Repotting bonsai: timing, root pruning and soil mixes with akadama, pumice and lava rock

TUREFF• •0

A bonsai lives in a small amount of soil. Over time, roots fill the pot, the soil breaks down and water stops draining properly. Repotting refreshes the soil and gives the roots room to grow. Done at the right time, it is routine; done at the wrong time, it can set a tree back badly.

How often

There is no fixed interval. Young, vigorous trees fill their pots faster and need repotting more often; older, slower trees less often. Signs it is time:

roots circling the inside of the pot or growing out of the drainage holes
water sitting on the surface instead of soaking in
soil that has broken down into a fine, compacted mass
the tree lifting out of the pot as one solid block of roots

When in the year

For most temperate trees, repotting is done in early spring, as buds begin to swell but before leaves open. At that stage the roots are about to grow strongly and repair quickly. Some growers have different preferred windows for particular species, such as some pines, so check your species. Tropical trees are often repotted in warmer months when they grow fastest. Avoid repotting a weak or stressed tree.

Getting ready

New soil, sieved to remove fine dust.
A pot with drainage holes, the same size or a little larger if the tree needs it.
Mesh to cover the drainage holes, and wire to tie the tree in.
A root hook or chopstick, sharp scissors and a watering can.

Step by step

Remove the tree from the pot. Cut any wire holding it in and run a knife around the edge if it is stuck.
Gently comb out the roots from the outside and underneath with a root hook or chopstick, working inwards.
Prune the roots. Remove long, thick, circling roots and trim back the outer roots. Many beginner guides suggest removing up to about a third of the root mass on a healthy tree; be more cautious with weak trees or species that dislike root disturbance.
Prepare the pot: mesh over the holes, tie-in wires threaded through, a layer of soil on the base.
Position the tree, then tie it in firmly so it cannot rock. A tree that moves cannot grow new roots.
Work soil in around and between the roots with a chopstick so there are no air pockets.
Water thoroughly until water runs clear from the drainage holes.

Aftercare

Keep the tree out of strong sun and wind for a few weeks while new roots form, and do not fertilise straight away. Hold off on heavy pruning until it is growing well again.

Bonsai soil: why it is different

Bonsai soil is mostly coarse granules, not garden or potting compost. It needs to drain fast, let air reach the roots, and still hold some moisture. Common ingredients:

Akadama: a Japanese baked clay granule that holds water while draining well. It is valued, but breaks down after a few years, which is one reason trees are repotted.
Pumice: light and porous, holds water and air and does not break down.
Lava rock: rough and porous, adds structure and drainage.

A common starting mix is equal parts of the three, adjusted by experience: more water-retaining material for trees that like moisture or for hot climates, more drainage for those that prefer drier roots. Many growers adapt mixes to their species and what is available locally.

Common mistakes

Repotting in midsummer or in the middle of winter.
Using garden soil that compacts and stays wet.
Leaving the tree loose in the pot.
Removing too many roots from a tree that was already weak.

When did you last repot, and what mix did you use? Share your recipe or ask about your species in the comments.