One of the ways that bonsai trees are kept small is by growing them in confined pots, where space for root growth is intentionally restricted. Over time, your tree’s soil will degrade, having absorbed salts, lime and insecticide, and so repotting bonsai trees regularly is an essential part of their care and maintenance.
For the tree to remain healthy under these conditions, the roots must be pruned and the tree repotted with new soil periodically. You should aim to repot your bonsai tree every two to three years.
When to repot bonsai trees
For deciduous trees, you should repot in early spring, just before the tree breaks dormancy.
Evergreen bonsai should be repotted in mid-spring, just before any spurt of new growth.
How to repot bonsai trees
1. Remove the tree from its pot
First, you will need to remove your tree from its pot. Check underneath to see if it has any wire securing the tree down, and if so, cut through or undo it.
If the tree is well established, the root cluster and compost should come out intact. You may find the tree easier to extract if you carefully run a blunt knife around the inside edge of the pot. If the drainage holes are large enough, use your fingers to help push your tree out.
Once your tree is removed from the pot, carefully tease away the compost from the root cluster. Use a bonsai rake to comb the roots from the trunk outward. Comb out and untangle a third of the root cluster.
2. Prune the roots
Next, take a clean pair of scissors and prune those roots which hang down. You should prune around a third of the roots, but no more, unless you are also performing major branch pruning.
3. Replace the Compost
Prepare your pot by placing a fine plastic mesh in the bottom. Next, add a layer of aggregate, as this will ensure good drainage.
Add a layer of compost and place your tree on top. The roots should radiate outward away from the trunk. Work the soil in between the roots with your fingers.
It is sometimes necessary to secure your tree to the pot with aluminum wire, which can be fed over the top of the root cluster and through the mesh and drainage holes. Finally, add a finishing touch to your bonsai with a surface dressing of fine horticultural grit or sphagnum moss.
4. Care for your repotted bonsai
Water your newly repotted bonsai tree immediately from the bottom by immersion. Do not water your bonsai again until the soil is half-dry.
Place your outdoor tree in a position of shade; begin exposing it to the sun gradually over a period of a month or so.
If your tree is the indoor variety, do not expose it to sources of dry air (i.e. heaters).
Do not feed your tree for two months after repotting.