Repotting a Bonsai tree

How do I repot a bonsai tree?

If you want to successfully create bonsai must first and foremost think about the care of bonsai.

Repotting bonsai is one of the most important care techniques and very crucial for the long-term health of bonsai trees. Unfortunately, it is often neglected or done incorrectly. Yet repotting bonsai trees is not complicated but a simple and basic horticultural technique.

Essentially, you need to repot at the right time, choose a suitable bonsai tray and use the right bonsai soil. On this page we have tried to gather the points that should be considered when repotting bonsai. Before we go into detail, we will first explain why bonsai need to be repotted in the first place.


Why is it important to repot bonsai?

Bonsai are living things. They are constantly changing, both above ground and below. To grow and thrive well they need energy (obtained through photosynthesis in the leaves and stored in sugars) and nutrients, which they take from the soil. To absorb the nutrients, they need many roots. These are constantly forming in the soil.

However, since there is only a limited space available in the bonsai tray, after a certain time the entire bonsai substrate in the tray is rooted. This results in a problem: There is no further space for new roots.

And an even bigger problem arises: all the pores in the soil are filled with roots. But the air in these pores is very important for the root. Root cells, like all cells, need oxygen to live. While oxygen is readily available above ground in the air, oxygen supply in the soil can be difficult. Without air in the soil, oxygen is lacking. Then even the roots that are already there will die and the bonsai will die.

The problem is compounded by the decay of the bonsai soil over time. Freshly potted, a decent bonsai soil has a coarse texture. Due to the influence of organisms in the soil and "freezing" in winter, the structure of the soil 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 further compacted by the newly formed roots. Finally, the necessary oxygen in the soil is always missing. A big problem for the bonsai.

And yet another problem arises with compacted bonsai soil: Oxygen is consumed in the cellular respiration of the root cells and carbon dioxide is produced. If the soil in the tray is dense and moist, the carbon dioxide dissolves in the water and carbonic acid is formed. This causes the soil to slowly and steadily acidify which most trees do not like.

All these problems can be solved only by regular repotting of bonsai. The decomposed, fine soil is replaced by new, coarser substrate. And in many cases, root pruning removes some of the roots, making room 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. You should not let it get that far. Besides, the longer you wait to repot, the more difficult the operation becomes for the tree and also for the bonsai enthusiast. Often it is extremely difficult and sweaty to remove the old bonsai soil.

But - no rule without exception: without repotting, the overall growth slows down over the years. The spacing of leaf nodes (internodes) on a shoot shortens. Often, the trees also bloom better. This can aid in the aged appearance of more mature trees. I.e. with more mature trees it can sometimes be beneficial to slow them down in their growth by postponing the repotting of such bonsai for 1 year.


When should bonsai be repotted?

First and foremost, a bonsai needs to be repotted when the soil becomes too compacted. 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 root growth and the type of bonsai soil, a bonsai will need to be repotted after about 3-5 years.

Young plants of fast growing tree species (most maple bonsai, elm and apple bonsai) often need repotting after 1-2 years. Older bonsai (shaping is largely complete) of tree species with slow root growth (e.g. pine bonsai, larch or juniper bonsai) can often be left in the same substrate 1-2 years longer.

Bonsai that have been potted in a fine or rapidly decaying substrate (e.g., peat, potting soil, reused akadama) need to be repotted sooner than bonsai that are in a structurally stable substrate (e.g., expanded shale, high quality akadama, kiryuzuna).

Sometimes salinization of the bonsai soil (e.g. after overfertilization with mineral nutrient salts) or excessive infestation of the soil with pests can also be a reason for repotting. However, both of these occur very rarely.

How can you tell if the bonsai soil is compacted ?
The best way is to carefully lift the bonsai out of the tray. If only roots can be seen on the outside of the bale, it is usually high time to repot.

If the bonsai was properly secured in the tray with bonsai wire during the last potting, it is difficult to lift it out. But there are other ways to assess the need for repotting.

  • If a deciduous tree has not been repotted for 3 years and a coniferous tree for 5 years, it is usually high time to change the substrate.
  • If you have bought a freshly imported tree, it will usually stand in the same soil for at least 2 years.  Most imported bonsai stand in an export nursery for this time, are checked regularly and are usually never repotted during this time.
  • If the tree grows slower than is typical for the tree species or variety, repotting the bonsai may be necessary.
  • If the soil is barely absorbing water when the bonsai is watered, the bonsai soil is often heavily compacted.
  • If the root ball slowly pushes up out of the tray, the root ball of the bonsai has not been properly secured with wire and usually too many roots have already formed. Repotting is usually advisable.
  • If, despite good fertilization and watering in an optimal location, the bonsai has yellowish leaves, the substrate is usually compacted.

However, all the arguments listed at the right time should not be implemented across the board. I.e. do not repot just because 1 point is fulfilled. Observe your bonsai carefully, analyze their growth and only after weighing all the important points should you decide. Too frequent repotting can also have negative consequences. All the above mentioned symptoms are basic indications to give the beginner facts for the decision. Just because 3 years have passed, a deciduous bonsai does not necessarily have to be repotted.

The question when to repot a bonsai can also be understood as a question about the right season.


When is the best time of year to repot?

For almost all bonsai, the best time to repot is early spring. By early spring we mean in Germany about the end of February-beginning of March. In southern European countries, the best time can be as early as the end of January, in northern European countries possibly as late as April. The decisive factor is that the winter with continuous frost should be over, the tree should be shortly before budding and it should not yet be too warm.

In individual cases, repotting can also be done at the end of the summer. Some evergreen tree species such as pines then form new roots. It is important here: The heat of midsummer should be over.

Some tree species such as azalea bonsai are often repotted in Asia even after flowering (i.e. around May-June). There the humidity is very high and the plants grow well even during this time. In Europe, however, repotting should only be done in summer if the bonsai can be placed in a greenhouse for a few weeks after potting to grow.


Why is early spring the best time to repot for most tree species?

The plants are then still in winter dormancy and evaporate little water without leaves. Therefore, they better survive the repotting phase with a partial loss of their roots. The wounds on the roots, which occur during repotting, can heal better. And most importantly - new root hairs can form until the leaves sprout, which ensure the supply of the tree in the summer. If you repot too early, the healing of the cuts is slowed 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 advice is: Do not repot a bonsai before you have informed yourself and weighed all arguments for and against. There is always 1 week time to read up, 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 bowl for many years. Except for massive overfertilization with nutrient salts, we know of no arguments why a bonsai needs to be repotted immediately. 1-2 weeks is always time to clarify everything.

Bonsai should not be repotted under the following circumstances:

  • Do not repot at the wrong time of year. Midsummer is usually not a good time to repot, as is winter.
  • Bonsai taken from the wild (yamadori) should not be repotted until they have shown significant growth in the previous year.
  • Do not repot several times a year. Even with very fast-growing bonsai species, this is not necessary in any case.
  • Do not repot just because 3 years have passed. If the bonsai soil is not well rooted, but otherwise still in order, you can dispense with repotting.
  • It is not necessary to repot a newly acquired bonsai immediately. It is better to observe the tree for a growth period and then decide.


Repotting bonsai - step by step



 1. selection of the bonsai bowl

Once we have decided to repot a bonsai, the next step is to choose the right bonsai tray. In many cases you will take the same bowl again, because it should have been selected during the last repotting. I.e. repotting a bonsai does not necessarily mean to take a new bonsai bowl for the tree. If the bonsai bowl fits optically well to the bonsai it will be cleaned and reused.

We are often asked how much larger a new bonsai bowl should be than the old one. Our answer is usually astonishing: In most cases it does not need to be bigger. Only if your tree has grown significantly larger and should remain, a larger bonsai bowl must be chosen. Otherwise, a bowl of the same size is chosen.

Why ? Bonsai bowls are chosen according to aesthetic principles. If you choose a larger bowl every time you pot your bonsai without changing its size, it will eventually look strange. A simple basic rule for tray size is: the length of the bonsai tray should be about 2/3 of the tree height.

If the root ball is too large for an otherwise suitable bowl it will be reduced. That is, the root ball of the bonsai is made to fit the bowl, not the bonsai bowl is selected to fit the root ball. If the root ball cannot be reduced in size at once, then the bonsai does not belong in the final bonsai tray.


2. selection of bonsai soil

When you hear the term soil, you often think of what you find in the garden: Very fine, containing humus, often loamy, with rotting components. This soil is unsuitable for bonsai. Therefore, experienced bonsai enthusiasts often do not use the term bonsai soil but usually speak of bonsai substrates. These are usually not fine and contain hardly any humus and few rotting components.

A bonsai tree often stands for many years in a bonsai tray without substrate change. Over this long period of time, the bonsai soil must ensure an optimal supply of nutrients, oxygen and water. A commercial garden soil, or even potting soil is not suitable for this. If you use such fine potting soil, your bonsai will be damaged in most cases.

A good bonsai substrate should meet the following requirements: Good crumb structure (aeration) and shape stability, good drainage with high water holding capacity and a sufficient pH buffering capacity.

The bonsai substrates we offer meet these criteria. Akadama can be used for almost all bonsai. For azalea bonsai, Kanuma with its low pH is more suitable. Kiryuzuna is structurally stable longer than Akadama and is therefore preferred for conifer bonsai that have longer repotting intervals. Expanded shale and pumice gravel can be used to further increase the structural stability.

On the Internet you can find many recipes for bonsai soils according to the following scheme: Soil mixture for deciduous trees: 25% pumice gravel, 50% akadama, 25% lava granules and for conifers: 33% pumice gravel, 33% akadama, 33% lava granules. Sometimes I wonder why 2 decimal places are not given. Whoever passes on such detailed cooking recipes should have scientifically examined the whole thing on many thousands of trees with statistical methods. However, the writers often have only a few bonsai standing at home.

If you look at a good soil for plants (e.g. by looking at it with a magnifying glass) you will see a porous, sponge-like structure. There are many air-filled cavities teeming with life. Every naturally occurring soil is unique and highly complex. But all soils in which plants can grow well have similar characteristics. One of them is large pore space. And that is exactly what we need in the bonsai tray, and for several years.

That is, whether 10% expanded shale and 13% pumice gravel is not so crucial.

Both pumice gravel and expanded shale can increase the structural stability and permeability of, for example, Akadama. However, this is only needed if the tree is to stand in the tray for a very long time. Summa summarum can be said: Take established bonsai soils such as Akadama or Kiryu and sift out the dust before potting. If the bonsai is to remain in the tray for more than 3 years, you can add substrates such as expanded shale and pumice gravel. To give exact percentages requires detailed research on many thousands of comparable bonsai under standardized conditions. We know of no bonsai nursery in Europe that has investigated this.

However, it is always important to remember: Fine, dusty components do not belong in a good bonsai soil. Decomposable components are also problematic because they decompose quickly. Akadama, Kanuma or Kiryuzuna pure and without dust fits for almost all cases. The whole thing combined with an organic fertilizer like Biogold or Hanagokoro - and the bonsai is happy. In our bonsai school we have been using a substrate with a lot of expanded shale for almost all bonsai (except azaleas) for 20 years. And we are very satisfied with the growth.

Trees in most cases live with mycorrhza fungi in symbiosis, that is, in a partnership for the benefit of both partners. Many trees are absolutely dependent on this symbiosis and cannot survive without the assistance of the fungus.

If we remove a large part of the old substrate some of the mycorrhiza will be lost. This is not a big problem. The mycorrhiza will re-form over the next few months. We can help the tree with the new formation of mycorrhiza by adding mycorrhiza to the substrate before potting.

One way is to add some (5-10% should be enough) of the old bonsai soil to the new substrate. The problem with this is: we mix in old, decayed substrate and immediately clog the all-important pores again.

Another option 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).


3. assemble accessories for repotting

When repotting a bonsai you should work quickly so that the roots with the fine root hairs do not dry out too much. Therefore, it is very useful to assemble all the accessories before starting the work so as not to have to interrupt the work later.

What accessories do you need for repotting bonsai?

  • In any case, you need a bonsai bowl. In many cases the current tray is reused
  • Bonsai soil and substrate and possibly mycorrhizal fungi are needed in sufficient quantity.
  • Sometimes a sieve is needed to sift the dust from the new substrates
    Cover grids for the drainage holes of the bonsai tray should be ready. Often you can reuse the old cover nets
  • Often the bonsai tree is fixed in the tray with wire. For cutting is well suited bonsai wire pliers
    Very important and indispensable are root hooks and root claws for removing the old bonsai soil
  • A hand brush for cleaning the root base is very helpful
  • Bonsai scissors and bonsai pliers are needed for removing roots
  • Aluminum bonsai wire to secure the tree in the tray is needed
    Earth scoops for filling 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 wetted with a spray bottle. In a pinch, a ball sprayer for bonsai can also be used.
  • Last but not least, you often need a watering can for watering the bonsai after potting.


4. potting the bonsai

The potting of the bonsai is sometimes not so easy. Especially if the bonsai soil is very solidified.

First, you often have to cut the fixing wire that holds the bonsai in the bowl. To do this, tilt the bonsai slightly to one side and cut the bonsai wire under the bottom of the bonsai bowl with wire cutters.

Then try to carefully pry the bonsai out of the bonsai bowl. If it is very tight in the bowl, you can use a root hook or a sickle knife to loosen the old substrate on the wall of the bowl a bit.

Before repotting, it is helpful to water the bonsai less. This often makes it easier for the root ball to come out of the tray.


5. fix the cover grid

If the old bonsai tray is going to be reused, it should be cleaned now. Then attach the cover grids over the drainage holes.

To do this, cover the drainage holes inside the bonsai tray with a cover grid and fix it in place with aluminum bonsai wire.

The cover grids prevent our substrate from falling out through the drainage holes. It also prevents some pests from entering the bonsai tray.


6. remove the old bonsai substrate.

After the bonsai has been freed from the tray, the old bonsai substrate is removed. This should be done much more carefully and sensitively with conifers than with deciduous trees. We describe here the procedure for a deciduous tree with strong roots, which has strongly rooted the bonsai soil.

It is best to start by clearing the root base of moss and weeds with a hand brush. This allows you to better see the surface roots and assess how the roots have developed in the visible area.

Then use a root hook or root claw to carefully pull the root ball apart. A hook is especially good if the root ball is very compacted.

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 root tongs. In very stubborn cases we use a saw and saw off the outer root layer. In this case, one should be very careful. Sometimes the origin of a large part of the bale is in a thick root. If you cut it, it can happen that this part of the bale is completely lost. Sometimes this is desirable, but often it is not.

Once the outer layer of roots is removed, you can usually rake the old bonsai soil out of the inner part of the bale well with the root claw.

How much old substrate to remove is difficult to answer. For healthy trees with strong roots (e.g. Chinese elm bonsai, three-pointed maple), in many cases you can remove the old soil completely. If the root ball is very compacted, it is often a good idea to remove only a portion and repot in 1-2 years to rake out the rest. Here it is really important to estimate the vigor of the tree correctly. The goal should be that the old soil is completely replaced after 2-3 repotting actions.

Often it is advised to wash out the root ball with a water jet. This is a very hard measure. The bonsai loses all the small root hairs and its mycorrhiza. That is why we do not do this, especially with conifers such as pines and larches. Often in stubborn cases it is enough to put the root ball in water for a few minutes. After that, it is often easier to tear open.

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 an enclosed space with high humidity (e.g. greenhouse, foil tent). Regular spraying of the root ball is very helpful from now on against drying out.


7. root pruning

Once the substrate is completely or partially removed, you can decide whether roots (or even more roots) should be removed. Often the question arises how the tree can continue to grow after a severe root pruning. Important is as so often in life: It is not the amount that matters, but rather the efficiency. A bonsai does not need strong roots for its stability. What is important is that there are many small roots for nutrient absorption. So bonsai can then thrive well for several hundred years in a bonsai bowl.

Root pruning is usually necessary for trees with vigorous growth. With many coniferous species (e.g. juniper bonsai, especially from the common juniper, girl pines) one should be very cautious with this and gain experience first. If in doubt, remove a little less and watch the development of the tree. If it thrives well, you can possibly bring forward the next repotting date.

Root pruning is mainly done to make room in the bonsai tray for new substrate and to force the tree to grow many new, small roots to supply the tree. The strong roots are not needed for the stability of a bonsai. What is most important are many fine roots with the all-important root hairs. However, these are only found at the ends of fine roots. Often there are hundreds per square millimeter.

It is often important to perform a root pruning at the superficially visible roots of the stem base. This involves the removal of aesthetically disturbing roots (e.g. crossed roots, strong roots growing towards the observer).

For root pruning, a separate pair of bonsai scissors or bonsai pliers should be used in any case. Due to small stones in the substrate, these tools wear out quickly. They are then usually no longer suitable for cutting the bonsai in the crown area.


8. positioning

Now it's time to start potting. First, put the wire to fix the root ball through the wire holes in the bottom of the tray. If there are no wire holes, you can also put the wire through the drainage holes.

After that, you put a flat layer of the new substrate in the tray and place the bonsai in the desired position on this layer. Since the subject of positioning is very complex I do not want to go into it here. It would go beyond the scope. In many cases, however, the position of the tree is not changed much. If you liked the bonsai before, you are welcome to pot it up again. In any case, the top of the tree should lean in the right direction towards the viewer. Usually a bonsai is also planted slightly off center. Especially in the inclined bonsai style in oval and rectangular bonsai bowls.

Once the tree is positioned as desired, it is held with one hand and with the other hand so much substrate is filled until it stands alone without holding. The substrate can now already be worked in a little with a wooden stick between the roots.


9. fixation

Fixing the bonsai in the tray 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 (for example, when carrying, cutting or watering the bonsai). And even if the bonsai has grown well, the fixation with bonsai wire has a great advantage: If you later lift the bonsai thoughtlessly on the trunk, the shell does not fall down and get broken.

To fix the bonsai, the aluminum wire that was previously inserted through the wire holes is bent around the root ball and shortened by twisting it with a pair of pliers or combination pliers so that it fits tightly to the root ball. 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. Better is 2mm, for large bonsai even 2.5mm. If the wire is too thin, it will easily break when tightened.

The place where it is bent around the root ball or roots should be chosen so that it is no longer visible after filling the missing bonsai substrate. If you use only one wire, it should be placed around the roots behind the trunk if possible.

The fixing wire remains in the tray until the next repotting. It does not normally disturb the tree.


10. fill substrate

Now we can fill the missing, new bonsai soil up to the top edge of the bonsai tray and carefully work it in between the roots with a chopstick and press it down well on the surface. The substrate at the edge of the bonsai tray should be pressed a little deeper than the edge of the tray so that when watering later the new bonsai soil is not washed out of the tray or the watering water remains where it should be - in the tray.

If the substrate is very coarse, you can possibly apply a thin cover layer (max. 1cm) of finer material. Then the undergrowth under the bonsai will grow better. But: During the 1st watering a part of this cover layer will be washed into our so important pores. This can be avoided by carefully dipping the bonsai to the edge of the tray the first few times.


11. watering

Once the bonsai is potted it should be watered thoroughly. Watering should be done carefully, otherwise the new bonsai soil will be washed out of the bowl. A watering can with a fine watering head is very suitable here. Also a ball sprayer is best suited or one dips the bonsai. When dipping bonsai freshly potted in kanuma, be careful that the kanuma does not float away. It is very light. Likewise, if you have added a lot of pumice gravel.

It should be watered until the water comes out of the bottom of the tray. If the new substrate in the tray sinks a little, you can immediately add some more substrate.


12. aftercare

After potting, place the bonsai in a wind-protected place with as much humidity as possible. Water the bonsai in the next few weeks only when the substrate surface has clearly dried. New root hairs must first form. Without root hairs, the bonsai cannot absorb the water anyway. If too much watering is done, you are constantly cooling the root ball. This slows down the new root formation significantly. So - water very sparingly after potting.

Constant spraying of the newly potted bonsai is also not advisable. Unless you have a professional misting system, the droplets are far too large and will constantly cool the root ball just like watering. A high humidity as in a greenhouse is good, constant wetting is not recommended.

Likewise, shady placement is usually not recommended. If repotted at the right time of year in spring the sun is not yet too hot. The sun warms the tray only a little and this is even desired in this situation. This allows new roots to form more quickly. Also, when potting, many bonsai do not yet have leaves and so evaporate little. Sun protection is not appropriate here. In our bonsai school we place all freshly potted bonsai in the greenhouse as sunny as possible.

The first 1-2 months you should also completely avoid fertilizing the bonsai. Without root hairs, the tree cannot absorb the fertilizer. In addition, the trees usually have to sprout first before they can do anything with the fertilizer. Fertilize the bonsai only when they are clearly sprouted.