pot with a wire cutter.
Then you try to carefully pry the bonsai out of the bonsai pot. If it sits very tightly in the pot, you can loosen up the old substrate on the pot wall with a root hook or a sickle knife.
Before repotting, it is helpful to water the bonsai less. This often makes it easier for the root ball to come out of the pot.
5. Fix the cover grille
If the old bonsai pot is used again, it should be cleaned now. Then the cover grids are attached over the drainage holes.
To do this, cover the drainage holes on the inside of the bonsai pot with a cover grille and fix it, preferably with aluminum bonsai wire.
The cover grids prevent our substrate from falling out through the drainage holes. In addition, some pests are prevented from entering the bonsai pot.
6. Remove the old bonsai substrate
After the bonsai has been freed from the pot, the old bonsai substrate is removed. You should be much more careful and empathetic with conifers than with deciduous trees. Here we describe the procedure for a deciduous tree with strong roots that has deeply rooted the bonsai soil.
It is best to start off with a hand brush to remove moss and weeds from the surface roots. This makes it easier to see the roots and assess how the roots have developed in the visible area.
Then the root ball is carefully pulled apart with a root hook or a root claw. A hook is particularly suitable when the bale is very solid.
If the bonsai has not been repotted for a long time, it is very difficult to get through the root mass on the outside of the root ball. In this case, you can cut off the outer 2-3cm of the root layer with a root cutter. In very stubborn cases, we use a saw and saw off the outer root layer. One should proceed very carefully here. Sometimes a large part of the bale originates from a thick root. If you cut it, it can happen that this part of the bale is completely lost. Sometimes this is desired, but often not.
As soon as the outer layer of the roots is removed, you can usually rake the old bonsai soil out of the inner part of the ball with the root claw.
How much old substrate should be removed is difficult to answer. In the case of healthy trees with strong roots (Chinese elm,trident maple), the old soil can be completely removed in many cases. If the bale is very solidified, it often makes sense to only remove a part and repot it again in 1-2 years to rake out the rest. Here it is really important to correctly assess the vigor of the tree. The aim should be that the old soil is completely replaced after 2-3 repotting actions.
It is often advised to wash out the root ball with a jet of water. It's a very hard procedure. In the process, the bonsai loses all of its small root hairs and its mycorrhiza. That is why we do without it, especially with conifers such as pines and larches. In stubborn cases, it is often enough to put the root ball in water for a few minutes. After that, it is often easier to remove the soil.
As soon as the substrate is partially or completely removed, you should continue working quickly as the remaining root hairs dry out quickly. It is best to repot in a closed room with high humidity (e.g. greenhouse, foil tent). Regular spraying of the root ball is now very helpful against drying out.
7. Root cut
If the substrate is completely or partially removed, you can decide whether roots (or better even more roots) should be removed. Often the question arises as to how the tree can continue to grow after a strong root cut. As is so often the case in life, it is important that it is not the quantity that counts, but rather the efficiency. A bonsai does not need the strong roots for its stability. It is important that there are many small roots for nutrient uptake. In this way, bonsai can thrive well in a bonsai pot for several hundred years.
A root pruning is usually necessary for vigorous trees. With many coniferous tree species (juniper, especially common juniper, white pine bonsai) you should be very cautious and gain experience first. If in doubt, you should remove a little less and look at the development of the tree. If it does well, the next repotting date can be brought forward a little.
A root pruning is mainly carried out to create space in the bonsai pot for new substrate and to force the tree to form many new, small roots to supply the tree. The strong roots are not required for the stability of a bonsai. Above all, many fine roots with the important root hairs are important. However, these are only located at the ends of fine roots. Often there are hundreds per square millimeter.
It is often important to make a root cut on the roots of the trunk that are visible on the surface. In the process, aesthetically disturbing roots (e.g. crossed roots, strong roots growing towards the viewer) are removed.
For the root cut you should definitely use separate scissors or bonsai cutters. These tools wear out quickly due to small stones in the substrate. They are then usually no longer suitable for cutting in the crown area.
Now it's time to potting. First, the wire to fix the root ball is put through the wire holes in the bottom of the pot. If there are no wire holes, you can push the wire through the drainage holes.
Then you put a flat layer of the new substrate in the pot and place the bonsai in the desired position on this layer. Since the topic of positioning is very complex, I don't want to go into it at this point. It would go beyond the scope. In many cases, however, not much is changed in the position of the tree. If you liked the bonsai before, you can repot it like that. In any case, the top of the tree should be tilted towards the viewer. Usually a bonsai is also planted slightly off the center. Especially with the inclined bonsai style in oval and rectangular bonsai pots.
If the tree is positioned as desired, it is held with one hand and with the other hand so much substrate is filled in until it stands alone without holding it. The substrate can now be worked a little between the roots with a wooden stick
Fixing the bonsai in the pot with wire is very important. The new substrate is still very loose for several months. If the tree is not properly fixed with wire, it will grow very poorly due to frequent movements of the trunk (e.g. when carrying, cutting or watering). And even if the bonsai has grown well, fixing it with bonsai wire has a big advantage: If you later lift the bonsai thoughtlessly up the trunk, the pot does not fall down and break.
To fix it, the aluminum wire previously inserted through the wire holes is bent around the bale and shortened with a pair of pliers by turning it so that it rests tightly on the bale. The protruding wire ends can be shortened.
It is best to use aluminum wire with a thickness of at least 1.5mm to secure the root ball. 2mm is better, for large bonsai also 2.5mm. If the wire is too thin, it will tear easily when tightened.
The place where it is bent around the ball or the roots should be chosen so that it can no longer be seen after filling the missing bonsai substrate. If you only use one wire, it should be placed around the roots behind the trunk, if possible.
The fixing wire remains in the pot until the next repotting. Normally it does not disturb the tree.
10. Fill up the substrate
Now we can fill in the new bonsai soil that is still missing up to the upper edge of the bonsai pot and work it carefully with a stick between the roots and press it firmly on the surface. The substrate on the edge of the bonsai pot should be pressed a little deeper than the edge of the pot so that the new bonsai soil is not washed out of the pot later on when watering and the irrigation water stays where it should - in the pot.
If the substrate is very coarse, a thin covering layer (max. 1cm) made of finer material can be applied. Then the undergrowth under the bonsai grows better. But: During the first casting, part of this covering layer is washed into our so important pores. This can be avoided by carefully dipping the bonsai up to the edge of the pot the first few times.
Once the bonsai is potted, it should be watered thoroughly. You should water carefully otherwise the new bonsai soil will be washed out of the pot. A watering can with a fine watering head is very suitable here. A ball shower is also ideal or you can dive the bonsai down to the edge of the pot. When diving bonsai freshly potted in Kanuma, you have to be careful that the Kanuma does not swim away. It's very light. Likewise if you have added a lot of pumice gravel.
It should be watered until the water comes out of the pot again. If the new substrate in the pot collapses a bit, you can immediately refill some more substrate.
12. Follow-up care
After potting, place the bonsai in a sheltered place with the highest possible humidity. In the next few weeks, water will only be poured if the substrate surface is clearly dry. New root hairs have to form first. Without root hairs, the bonsai cannot absorb the water anyway. If too much is poured, the root ball is constantly being cooled. This significantly slows down the formation of new roots. So - water very sparingly after potting.
Constantly spraying the newly potted bonsai is also not advisable. If you do not have a professional nebulization system, the droplets are much too big and constantly cool the root ball, just like when watering. A high level of humidity like in a greenhouse is good, constant wetting is not recommended.
A shady set-up is also usually not recommended. If the pots are repotted at the right time of year in spring, the sun is not yet too hot. The sun only warms the pot a little, which is actually desirable in this situation. This way, new roots form faster. In addition, many bonsai do not have any leaves when potted and so little evaporate. Sun protection is not appropriate here. In our bonsai nursery, we place all freshly potted bonsai in the greenhouse in the most sunny spot possible.
You should refrain from fertilizing for the first 1-2 months. Without root hairs, the tree cannot absorb the fertilizer. In addition, the trees usually have to sprout out first before they can do anything with the fertilizer. Only fertilize the bonsai when they show new growth.