How to Repot a Bonsai Tree

The objective when growing bonsai trees is to keep them small, when they are trying to grow massive. To achieve this, bonsai trees are grown in confined pots, where space for root growth is intentionally restricted.

Over time, your tree’s soil will degrade. For the tree to remain healthy under these conditions, the roots must be pruned and the tree repotted with new soil periodically. Most bonsai trees will require repotting every two to three years.

When to Repot Bonsai

The best time to repot bonsai depends on whether it is evergreen or deciduous:

• For deciduous trees, you should repot in early spring, just before it breaks dormancy.
• Evergreen bonsai should be repotted in mid-spring, just before any spurt of new growth.

Bonsai Root Pruning

To prune your bonsai’s roots 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.

Next, take a clean pair of scissors and prune those roots which hang down (see right). You should prune around a third of the roots, but no more, unless you are also performing major branch pruning.

Replacing the Compost

While special bonsai compost mixes are available, you may also mix your own soil if you wish. The secret to a healthy compost mix is that it must be free draining, while have the ability to remain damp without water logging. It is, therefore, important to sift the new soil to remove particles smaller than 1mm.

For deciduous trees, use a mix of 60% open textured organic compost, with 40% horticultural grit or aggregate. Grit particles should be between 2mm and 6mm in size. A suitable mix for conifers is 30% organic and 70% aggregate. For Azaleas, use a mix 50% organic with 50% aggregate.

For indoor bonsai, us a 70% organic with 30% aggregate.

It is important to wash grit or aggregate to remove dust. Ensure that you buy materials suitable for horticulture from garden centers. Never use grit or sand from a builders yard, as it will have a high lime content.

Repotting the Bonsai Tree

1. Prepare your pot by placing a fine plastic mesh on the bottom. Next add a layer of aggregate, as this will ensure good drainage.
2. 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.
3. It is sometimes necessary to secure your tree to the pot with aluminium wire, which can be fed over the top of the root cluster and through mesh and drainage holes.
4. Finally, add a finishing touch to your bonsai with a surface dressing of fine horticultural grit or ground cover plants such as moss.

Post Repotting Care

Water your tree immediately from the bottom by immersion. Do not water again until the soil is half-dry. Do not feed your tree for two months after repotting.

Place your outdoor tree in a position of shade; begin exposing it to 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).