Larch is one of my favourite bonsai species and are one of the most common species of tree found in my garden. They are fantastic for bonsai and when you repot larch bonsai at the correct time they are extremely tolerant of a lot of work, so in this blog we'll take a look at when to repot larch bonsai.
This blog is going to cover the question of larch bonsai repot timing in two ways - what time of year should you repot a larch, and what are the indicators that your tree needs a repot.
Here's what we'll cover in this blog:
- Larch fundamentals
- How do I know if my larch should be repotted
- What time of year should I repot my larch
- Larch repotting best practices
- Best soil for larch bonsai
Larch bonsai fundamentals
Larch is a deciduous conifer, meaning that it is a needle based tree (doesn't have broad leaves) and it sheds its foliage before winter each year. The deciduous aspect of larch make them fast growing and a bit more thirsty than other conifers. They have softer, more fleshy needles that loose more water and as a result move more resources through the tree.
The deciduous nature of larch also lends the species to a very varied aesthetic through the year. Most people agree they look their best in spring, when the green of early foliage growth is just starting to peep through. They then become quite dense and heavy through the summer as they accumulate foliage. Finally, after they have lost their needles you can achieve a really stunning winter silhouette on a well ramified tree.
Given the aesthetic value of larch, as well as their hardy nature, they are one of the best bonsai tree types for beginners.
Larch produce two types of foliage - short shoots (know as 'spurs') and long shoots.
The spurs will act as energy producing solar panels, continually fuelling the tree, while the long shoots grow out into new branches with fresh buds along their length.
Understanding these two growth types is vital to pruning your larch properly and maximising its ramification, but that's a whole other topic for another time.
As with other deciduous trees, larch build up an energy bank through the year, particularly over the autumn season. This is stored as sugars in the vascular tissue of the tree, which makes them frost tolerant over the winter.
Once winter has passed, the tree will then invest its stores of energy on new foliage to start the cycle again.
Understanding this is really important. In spring, just as trees are starting to wake up, your larch will be bursting with energy. This means it has the best possible chance of repairing any damage caused to roots or branches from repotting or styling.
How do I know if my larch bonsai needs repotted?
First off, let me say that you should not be repotting your trees on a schedule. It is true that many trees will need a repot every 'X' number of years due to their growth habits, soil substrate, environment etc, but the decision to repot should be made on what is happening with that individual tree, not the number of years since it was last done.
There are three basic reasons that you should be repotting a bonsai tree.
Loss of percolation
Bonsai containers are an intentionally small space, in order to constrict root growth and develop a well branched root structure.
For that reason, a tree will eventually fill the pot entirely with roots. This can stop the flow of water through the root ball, leaving dry spots in the soil mass (particularly in the core of the tree, right under the trunk).
If you are watering your tree and noticing that water is mostly running off the sides rather than soaking through the rootball, then you likely have a percolation issue and need to consider a repot next spring.
Soil in your bonsai container will break down over time as a result of root growth and microbial activity. As soil breaks down, it starts to hold more water and have less space for oxygen circulation.
Oxygen in the root system is vital for your bonsai's health as it stimulates the production of new roots. In soil that is overly decomposed you will notice the health of the tree decline as less roots are produced. In the worst case your tree will develop root rot as a result of the festering water in the container.
The larch forest pictured above is a good example of a tree suffering from excessive soil decomposition. The soil is muddy and seldom dries out. You may also notice a general loss of health and vigour in your tree, as was the case with this forest that had lots of branch die back over the winter. It's important to repot your bonsai when you notice and decomposition or loss of percolation, because these can imapct your tree's health and potentially limit how long your bonsai lives.
Change of style
Bonsai is an artistic endeavour, so we have to make sure we are making decisions based on both horticulture and stylistic reasons.
If you want to alter the planting angle of the tree, select a new front or change the pot entirely, then you should repot your tree to make sure it is looking its best and you are getting maximum enjoyment from it.
If you repotted your bonsai the previous year, or if it is experiencing some health issues, then I would recommend you wait a year to re-establish some roots before repotting for aesthetic reasons.
What time of year should I repot my larch bonsai?
Like with most other non-tropical bonsai trees, the best time of year to repot your larch will be late winter or early spring.
As mentioned above, your bonsai will enter spring bursting with stored energy and ready to grow. This means that the tree can easily heal and repair any cuts we make to the roots.
Winter and spring can vary a lot in temperature, day light levels and rainfall based on where you are in the world, so to make sure you are repotting your larch at the best time you should let your tree tell you.
As your bonsai starts to wake up from its winter dormancy, its buds will start to swell. They will initially become a golden colour, moving onto green just prior to foliage emerging.
The best time to repot your larch bonsai is when the buds have swollen and are showing green.
After growth starts to emerge, the general consensus is you have around 7 days to repot your tree before you have missed your chance and will need to wait until the following year.
It's important to state that you need to repot your tree before it grows any foliage. There are a few reasons for this.
Firstly, the foliage that a tree produces is based on the roots that it has. More roots means more foliage growth. If you let your bonsai start growing, then the tree has committed itself to supporting that foliage growth with the roots it currently has. If you repot and remove half the roots, the tree is going to struggle to keep the foliage mass properly supplied with water, which may result in the loss of some branches or even the death of the tree.
Secondly, repotting a tree generally involves a fair amount of trauma. Like having surgery, it will improve the health of the tree in the long run, but it needs its stored energy supply to heal any wounds and to get some new roots growing. If we let the tree start growing foliage before the repot, it will have already spent a good portion of its stored energy, which means it will be weaker when we repot.
Larch bonsai repotting best practices
So you've decided your bonsai needs a repot based on one or more of the three reasons above, and the buds are swelling which tells you it is time to do the repot. Here are a few best practices to follow to make sure you give your tree the best chance of success.
Always leave a portion of the roots untouched
If you are removing nasty soil, or trying to get a tree from a nursery container into a small bonsai pot, there is always the temptation to wash the soil off the rootball with a hose. This can be an ok approach for broadleaf deciduous trees, but since larch is a conifer hosing the rootball is a lot more risky. Conifers rely on a relationship between roots, microscopic organisms and the soil. If you wash the soil and microbial organisms away, it will take years for the tree to build up that system again.
Make sure the tree is secure in the pot
For roots to grow, your bonsai needs to be immobile. If the tree is wobbling around in the container it prevents the growth of new fine roots, which could mean the end of your tree.
Make sure you tie your bonsai into the pot securely, ideally in 2-3 places depending on the size of the tree.
Water thoroughly after repotting, then be patient
After you have repotted your bonsai, it's really important to give it a very thorough watering. This will help rehydrate the roots, as well as flush out any small soil particles or debris in the pot following the repotting.
There is always a temptation to water our trees more than we should, and in doing so we can sometimes love our trees to death. This is particularly important to consider in a newly repotted tree, which has open cuts in its roots. In moist but aerated conditions these cuts will callus and heal. In wet and airless conditions, these cuts can introduce root rot.
Roots grow best in well aerated soil, so to encourage new root growth you need to allow the tree to dry out slightly after its first watering. I don't mean let it get completely bone dry, but you must wait until the top dressing is dry and you start to see the soil particles start to change colour. Let the tree get slightly drier than you normally would, and the new root growth following the repot will be much better.
After that you should resume your normal watering practices. I've created a Bonsai Watering Checklist to help with this, which you can find here: https://www.bonsaiable.com/blog/six-step-checklist-to-improve-your-bonsai-watering
Best soil for larch bonsai
As mentioned above, the best soil for larch bonsai is a well-draining mix that allows good aeration of the root system.
I use 100% akadama for my larch to help create a refined and well-scaled root system. Others prefer to include differing proportions of pumice and lava to reduce soil breakdown and maintain percolation through the root ball over a longer period of time. Akadama can also be substituted for another organic component such as pine bark if it is too expensive for you or difficult to get hold of in your area.
Time to get repotting!
I hope this blog has helped, if you're interested in this species you should check out our full larch bonsai tree care guide. I'd love to hear from you so drop me a tweet with a picture of your larch or any questions and I'll get back to you!