Understanding How Trees Grow Through The Seasons – A Simple Explanation To Help With Bonsai.

To grow a bonsai tree you need to understand how trees grow in nature.

I’m not talking about the exact scientific reasons and explanations. A deep knowledge of Biology has its place, but here I’m talking more about simply know the life cycle of a tree and how it changes through the four seasons of the year.

If you have a basic grasp of this you can understand what the tree is doing, which means you can understand what the tree is “telling you”. This will allow you to make informed decisions about what techniques you can perform on a tree at that time.

The following information is mainly based around deciduous trees, but aspects of it can still apply to other types of trees.

Trees go through a year long growth cycle corresponding with the 4 seasons, so let’s start with the part of the cycle we all enjoy the most – Summer

Summer – Growth

Summer is the season of growth or your trees.

Trees use photosynthesis to grow and hopefully you can still remember this from biology class at school. But if not here is a quick basic recap.

The leaves take in sunlight and absorb carbon dioxide from the air while the roots suck up water. The tree then mixes these three and turns them into sugars which it can use to grow. These sugars can also be called Food or Energy.

This is the most basic explanation of photosynthesis and it really doesn’t need to get any more complicated than this. Really all we need to know is that the leaves and roots are important to produce energy, so the tree can grow. More importantly growth happens in a cycle.

The energy that is created in the leaves is sent down into the roots. This allows the roots grow bigger. The bigger roots can now take up more water and can now send more water back up into the leaves. The extra water means the tree can grow more leaves. More leaves can take in more light. So the plant can make even more food and this just keeps happening and your tree gets bigger and bigger.

That’s it. That’s the circle of life. That’s the growth cycle of trees and it can be a good time to work on your trees at this time.

Summer Pruning

If we interrupt this cycle and decide to prune the tree, it is going to cause the tree to respond in a way that favours us. The roots will be pushing water up the tree, but since we will have removed some leaves this growth loop is not going to be complete.

This causes the tree to force out new shoots, so that it has the leaves it needs to get the cycle back in harmony.

Usually this energy is not as strong the initial growth in spring, meaning the new leaves are smaller. You will also find that the tree sends out a number of shoots where you prune, not just one.

This now means you have more branches with smaller leaves, which is of course the difference between a bonsai and a full size tree. Doing this year after year will get you a small dense tree, rather than a full size one.

Hard Cuts

The start of summer can also be a good time to make hard cuts and remove large branches. Doing work like this can be stressful for the tree, but the fact that it is actively growing means it’s should recover quickly.  

Some trees may bleed out a lot of sap when you do this, so for these species it may be better to do this work when the tree is dormant in winter.

Autumn – Slowing Down and Preparing For Winter Dormancy

After summer comes Autumn (or Fall for my American friends) and change in sunlight levels causes the growth rate to slow down and the tree realises winter will be coming.

Many people start working on their trees in autumn, which I feel is unproductive so let’s look at what’s happening in the tree and then I’ll explain why I don’t like doing work on them at this time of year.

So as we all know in autumn the growth stops and the leaves change colour.

Again, I will keep it very simple. As the days get shorter, the tree gets less light. It knows from this change that winter is coming and starts to prepare itself for this.

The summer growth cycle stops and the tree stops making energy. It instead starts focusing on storing all its resources and going into hibernation.

Winter Preparation

All the energy in the tree is literally sucked back down into the tree roots during autumn.

The changing colour of the leaves is basically the tree sucking out all the useful components to reallocate the energy elsewhere. That’s why they go brown, dry out and drop off. The tree has sucked them dry and thrown them away. Obviously it’s a bit more scientific than that, but that is basically what is happening.

The relocation of the energy has two main effects on the tree:

Branch Thickening

Firstly, this relocation of energy really thickens the branches and trunk. A lot of the energy goes to roots to be stored. But there is still a lot kept in the branches and main trunk.

This is little but of information is very important to know as a bonsai grower. I continually see people growing out branches to thicken their trunks, but they then cut the branch off at the end of summer saying “this branch has done its job now, it can be removed”.

I’m sure the branch will have thickened over summer, but if only they waited a few more weeks, they would get some extra thickness. What are a few more weeks in the grand scale of bonsai? I really wish people would just wait a little bit longer before cutting their trees back.

I’ve found this time of year to be the most productive for trunk thickening. I’ve had wire on trees that has gone from loose to fully cutting in to the branch, in what seems like overnight.

My advice for anyone wanting to thicken a branch is to let your tree go fully dormant before cutting any sacrificial branches off. Once all the leaves have fallen off, your tree is now ready for winter and it has moved out of the autumn stage.

Hardening Off

The second thing that’s happens at this time of year is, well we’ve sort of already said. The energy goes back down into the roots.

You will often hear gardeners talk about the roots “hardening off” for winter.

Basically the tree pumps the roots full of energy so they can survive the winter. As they are underground they going to be better protected from the cold. Think of it like a bear preparing to hibernate in its den.

The tree knows winters can be harsh, it knows frost and ice can be especially dangerous to its health. So it hedges it bets and puts most of its effort into keeping the roots safe.

That’s why it looses it leaves and that’s why it can survive when the thin branches are lost to frost. Even if the main branches or even the trunk gets damaged it can still survive since most of its energy is safely stored in the roots.

Avoid Work At This Time

Getting back to why I think it’s bad to do work at this time.

You may see people repot their trees in autumn. The science behind this is the tree should be pushing a lot of energy down into the roots at this time. So those freshly cuts roots should heal up and re-establish fairly quickly.

Many people repot at this time of year and have success, but I would advise you to air on the side of caution. If you can wait until spring, wait.

Autumn can be an okay time to repot, but it’s not ideal. It’s like the second choice as such.

My main reason for being against repoting now it is the fact that you are cutting it fine. The time the tree has to recover is short before winter hits.

The tree may also recover, but it’s not going to grow after it does, as it’s going to hit dormancy. It’s basically going to heal itself and then have to sit through winter. If you are going to repot now you should really be putting your tree somewhere sheltered like a greenhouse. To leave them exposed to the elements may be a little too risky. One early frost could wipe the tree out if it hasn’t recovered fully.

I would always suggest to just leave your trees alone in Autumn. Just sit back and enjoy their colours. I would rather leave my trees to prepare themselves for winter and do no work on them.

A little bit of patient and waiting is what autumn is all about for me. We will get to the benefits of why you should wait until spring in a minute.

First we need to explain winter.

Winter – Dormancy

Once the tree has finished sucking all its energy down into the roots and storing it there, it shuts down.

This is dormancy. The tree is alive. But nothing happens. It’s a sleep. It’s hibernating as such.

At this time you should leave your trees alone. However, as the winter comes to an end this can be a good time to work out your trees.

(You can read more about  – How To  Overwinter Your Bonsai Trees)

Hard Cuts At The End Of Winter

Late winter can be a good time for hard cuts. Remember when I said that trees like to store all their energy in the roots, to keep it safe, well this now means that every other part of the tree is basically expendable and we can use this to our advantage.

We can make hard cut backs and the tree will not be bothered in the slightest. I can’t give you exact dates on when to do this, as everyone’s climate is slightly different. But about a month before the tree wakes up you can start to do work. For me this is about mid February. This is when I cut all my branches back and make hard cuts.

You’ll see people do this is autumn and the sensible ones do it in between the leaves dropping and dormancy. Professionals say it’s fine in autumn, so we have to trust them, but as I already mentioned, I’d rather wait until the end of winter, rather than the start.

For me it feels like there is less risk to wait until the end of winter. I make my cuts and in a few weeks the tree will start to grow and heal. The window of opportunity for infection to get into the cuts is a lot smaller.

I also find winter can be harsh. Branches get damaged by wind and frost. If I leave my branches long during winter, who cares if the tips get damaged, I will be cutting them back soon anyway.

If I make cuts at the start of winter, and those cut ends get damaged, that’s a problem. This is why some people do not prune in winter. The fresh cuts can dry out and make the wounds larger. As I make my cuts right at the end of winter I find this isn’t really a problem.

I guess it’s up to you to experiment or take the risk, but I just feel like it’s safer to do the work near the end of winter.

However, once your tree hardens off at the end of spring, you can again start to make hard cuts (As I discussed in the summer section). Many professionals prune at this time of year, but I prefer to work when the trees are free from leaves.

So hopefully a few weeks after you make any hard cuts the tree will wake up and you will start to notice some changes. “Spring is in the air”.

Spring – The Trees Wake Up

Spring is a great time of year as everything starts to wake up and come back to life.  

Buds Swell

The first thing to happen is the tree will start pushing the stored energy out of the roots up into the trunk and branches. You’ll notice this as the buds will start to swell.

This is another reason why winter cut backs are preferred. You don’t want your tree to start pumping energy into a branch, only for you to cut it off. That’s a waste of energy that could have been used to develop a different section of the tree. Let alone the further waste of energy from the bleeding that I mentioned.

So when people say you can “read” the tree or the “tree will tell you when it needs work”. They are probably talking about this time of year and repotting.


As a tree pushes its energy into the branches and new buds form, you will see them swell. This swelling is the tree telling you that it’s safe to repot.

Yes, you can do it before and yes you can do it after. But it’s not as productive and can set the tree back, sometimes even killing it.

Finding that sweet spot will make sure your tree doesn’t skip a beat when you repot it.

You just need to learn what the buds look like as they swell and repot just before they open. Once they open up and the leaves are open, it’s too late. You have to repot in this small window of opportunity for best results.

The best way to imagine this and work it out is it by picturing the tree in winter. Imagine the roots having an energy percentage of 100% and the upper tree as 0%. Then as spring comes along the tree starts to empty its roots and push some of the energy up into the tree.

Once the leaves have opened the roots have removed all its energy down to 0% and the upper tree has 100% of the energy. The Growth cycle (that I talked about in the summer section) will now start. Once it’s begun, you shouldn’t break the cycle (well you can prune the leaves, but not the roots).

So you pretty much need to repot when the roots have emptied their energy supplies and are down to the last 1% and the tree nearly full and is at 99%. Just before its starts opening it’s leaves and starts the growth cycle.

It’s a small window, but you will be able to get it right.

Don’t Repot Too Early  

So repotting too late is a dangerous problem, but it’s usually doing it a bit too early that’s most people’s crime.

If we imagine those roots again, sitting at 100% energy in winter .Now imagine I come along and rake all the roots out and cut them back hard (If you’ve seen any of Youtube my videos you will know I love doing that)

Say you cut 80% of the roots off. Well you’ve just cut 80% of the trees energy off!! So now in spring, the tree can only push up the remaining 20%. And guess what. That’s not enough to go around the whole tree. You might only get 1 or 2 buds and even they might not be very strong.

You’ve also cut the roots and it might be a good few weeks before spring and before the growth cycle begins. The roots being sat, not getting any energy sent down means they will not heal.

Then when you add on the fact that you’ve only got 2 buds, it’s not going to be a very strong growth cycle at first, so when it does send energy down to heal the roots, it won’t be much. So, it’s going to take a long time to recover get back to where it was before.

So basically don’t repot too early. It doesn’t end well.

What Happens If You Repot At The Right Time?

If you manage to repot at the right time then you will find roots are petty much empty of energy. This is perfect as you can cut away at them like crazy and no energy will be lost, since its now all up in the tree.

A few days later the leaves will open and the growth cycle will begin. Energy is going to start heading back down to your freshly cut roots pretty quickly and they are going to start growing and healing straight away.

Also as all the energy is up in the tree, there should be lots of new leaves, meaning lots of new energy is being made and pushed back down into the roots.

Timing this right means that no energy is lost and it will be like nothing has ever happened to the tree. After this the tree enters the summer growth cycle and the full year long cycle is complete.


No matter when you read this, you can work out what part of the cycle your tree is in and plan for the next step in the cycle. After a few years of owning a tree you will get to know its rhythm as it moves through the seasons.

So hopefully this gives you an overall picture of how trees grow and can help you understand what is going on so you can make the best decisions for your tree.

 Just remember it takes time and tree growth can be slow. There is no need to rush.