Repotting is one of the fundamental skills of bonsai and an essential part of keeping your tree healthy over many years. However, it can also be one of the most dangerous operations you can perform on your bonsai. Late winter through to mid-spring is the safe repotting window, but sometimes situations arise which make people consider repotting bonsai in summer.
In this blog, we'll tell you why that might not be a good idea and show you some tricks you can use to avoid repotting your bonsai in the hottest part of the year.
Reasons to repot your bonsai
Bonsai trees are grown in small containers with the intention of restricting large root growth. This helps to reduce internode length and leaf size and generally facilitates the refinement of a bonsai tree.
To make best use of the bonsai container, you need to repot your bonsai as infrequently as you can. Each time you repot you create space for roots to grow, which will impact the quality of the top growth you see for the next couple of years.
Most bonsai hobbyists repot too frequently and on a schedule, but there are clear signs you can look for that will tell you if your tree needs repotted.
The first is decomposition of the soil, which leaves your tree growing in a wet, muddy mix that can increase the risk of pathogens such as root rot in your bonsai.
The other is loss of percolation, where the root ball has become so compacted that water can no longer penetrate to the center of the pot. This can make watering extremely difficult.
Loss of percolation is particularly relevant for us discussing summer repotting because bonsai forums are frequently filled with questions about repotting pot-bound trees out of season.
Even though watering can be harder for a tree with compacted soil, I would still advise against a summer repot. There are other things you can do to help your tree along until spring, and your tree will do better in the long term if you wait to repot at a safer time.
Why we repot bonsai in the spring
Throughout the growing season, your bonsai is creating and storing energy via photosynthesis. This energy is stored as sugars, which reduce the freezing point of the water in the plant and help it to survive the winter.
When spring comes around, the tree has big stores of energy ready to invest in the coming year's growth. The timing of root reduction in relation to foliage growth is crucial for your bonsai.
The hormonal exchange between foliage tips and the roots of your bonsai results in it growing the correct amount of foliage that can be supported by its root mass. The foliage that your bonsai grows is an investment of its stored energy, and it won't see a return on that investment until the foliage hardens off and starts photosynthesizing at full capacity.
The reason it is so important to repot before your tree starts growing is it allows the tree to invest its energy in growth that it can support. If we let it grow first, then reduce the root mass by 50%, there is a high chance that the tree may not be able to fully hydrate its foliage anymore.
Can you repot bonsai in summer?
It is generally not a good idea to repot a bonsai tree in summer. Repotting in summer greatly increases the risk of the tree dying, and there are other methods you can use to help a tree without repotting it in the heat of summer.
One of the main functions of water for a tree is to cool it. As water is taken up by roots, it passes through the tree taking up heat before evaporating at the leaf stomata, releasing heat from the tree.
If you repot your tree and reduce roots in the middle of summer, your bonsai may not be able to take up enough water to cool itself properly. This can result in foliage dying off, branches dying back or in the worst case the entire tree dying.
But while it is a bad idea to repot in summer for most bonsai, some trees can safely be repotted in summer.
Tropical trees (such as Jade) require warmer temperatures when repotting, so feel free to repot these in summer.
Alternatives to repotting in summer
There are some horticultural techniques you can use to help maintain your bonsai's water balance over the summer and autumn, helping you to wait until next spring to perform your repot. This applies to bonsai that either have loss of percolation problems or decomposition problems.
Bonsai topsoil replacement
Often when a bonsai tree is suffering from a loss of water percolation, the problem lies at the very top of the root ball. Rain and watering can break down the surface layer of soil, while fertilization can add a compacted layer of organic matter.
In many cases, the soil underneath all this is still intact and will drain well. To sort this problem without repotting your tree, you can remove just the surface layer of soil and replace it with new, free-draining soil.
This technique usually involves very little root reduction or sometimes almost none at all, so it's a great option if you need to tackle a tree during summer.
Here's how you do it:
- First, remove any moss growing on your bonsai.
- Take a chopstick and scrape away the compacted surface layer of soil.
- Use a vacuum cleaner to remove soil from the pot as you loosen it.
- Continue working down through the root mass until you find soil particles that haven't broken down and still look intact.
- Fill the pot with new soil.
- Apply a moss topdressing to protect the soil from breaking down again.
This technique can add 2-3 years to your tree's time in a pot, which can massively help with refinement. If you're struggling with a tree that you can't water properly, I'd strongly recommend this as your first approach.
Duct tape your bonsai pot to help with watering
If you have a bonsai with a loss of percolation that you're struggling to water, you might want to try this trick. It might not look pretty, but it can ensure your bonsai remains well hydrated and avoids a dreaded summer repot.
Simply add a wall of duct tape an inch high around the bonsai pot. When watering, you can fill the pot with water and allow it time to drain through the root mass without running off the sides.
This doesn't look good, but it can make a big difference in an emergency to help water your trees.
Tipping your pot on an angle
In the case of a tree with decomposed soil mass, overwatering your bonsai is usually the main concern.
To help avoid this, you want to drain your bonsai pot of as much excess water as you can when you are watering. There is a really easy trick you can use to help you do this.
After you've watered your bonsai, place a wooden block under one side of the pot to tip it at an angle. This increases the gravity column working on the pot, helping water to drain more quickly.
Leave your tree for 5 minutes, then swap the block to the other side. If you experience a lot of rain you may just want to leave the block in place, swapping sides once a day to stop water pooling in the corners of the pot.
This simple solution can work wonders on a soggy rootball and help your tree to build up strength before you repot it in the spring.