Repotting Bonsai Trees
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How to repot your Bonsai
Not sure if your bonsai tree needs to be repotted?
It’s pot bound Many roots have started to grow out of the drainage holes, and when lifting the tree from the pot the roots have formed a dense mat along the pot edges
Yearly repotting for younger trees If the tree is still young and hasn’t yet reached a “finished” and adult stage, repotting is often recommended once a year in the beginning of the growing season. Old and mature bonsai trees can be repotted every second, third or up to fifth year depending on species. Deciduous species are often repotted more frequently than conifers.
The soil is dense It’s common for some substrates to degrade and become more dense which in turnm makes the water drain slower from the pot as it clogs the drainage. This will affect the roots and the tree’s health overall.
It’s planted in the wrong soil type If the bonsai comes planted in either peat based soil, or a clay rich, dense soil it can easily lead to issues like overwatering. This is common when buying a tree in a store. Repotting a tree because it's in the wrong soil type can be done most parts of the year as long as it has good growing conditions.
Repotting a bonsai tree might be needed regardless of the health of the tree or how well it grows. When you repot due to an improper soil type, your bonsai may also require a root trim. Pruning roots are mainly meant for fully healthy trees with a lot of vigorous growth. We’ll explain it further below in this article.
What pot should I repot it into?
In growing a bonsai you'll use a combination of techniques to keep it at a smaller size. The idea is that both the tree and the pot should stay small regardless of the plant’s age. This means that you need to prune the twigs of the tree, but also regularly prune the roots in order to open up space in the pot for new roots to grow.
Since growing bonsai is equal parts an expression of art as it’s horticulture, you might want to change the pot to better highlight and represent the tree. If you think the current pot is nice and suits the tree, using the same pot as before is okay and is the most common approach.
Traditionally there are a number of different rules and guidelines when it comes to choosing the right pot for your bonsai tree. The width of the trunk correlates to what height of pot you should choose, while the type of tree (deciduous, evergreen or conifer) determines if a glazed or unglazed pot is better suited. These guidelines can be interesting to know about, but they might not be necessary to follow blindly and you should choose what you think looks nice. It’s your tree after all.
There’s also a difference between a “finished bonsai” and a bonsai that is still early in training. You can see this as two different stages. With trees in early training you want them to grow and thicken the trunk and branches rather fast, so you have a base to work with. These plants are better grown in bigger training pots where they can get a bigger root system that enables them to absorb more nutrients and water. It’s only once the basic shape of the tree is set as it’s ready to be planted in a smaller true bonsai pot.
Recommended soil
There are a number of different soil types that can be used when growing bonsai. Traditionally in Japan there are two main types of soil used. Akadama and Kiryu.
Akadama is a granular clay soil that is good at absorbing water, nutrients and is rather porous. It will however degrade with age and the porosity will slowly be reduced in the pot the longer the tree grows in the substrate. It’s therefore recommended to repot trees planted in akadama yearly or every other year for a healthy root system. When planting in akadama, it’s often used on its own, but depending on the plant you might want to mix in a small portion of peat based soil too (for species that require a very even moisture level).
Kiryu is a similar substrate to akadama, but it’s usually used for pines, junipers and other trees that needs to be repotted less frequently. Kiryu decays more slowly and doesn’t clog the pot as fast.
Apart from akadama, there are a number of similar substrates like burnt clay granules, pumice gravel, volcanic rock etc. These won’t wither as much as akadama and can work well as a substitute if it’s easier for you to get hold of. Just make sure that it doesn’t have a pH-level or high mineral content your tree won’t tolerate.
How to repot your Bonsai
Water the plant Water your plant to loosen up the soil from the pot.
Remove the pot Gently take hold of the plant and start to loosen the pot. Avoid pulling on the trunk or branches too much as it may harm them or the roots. If it’s hard to get the pot to come off, try to cut the soil loose from the edges of the pot with a small knife. You can also gently tap on the outside of the pot with a tool or on the bench. Note that bonsai trees often are anchored to the pot with a wire. Make sure that you first release the anchoring before trying to pull the tree from the pot. This anchoring wire is often visible in the bottom of the pot, by the drainage holes.
Comb out the roots Use a small rake or a fork to gently comb out the roots and remove most of the old soil. Try to be gentle with the finer roots. Rinsing them in water can also be a gentle alternative. Another tip is to have a spray bottle ready to mist the roots regularly as long as they’re exposed to the air. You don’t want the finer root threads to dry and get damaged in the process.
Prune the roots A healthy bonsai can get about 20 - 30% of the roots pruned away in order to leave space in the pot for new roots to form. This is also a great time to prune away any bigger tap roots searching its way down the pot as they won’t be needed. Note that pruning the roots can be skipped if the tree isn’t in best health or if the reason for repotting it is only to change the soil type.
Add pieces of mesh in the bottom to cover the drainage holes. This will keep the soil from falling through. Also add a piece of aluminum bonsai wire you will use to anchor the tree to the pot later on.
Pour soil in the bottom of the pot - Add soil to the bottom of the pot so the rootball is not sitting directly on the pot surface. The bottom level of soil should be about the same as before. If the tree was previously planted rather deeply, you might want to plant it higher and slightly expose some of the bigger roots on the surface of the pot. These shallow visible roots are called nebari and exposing them slightly helps the tree look more mature.
Put down the plant Place the plant on the soil and check that it’s placed as you want it to before you start adding more soil. When you feel that you’ve placed it as you want, anchor it to the pot with the wire. This will keep it from falling over later on once it's planted. Traditionally it’s often recommended to place it so that the trunk is placed a little off center in the pot.
Add soil Add soil around the plant: tap the pot and use a chopstick or a skewer to really work the soil into every crevice in between the roots.
Water Water the plant until water starts to exit through the drainage holes. When planting in akadama and kiryu the excess water will often be very dirty the first couple of waterings. This is normal.
After repotting
Repotting is stressful for any plant and especially a bonsai tree you’ve pruned the roots. It may take a little while for the tree to start growing fully again. This is normal.
During this period make sure to
Place the plant in a bright, but sheltered spot.
Water it regularly and make sure it doesn’t dry too much in between. The schedule set by Planta will help you keep the watering regular.