Repotting bonsai trees is essential in achieving healthy growth and good aesthetics. All too often bonsai tree growth is stunted by repotting which takes too long or is simply not done at all. Conversely, however, there is a certain prudence to be had regarding hasty repotting. We will examine these issues, and more.
Although we will expand on it later, the basic procedure to repotting bonsai trees is as follows:
- Select a pot. Large pots allow for more growth. It is important to understand, regardless, that when repotting bonsai trees we don’t need to change the pot if we don’t want to.
- Set a mesh screen on the bottom of the pot and wire it in place. Prepare some wiring to set your bonsai tree in place as well.
- Carefully remove the bonsai tree from the old pot and trim excess roots. Avoid removing more than 30% of these roots, more or less.
- Re-soil your bonsai tree. There are several different mixtures that can be used. A mixture of akadama, gravel and compost is quite typical.
- Water the bonsai tree and let it rest, protected from winds.
When to Repot Bonsai Trees
It is important not to take too long to repot. There are several factors that will tell a bonsai owner when to make this happen.
The size of the bonsai pot and the tree species greatly influence the timing of repotting. Fast growing species must be repotted every couple years. Younger bonsai trees need to be repotted once a year. They evidently grow fast and roots can become a problem quite quickly. Alternatively, other species, and particularly mature trees, are to be repotted every four years or so. Note that these are rough guidelines.
Some of the signs that suggest repotting a bonsai tree is necessary are:
- Slow rate of growth compared to rate of growth previously.
- Reduced water uptake (too much leakage underneath).
- Reduction of aesthetics: noticeably smaller leaves, for instance, or discoloration/yellowing of leaves.
- Falling of leaves which occurs too early / suddenly.
What to look for before repotting
Repotting bonsai trees is not to be done haphazardly. It is vital that we observe our trees carefully and the signs that point towards the need to repot. Repotting will essentially protect our tree from choking itself off from the soil, water and nutrients through excess rooting.
Early spring is an ideal moment to check on a bonsai tree’s roots. Remove the tree carefully from its pot and observe the roots. If the roots circle around each other, and constitute a sort of block, so to speak, it is time to repot. Such mesh of roots displace the soil, pushing it away from the tree and thus essentially starving the bonsai tree. If, however, the roots are still within soil, repotting need not take place just yet.
Having the right materials is very handy when repotting bonsai trees. Things such as root rakes, scissors, wire cutters and chopsticks are standard. A roothook is quite useful to remove excess roots and soil when we lift the tree from its pot. As usual, wire is a handy material (copper or aluminum).
Pots are evidently another item to consider. Sometimes it will be possible to repot the bonsai tree in the same old pot. Other times, we may want to repot in an entirely new pot.
Finally, there are several types of soil which can be used when repotting bonsai trees. Compost soil, gravel, akadama and obviously water are a typical mix.
Steps to Repotting Bonsai Trees: In Detail
- If the bonsai tree is anchored, cut the wire from beneath the pot.
- Remove the bonsai tree from its pot.
- Evaluate the roots: Are the roots still inside the soil? Conversely, are they circling around the pot? Have they formed a solid block, without access to soil or even water?
- Use the chopstick to remove the old soil around the bonsai tree and beneath it. Be careful with the roots and don’t place any unnecessary stress on them. Note that this cleansing must be prudent; pines for one, require about half of the roots to be left untouched for proper health.
- Trim very long roots with the scissors. In general, however, be careful both with this trimming and with the aforementioned cleansing of roots. It is best not to prune more than 30% of roots at one time. Also, cleanse soil, but don’t worry about getting rid of it all. In any case, observation and further trimming may take place the following spring.
- Once pruning of roots has taken place, cover the drainage holes with mesh first, to avoid the new soil from seeping out.
- Set a wire to anchor the bonsai tree in the new pot.
- Now, re-soil the pot. This is best done in several stages:
- Set a thin layer of heavy soil, below, such as lava rock or akadama. This will be our main drainage layer.
- On top of this drainage soil, set another thin layer of Bonsai soil as such.
- Place the bonsai tree back in the pot. Use the wires set before to better support it in place.
- Add soil around the bonsai tree and use the chopstick to make sure this soil seeps in correctly.
- After re-soiling is over, water the bonsai tree and let it rest. Protect it from strong winds for a couple months.
- Monitor the bonsai tree in the weeks and months after repotting and notice its progress.
Bonsai Repotting Soil Mix
Soil mixture is a major consideration when repotting bonsai trees. There are two guidelines that we should understand.
First, the soil mix must be able to drain and prevent rotting of the roots. Second, this soil mixture needs to be able to absorb enough water to nourish the bonsai tree.
Varying the soil mix and its ratios of mixture also depends on the environment. Wet climates, for one, can make do with mixtures that are more draining (such as mixture with more lava rock).
A typical soil setup is something like this: akadama, gravel and compost in a 2:1:1 ratio. Note, however, that there are several types of mix that are possible. Pumice, akadama and lava rock are another fairly popular mix.
Do bonsai trees need special pots?
A bonsai tree’s aesthetics are directly related to its pot. There are some species which will look much better in specific pots. There are others species which grow fast and can profit from a large, more spacious pot; this is the case with junipers.
Some people use rectangular pots for conifers. Round pots, in turn, are often used for more delicate trees in general. Oval pots are often associated with delicate bonsais too.
Now then, does repotting bonsai trees always mean changing the pot? Not quite. Sometimes repotting may mean taking care of the soil, re-soiling, pruning roots and what not, but also re-using the same pot.
Are there tips on choosing the right pot?
As pointed out, we need not change the pot every single time. Having said that, however, here are some tips on choosing the right pot:
- Consider the depth and general dimensions of the bonsai that will be repotted.
- The idea is to have a pot that is large enough to let the bonsai tree stretch.
- Alternatively, there will be many times when we simply don’t have to change the pot. Increasing the size of the pot is not mandatory. It is entirely possible to retain the same size of the pot and tree. Note that a larger pot, however, carries certain benefits: Larger pots will be able to provide water to a bonsai tree for longer.
- Having said that, there is a simple rule of thumb to all this. The bonsai pot’s length will usually be around 2/3 of the height of the bonsai tree itself. Sometimes a bonsai tree will be shorter in height than it is wide. In such a situation make the pot length 2/3 of the width of the bonsai tree.
If you would like to learn more details, check our Bonsai Pot Guide
Can I repot a bonsai in summer?
Ideally, repotting bonsai trees should take place in early spring. There are several reasons for this. Early spring will create less stress on a bonsai tree and on its root system. This is because a tree is still in dormancy at this stage of the year. Any damage is lessened because the bonsai tree does not have to provide for full-grown foliage. Moreover, as soon as the tree starts growing, roots will be repaired.
Note that some bonsai experts recommend repotting during the winter. They point out that during winter, bonsai trees are dormant and thus the stress of repotting is diminished.
The main takeaway from all this is that repotting bonsai trees should definitely not be done in the summer. Heat is quite problematic when we are repotting bonsai trees. Exposing roots to sunlight or indeed to wind, will dry them up fast. This is to be avoided when repotting bonsai trees.