When Do You Need To Repot A Bonsai?

Knowing when you need to repot a bonsai is extremely important, as getting it wrong could seriously affect the health of the tree.

Repotting is an important process in the trees overall development as it also allows us to keep the tree in a small pot while still keeping it very healthy. It also allows us to work on the roots aesthetically, so that over time it develops the roots to look like a powerful full size tree that is anchored into the ground, rather than just a stick poking out of the soil.

This can be a drastic and stressful operation for the tree so it needs to be done correctly and at the right time. It is often done wrong, and this is probably one of the major causes of trees being killed.

However, if you follow some simple rules, it is relatively safe and its easy to work out when you need to repot your bonsai.

Reasons To Repot A Bonsai

Repotting a bonsai can be done for two main reasons. Either the tree needs it for horticultural reasons, or you want to work on the roots to make them aesthetically better looking.

In both cases, the health of the bonsai is the most important factor. That is non-negotiable. If you start messing around with trees roots when it is not in full health, it will die.

Before we get started I want to point out that this just a very general overview on when you should be repotting a bonsai.

You will need to do research on the individual species you own to make sure you are repotting at the correct point. Don’t use this guide to repot every tree you own without doing further research on your exact species.  

How To Know If A Bonsai Tree Needs Repotted?

Knowing when a Bonsai needs to repotted is something you need to learn. You need to look at the tree and see what it is telling you. The tree will give you signs that it is in need of a repot, which I will discuss shortly.

The Time Based Approach Is Wrong

Some people seem to repot on a sort of calendar time based system. They say something like “This tree was repotted 12 months ago, so it’s time to do it again”.

This is not the right approach. You need to evaluate the tree and assess its individual needs and its requirements for development.

When you have a tree that is young, or early on it its development it will probably have terrible roots. This is very true for seedlings and nursery stock. Trees grown in this way will have wild, long, thick roots. For the first couple of years of their development it is good to focus on the roots and start to tame them away from the wild mess you have started with, so you will find a yearly repot is needed.

Each year you slowly correct all the errors and start growing something more appropriate. This is probably where the “yearly repot” idea comes from.

This is fine, if the tree is healthy and strong, which young trees usually are, then this is perfectly acceptable to do and a young tree should be able to handle this frequency of work, but it is still not the most ideal way.

Older Bonsai Do Not Work On A Schedule

Once the bonsai tree starts to develop and age, you will find that the aesthetic side of the tree needs less work done. The roots will look great and are no longer out of control, so you can repot less often for this reason and you will only need to repot it for health reasons.

When this happens you will find that the tree tells you when it needs to be repotted.   

Yamadori Need Even More Care

Yamadori, or collected trees need more care when being repotted. They are already old and delicate trees, trying to repot them on a yearly approach would not be ideal for them. Any aesthetic work would need to be done with more caution and they would need much more time to recover.

You really need to work with these trees and go at their speed, not your calendar.

Repotting To Change A Pot

You might also be repotting a tree to just change its pot, which would just be for aesthetics and does not really fit in with a time schedule.

Changing a pot can play an important role in the overall image of the tree. The size, shape, colour and even the texture of the pot can all play a part in the overall design of the tree and there are no real rules about when you should, or can change this.

The tree may also be repotted to change the angle that the tree is planted in, or rotated to change the orientation. This is again something that is not going to fit in with a schedule and should just be done when the tree is healthy enough to handle such an operation.

How A Bonsai Tree Will Tell You That It Needs Repotted

Repotting should be done to keep the bonsai tree healthy and the tree will start to tell you when it is time to be repotted.

Usually a tree will start to display a number of the following indicators when it becomes root bound and needs to be repotted, so identifying when a tree is ready for repotting is fairly easy to see.    

Root Bound/ Pot Bound

Bonsai pots are small, so obviously there isn’t much space in them. A healthy growing bonsai will fill a pot with roots fairly quickly. Which is great, but eventually it will run out of room.

The tree doesn’t understand this and just keeps growing, so the pot just keeps getting fuller and fuller. This causes a problem as there is now less space for air and water to be in the pot.

If left untouched the tree will basically kill itself by suffocating its self with its own roots.  You need to repot a root bound tree the following spring in order to keep it healthy.

Signs ThatYyour Bonsai Is Root Bound

A sign that your tree is root bound is that you might see roots growing out of the bottom of the pot through the drainage holes.

You will also see that the tree starts to lift up in the pot. You will see the soil line rise above the lip of the pot. There are so many roots in the pot that they start pushing the tree upwards as they have reached the bottom and cannot grow any further.

Poor Soil

You may have poor soil as the components you are using have started to break down over time, or you could have just bought some nursery material or a tree that is just using bad soil.

Bonsai soil needs to loose and full of air gaps to keep the tree healthy. As the soil breaks down, it clogs these air gaps causing the soil to become less effective. Water and air are now no longer to fill these gaps and if left for too long will start affecting the health of the tree.

It can be hard to tell if you have poor soil, but basically if it doesn’t look loose and airy, then its probably not great.

Poor Drainage

Poor drainage basically means that the water is not soaking into the soil as easily or quickly as it should. This is usually caused by either poor soil or the tree being pot bound, its normally a combination of both.

If you are noticing any changes in watering, or just that the water doesn’t seem to be absorbing as well, its probably a clear sign that you will need to repot soon.

Growth Slowing Down

A tress growth may slow down for many reasons, including the fact that this is totally normal at times. However, if you think it should be activity growing and it seems to be slow, it could be potentially in need of a repot.

Basically the above factors I have mentioned all effect the trees ability to remain healthy and this will start to manifest itself in poor growth on the top of tree.

When Should You Repot Your Bonsai?

All repotting should take in place in spring, some tropical’s and other trees can be repotted in summer, but for the majority the spring is the best time.

The exact time is dependent on the individual tree. If you study your tree and watch them grow, you will know when it is time to repot.

For deciduous trees the best time to repot is just as the new leaf buds are swelling in spring and are about to open up.    

Why Do We Wait For The Buds To Swell To Repot A Bonsai?

When the new buds on a bonsai are swelling in spring it means the energy has moved up from the roots and is now in the tree. You can safely remove roots from your bonsai at this point. If you remove roots while the tree is still dormant, you are basically removing its energy supply. 

So guess what happens in spring? …It has no energy and then struggles to grow.

Makes sense, right? No energy, means no growth. This is the main killer of trees. Some tough trees will scrape through, but they will still struggle and will take a while to recover.  A lot of trees will just lack the energy to get going again and will die.

This is why you need to wait until those buds are about to burst. I can’t stress that enough.

As soon as the leaves open, it will start sending energy back down into the tree, which will heal any cuts and start to grow new roots. So the closer you can get to the leaves opening the better. It reduces the amount of time that the roots are just sat doing nothing after being cut.

What Happens If You Repot A Bonsai To Late?

So what happens if you are too slow and the buds have opened? Repotting when the leaves have already opened isn’t ideal either. I’ve personally done it a few days after they have opened and it’s been fine, but I wouldn’t recommend it. 

The leaves will want water, and the roots provide this. If you cut the roots off, the leaves will not get the correct amount water that it needs and they may wilt and die.

Basically once the leaves have opened the tree is in the growth cycle. By removing the roots at this point you unbalance this. Any sort of imbalance is going to stress the tree and can cause it to die.

If you have a new tree, you might not be aware of how the buds grow and you might not know at what point they are about to open. You really need to study your trees at this time of year.

Taking pictures of the buds every few days will really help you learn and understand how to read your tree and know when the buds are about to open and it is ready for a repot.  

(You can read more about – What Are Some Common Mistakes To Avoid When Repotting A Bonsai?)

Considering Frost When Repotting A Bonsai

If you get the bud timing slightly off, you will probably be okay if it’s only a few days either side, however, one huge factor to consider is frost. If you cut the roots and then they get hit by frost. Your tree is dead.

I’ve had trees with buds at the perfect point for a repot, but this has been in the middle of February. Looking at the forecast there was snow and frost due in about a weeks’ time. So, I waited.

It was the right decision, about 5 days later, it was snowy and frosty. If I had repotted when the buds where ready, my tree would have died.

 I had to wait about 2 weeks to know I was in the clear and there would be no more frost.

The leaves had opened by this point, so not ideal. It defiantly stressed the tree when I repotted it and it took some time to recover. A stressed tree is not good, but i’d rather risk having a stressed tree than a guarantee of having a dead one. 

So that repot was a bit risky, but I knew I could get away with it. I knew this indivudal tree would be strong and healthy enough to take the stress of being repotted like this. It’s very important to think about this and factor it into your decision. If it was another tree, I might have just had to skip repotting and wait a full year.

I also could have repotted when the buds where about to open. Then kept the tree somewhere sheltered for a few weeks, but I decided to wait until the frost had passed.

A lot of trees will get too excited for spring and bud a little early. This is fairly normal, at worst the buds will die and when spring actually comes, the buds regrow. The tree will have loads of energy in its roots, so don’t worry about this. Its better this happens and you wait. Rather than you repotting it too early and its freezes and dies.

Repotting A Bonsai Out Of Season/ Emergency Repots

There are very few exceptions for repotting a bonsai tree outside of spring.

The main one would be when you collect a tree from nature. Although, I would argue that this is a whole different thing. You are not really repotting. You are digging a tree up from the ground and putting it in a pot (well usually a large crate) so I don’t think this counts as a repot. The skill required and timing are a whole different thing from repotting a tree.

Another popular reason is the “Emergency Repot”. Even these are not really a thing. I can’t quite think of an appropriate situation that you would be classed as an “emergency” and you’d need to repot.

If a tree is a state where it needs repotted, you can manage the tree until spring and repot then. Maybe the soil is awful, its staying to dry, or too wet. Then you need to water more, or less until spring. Either way, I feel this is not an “emergency” and you should wait until spring.

The final reason I see people “emergency repotting” is when the pot breaks. For example it gets blown over in a storm and smashes.

I don’t think you need to repot in this instant. You can just slip pot the tree into a different pot. You shouldn’t need to disturb the roots. I would not class this as a repot. it’s simply managing a situation. In spring you get a replacement pot and repot it properly.

Slip Potting

I should mention slip potting here. You can pretty much do this at any point of the year. All you are doing is taking the root ball and putting it in a larger pot and back filling the gaps. You should not be disturbing the roots during this, so it is pretty safe to do at any point.

I would just ask why you actually need to slip pot a tree? Unless its pot is smashed into a 1000 pieces, I can’t see the need and would just wait until spring to repot as normal.   

You will see Japanese masters slip pot trees for competitions. They lift the pot out of an okay pot and slip it into a very old and beautiful pot for the exhibition.

The two pots are usually around the same size, so they don’t need to back fill with much soil and of course the root ball is not disturbed. Once the show is over they slip it back into the original pot and they put their fancy pot away again for safe keeping. I would imagine not many of us are displaying or trees like this, or own very old and rare pots that we need to keep safe, so this isn’t something we need to worry about.

(You can read more about – Frequently Asked Questions About – Repotting Bonsai)

Conclusion

There is quite a lot of detail here, but hopefully now you will be able to tell when you tree needs to be repotted and will stop trying to do it when you feel is the right time.

Getting the timing right and knowing when a tree needs repotted is really is half of the battle

…Now you need to make sure you are doing the right techniques, which you can read how to do in my guide on – How To Repot A Bonsai Tree.