A bonsai needs repotting rather often, every 1 to 3 years, to ensure the tree gets the nutrients it needs – even if you use the same pot every time.
Indeed, when the the bonsai is mature and formed, its size won’t change anymore. Don’t repot to a larger pot since the aim is to maintain a harmonious balance between the size of the tree and that of its pot. However, one thing must absolutely be replaced: the substrate.
- See also: Video tutorial on repotting a bonsai
1- Why repot a bonsai?
The tree’s roots end up depleting all the nutrients and minerals from the substrate. Fertilizing regularly isn’t enough, because the substrate cakes up in time, becoming more compact. Roots can’t breathe anymore and start dying off. The tree would end up dying.
How often is this required?
- Trees that grow fast like willow and young bonsai that aren’t yet formed need repotting every year, as do fruit and flower trees (fruit trees, meadowsweet, Wisteria…)
- the other deciduous bonsai need repotting ever 2-3 years when still young, then only every 3-4 years,
- whereas conifers can wait 3-4 years while young, then longer when they age: 5-6 years, even up to every 8 years for the older ones.
A simple trick to know whether a bonsai needs repotting or not is to tease the clump out from the pot: if roots start circling around the pot and begin to tangle at the surface of the clump, time to repot!
When to repot?
The ideal season is when the tree starts budding, usually between March and May, depending on the species. In rare cases, October is better. The winter period isn’t a good choice, because the tree needs to be able to send new roots out to quickly recover. Summer also isn’t good because it gets too dry and hot.
- Early March for larch,
- Between March and May for other conifers,
- March-April for deciduous trees,
- right after the blooming for fruit trees that bloom on bare branches (Prunus, apple tree…),
- end of April for greenhouse bonsai (olive tree, pomegranate, citrus, Araucaria, bamboo, palms…)
- May or after the blooming (whichever is later) for kanuma bonsai (Azalea japonica and other heath soil bonsai)
- May to June for tropical species like Serissa and Ficus.
- October for early spring bloomers like maule’s quince.
2- Substrate and soil mix
There are special substrates for bonsai derived from actual Japanese soil: akadama, kanuma (for acidic-soil-loving plants) and kiriu (for conifers). You can use these pure, or mix them together. These dehydrated soil types have the advantage of being very stable and ensure great drainage. However, they don’t contain any nutrients. Adding slow-release fertilizer pellets is recommended. You can replace all of these specialty items with more conventional products, too. Getting the perfect substrate for each bonsai would require pages and pages of information! At least, with these hints, you can get started.
Just remember this:
- a conifer requires a very well-draining mix to avoid root rot, and it should be a bit acidic. For instance: 60% river sand, 20% akadama and 20% composted bark shavings; or, equally good, 75% garden soil and 25% non-chalky sand.
- At the opposite end of the range, a deciduous tree will appreciate humus-rich, cool soil with lots of decaying leaf matter. For instance: 50% garden soil, 25% non-chalky sand (fine-grained) and 25% vegetable growing soil mix; or, for a more vigorous grower: 50% garden dirt and 50% growing soil mix.
- If your bonsai is an azalea, acidic and well-draining soil with 80% kanuma and 20% akadama is a nice balance.
- An indoor bonsai loves 25% garden soil and 75% nutritious soil mix.
- Young bonsai still being shaped require moist substrate with large-sized grains. For instance, for a deciduous tree, 60% pine bark, 20% river sand and 20% akadama.
3- Steps for repotting
- Step 1:
Stop watering for a few days before repotting. This makes removing the root ball much easier: all you must to is tap the bottom of the pot a bit. Select a pot for which the depth doesn’t exceed the width of the trunk. Make an exception for young, still-growing trees: these need more soil.
- Step 2:
Soak the new pot in water to let it absorb moisture. Use chopsticks and soft brushes to untangle the tree’s roots and remove the soil from within the root ball.
- Step 3:
With sturdy scissors or pruning shears, remove at least ¼th of the root length on the sides and bottom.
Also cut out malformed roots and thin some out if they’re too abundant.
However, don’t cut any of the largest roots off. At most, shorten them to fit the pot.
- Step 4:
If needed, wind a galvanized copper wire through the drainage holes to tie the tree down in its new pot.
Pour a layer of drainage along the bottom of the pot, and rest the bonsai atop it. Stuff the new soil through the roots with the brush or chopstick.
Tie the copper wire around the larger roots.
Cover with sieved, fine soil and press it down with your hand or a wood block.
- Step 5:
Soak the entire pot in a basin with water for a few minutes.
Optional step: Add a fine layer of mulching pebbles or moss to finish the job!
- See also: Video tutorial on repotting a bonsai