Very important and to be adapted according to the species...


Bonsai live in pots, which fill up quickly with roots. Therefore, in order for the roots to live well, bonsai are grown in various substrates in the form of grains. In this way, they have enough space between the grains of soil for air to enter. This is necessary for the roots of the bonsai to breathe without difficulty. But as we have already said, this influences the frequency of watering. As a solution to this problem, we find many bonsai species grown in substrates that retain more or less water. With this type of substrate, beginners have less difficulty in terms of watering frequency.

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The repotting of bonsai is one of the most important maintenance techniques and is fundamental to keeping bonsai in good health over the long term. Unfortunately, it is often neglected or postponed. The repotting of bonsai is not complicated. It is a simple and basic horticultural technique.

Essentially, you need to repot at the right time, choose an appropriate bonsai pot and use the right bonsai substrate. In this page, I have tried to summarize the points to consider when repotting of bonsai

Why is repotting bonsai important?

Bonsai are living beings. They are in constant evolution, both above and below ground. To grow and prosper, they need energy (obtained by photosynthesis in the leaves and storage of sugars) and nutrients, which they get from the substrate and fertilizers used. In order to absorb the nutrients, they need many roots. 

Since there is only limited space available in the pot, all the bonsai substrate in the pot will be filled with roots after a while. This creates a problem: there is no space for new roots.

And there is an even bigger problem: all macropores in the substrate are filled with roots. But the air in these macropores is very important for the roots. Like all cells, root cells need oxygen to live. While oxygen is readily available in the air above the substrate, getting oxygen into the substrate can be difficult. Without air in the substrate, there is no oxygen. Then even the existing roots die and the bonsai tree dies.

The problem is compounded by the deterioration of the bonsai substrate over time. When newly repotted, a good bonsai substrate has a granular structure. Due to the influence of organisms in the substrate and "freezing" in winter, the structure of the soil becomes finer and finer over the course of 2-3 years. However, the finer the substrate, the more the space for macropores in the substrate decreases, which is then compacted by the newly formed roots. In the end, the necessary oxygen is always missing in the substrate. A big problem for bonsai.

Another problem with a compacted bonsai substrate is that when the root cells breathe, oxygen is consumed and carbon dioxide is produced. If the substrate in the pot is dense and moist, the carbon dioxide dissolves in the water and carbonic acid is produced. As a result, the substrate becomes slowly and steadily more acidic, which most trees do not like.

We can solve all these problems by repotting the bonsai regularly. The fine, decomposed soil is replaced with a new, more granular substrate. And in many cases, some of the roots are removed with pruning, creating space for new growth.

If a bonsai is not repotted regularlyIt will become weaker and weaker as the years go by and it will die, sooner or later. It shouldn't go that far. Moreover, the longer you wait before repotThe more difficult it will be for the tree and also for the bonsaika. It is then often extremely difficult and annoying to remove the old bonsai substrate.

But no rule without exception: without repotting, all growth slows down over the years. The distances between the leaf nodes (internodes) of a shoot are shortened. Often the trees even flower better. This can increase the aged appearance of more mature trees. This means that with mature trees, it can sometimes be beneficial to slow down their growth a bit by delaying the repotting of such a bonsai for 1 year.

When should I repot a bonsai?

Above all, a bonsai must be repotted when the substrate becomes too compact. Substrate compaction increases mainly due to root growth and the slow decomposition of the bonsai substrate. Therefore, depending on the speed of root growth and the type of bonsai substrate, a bonsai should be repotted after about 3 to 5 years.

Young plants of fast-growing tree species (most maple, elm and apple bonsai) often need to be repotted after 1 or 2 years. Older bonsai (largely finished shaping) of slow-growing tree species (e.g., pine, larch, or juniper bonsai) can often be left in the same substrate for an additional 1-2 years.

Bonsai that have been repotted into a thin or rapidly decomposing substrate (e.g., peat moss, potting soil, reused akadama) should be repotted earlier than bonsai which are in a structurally stable substrate (e.g. expanded shale, high quality akadama, Kiryuzuna).

Sometimes salinization of the bonsai substrate (for example after overfertilization with mineral nutrients) or excessive infestation of the substrate by pests can be a reason for repotting. However, these two phenomena are very rare.

How do you know if the bonsai substrate is compacted?

The best way to be sure is to carefully lift the bonsai out of the pot. If only the roots can be seen outside the root ball, it is usually high time to repot.

If the bonsai was properly anchored in the pot with bonsai wire when last repotted, it is difficult to lift. But there are other ways to assess 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 purchased a tree that has just been imported, it has usually been in the same substrate for at least 2 years. As long as most imported bonsai are in an export nursery, they are regularly checked and usually never repotted during this period.
  • If the tree grows more slowly than is typical for the tree species or variety, the repotting of bonsai may be necessary.
  • If the substrate barely absorbs water when the bonsai is watered, the bonsai substrate is often heavily compacted.
  • If the root ball grows slowly out of the pot, the bonsai root ball was not properly anchored with wire and too many roots have already formed. 
  • If, despite good fertilization and watering, the bonsai placed in an optimal location has yellowish leaves, the substrate is usually compacted.

However, all the arguments listed, even if at the right time, should not be implemented across the board. That is, do not repot just because one point is filled. Observe your bonsai carefully, analyze their growth and decide only after weighing all the important points. Repotting too often can also have negative consequences. All the above mentioned symptoms are basic indications to give beginners the necessary elements to make a decision. Just because a deciduous bonsai has not been repotted for 3 years does not mean it should be repotted.

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

What is the best season to repot?

For almost all bonsai, early spring is the best time to repot.  

In some cases, it can also be repotted in late summer. Some species of evergreen trees such as pines then form new roots. The important thing here is: the mid-summer heat should be over.

In Asia, some species, such as azaleas, are often repotted after flowering (i.e. around May-June). The humidity is very high there and the plants grow well even during this period. In Europe, however, you should not repot in summer only if the bonsai can be put in a greenhouse for a few weeks after repotting.

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

The plants are then still in winter dormancy and let little water evaporate without leaves. They therefore survive the repotting phase better with a partial loss of their roots. The wounds on the roots that occur during repotting can heal better. Most importantly, new rootlets can form until the leaves come out, ensuring the tree's supply in summer. If you repot too early, the healing of cuts is slowed, if you repot too late, the tree's water and nutrient supply may suffer.

When not to repot bonsai?

The most important tip is this: do not repot a bonsai before you have informed yourself and weighed all the arguments for and against. There is always a week to get information, ask a bonsai professional or take the tree to a bonsai working group meeting to analyze it together. A bonsai tree will last for many years in a bonsai pot. With the exception of massive overfertilization with nutrient saltst. One or two weeks are always necessary to clarify everything.The bonsai should not be repotted under the following circumstances:

  • Don't repot at the wrong time of year. Mid-summer is generally not a good time to repot, nor is winter.
  • Wild-caught bonsai (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 species, it is not necessary.
  • Do not repot just because 3 years have passed. If the bonsai's substrate is not full of roots, but is otherwise in good condition, you can put off repotting.
  • A newly purchased bonsai does not necessarily need to be repotted immediately. It is best to observe the tree for one growing season and then decide.

Potting bonsai - step by step

1. Choosing a bonsai pot

When we decided that a bonsai must be repottedChoosing the right bonsai pot is the order of the day. In many cases, you will use the same pot again, as it was probably chosen the last time you repotted your bonsai. For example, repotting a bonsai does not necessarily mean getting a new pot for the tree. If the bonsai pot fits the bonsai well, it will be cleaned and reused.

I am often asked how big a new bonsai pot should be compared to the old one. My answer is usually surprising: in most cases, it doesn't need to be bigger. Only if your tree has grown significantly and needs to stay that way, should you choose a larger bonsai pot. Otherwise, you should choose a pot of the same size.

Why is this so? Bonsai pots are chosen based on aesthetic principles. If you choose a larger pot every time you repot your bonsai without it having grown, it will end up looking strange. A simple rule of thumb for pot size is that the length of the bonsai pot should be about 2/3 the height of the tree.

If the root ball is too large for a suitable pot, it will be reduced. That is, the bonsai root ball is made to fit the pot and a bonsai pot is not chosen to fit the root ball. If the root ball cannot be reduced at one time, the bonsai should not yet be placed in the final bonsai pot.