This article is in the CBS Knowledgebase. It covers key aspects of the why, when and how to go about re-potting and root-pruning a bonsai.

Why re-pot?

The short answer to the question ‘why re-pot?’ is that plants in pots will in time become pot-bound. The roots will grow to the point where they fill the pot and the role they play in feeding the tree is compromised. Trimming the roots and replenishing the potting mix will facilitate strong root growth, a pre-requisite for a healthy tree.

When a plant is pot bound water does not penetrate or drain easily. This makes it difficult to maintain a good moisture level over the full depth of the pot and to ensure the roots have the oxygen they need to function effectively. Poor penetration of liquids also makes it hard for the tree to absorb nutrients applied in liquid form.

For more information about the role of roots, see the article titled Fertilising (What is fertiliser? How does it work?) in the CBS Knowledgebase. If you would like to understand more about this (and plant biology generally), see Plants in Action an online text book produced by the Australian and New Zealand societies of plant sciences from where the following information is largely drawn.

As explained in Plants in Action, the soil: root interface is a chemically complex ‘factory’ called the rhizosphere (see Picture 1).

Picture 1: Rhizosphere (source, Plants in Action). Note: AMF = Arbuscular mycorrhizal fungi.

The rhizosphere is the narrow zone of soil surrounding plant roots that may be only a few hundred micrometres thick. It is rich in bacteria and fungi (some beneficial to the plant; others are pathogenic) and substances secreted from the roots.

Further information on roots is provided in the article What keeps a bonsai small? in the CBS Knowledgebase, some of which is repeated below.

Roots play a critical role in plant growth. Plants not only extract nutrients from the soil surrounding their roots, they interact chemically with it. Some of the photoassimilate (the sugars produced by photosynthesis) are stored in the roots and later diffused into the rhizosphere where it is either taken up by micro-organisms or deposited as organic carbon.

Roots exude amino acids, organic acids, carbohydrates, sugars, vitamins, mucilage and proteins which act as messengers that stimulate biological and physical interactions between roots and soil organisms. They modify the biochemical and physical properties of the rhizosphere and contribute to root growth and plant survival. However, the fate of the matter exuded by the roots and the nature of their reactions in the soil remain poorly understood.

It is known, however, that the root itself strongly controls the means by which it extracts nutrients from the soil effectively. The rhizosphere encases the root (most importantly the youngest, most active parts of a root) in a chemical milieu of the root’s making. For example, the release of organic acids from roots can react with certain insoluble mineral deposits to make important nutrients such as phosphorus and iron available to the tree.

While it is possible to have a healthy rhizosphere in a bonsai pot, the rhizosphere is compromised when tree becomes pot-bound. Where plants are growing in nutrient-limiting conditions, including in small pots, root growth is limited, as is the plant’s capacity to store energy.

As Plants in Action explains, young roots absorb nutrients more rapidly than old roots. The roots of perennials have a large turnover of the fine, high-order lateral roots that emerge from the secondarily thickened framework each year. The production of these fine (and often short-lived) roots ensures the plant can absorb nutrients over many years.

When to re-pot

There are two parts to the question ‘when to re-pot’: how often; and what time of the year is safe to re-pot (root-prune) a bonsai.

How often?

It is extremely difficult to generalise about how often to re-pot a bonsai. Young, fast growing trees need to be re-potted more often than older trees. Some species grow roots quite quickly. Other species produce roots more slowly. Pot size is also relevant. Kaizen Bonsai suggests the following signs MIGHT indicate re-potting is on the cards:

a slowing rate of growth over previous seasons;
difficulty wetting the tree’s soil;
reduced uptake of water in summer;
rapidly reducing leaf size;
early leaf-drop in autumn;
slight yellowing of foliage colour;
reduced gloss on the foliage;
die back of fine twigging in winter;
liverwort and algal slime forming on the soils surface;
root ball gradually rising in the pot;
reduced leaf viability (discolouring and dropping after a few weeks in summer).

What time of the year?

In relation to the time of the year to re-pot a bonsai, in Canberra there are two main re-potting opportunities: late winter to early summer; and early autumn. As a general rule, the safest time to re-pot a bonsai is at bud-burst, but CBS suggests you re-pot your bonsai to the following timetable:

Deciduous trees: August – September;
Evergreens: September – October; and March (for many species);
Tropicals (and some Australian natives): October – December;
Cedars: August only.

Check the Care Calendars in the Knowledge-base on the CBS website (in the members-only part) for species-specific advice.

How to re-pot

Preparation:

The main tools you will need are a root rake, root hook, root-pruning scissors wire cutters and a bin to collect the waste. A saw may be required for larger plants with a woody root system. To save mess, and your back, it is a good idea to set up your equipment at a convenient height.

Photograph 1 shows a plastic bin with a piece of weld-mesh on top (hardware stores sell the mesh in various sizes). Tools (from L to R) are root rake, root hook, root trimming scissors and bonsai wire cutters.

Photograph 2 shows Nuoi’s idea described in the July 2018 edition of Wirrabara which does a similar job but holds the tree in place better than the mesh. The gap in the middle is wide enough to hold the root base steady.

In both cases, the waste is collected in a plastic bin.

Remove the tree from its pot:

Cut any wires used to hold the tree securely in its pot. Be aware that sometimes the roots are attached to the sides of the pot and you may need to run a knife around the sides of the pot, perhaps several times, in order to remove the tree without using undue force that could damage the tree or the pot.

Trim the roots:

Use the root rake, root hook and root-pruning scissors to tease out and trim the roots (see photograph 3). As a general rule, you can safely remove half of the root mass, but if you prefer to be conservative, remove just one third. If re-potting in March, remember that there are only about 6 weeks of warm weather remaining for the new roots to grow, so the more conservative approach may be safer.

Prepare the pot:

If you are putting the tree back into the same pot, clean out the old potting mix, wash the pot and check the mesh covering the drainage holes. You may or may not need to replace the mesh.

Covering the drainage holes with mesh:

If new mesh is required, you need to decide how best to secure it over the drainage holes. A ‘butterfly’ made from aluminium bonsai wire is commonly used (see Wirrabara April 2015 for more on this). First, create a figure of 8 ‘butterfly’ and then bend the ends of the wire 90 degrees so they can be passed through the drainage hole (photograph 4).

Photographs 5 and 6 shows the mesh held in place. Note that you can use it with the ‘butterfly’ inside or underneath the pot. It is important, however, that the mesh sits flat against the pot to prevent entry of bugs such as slaters.

Photograph 7: drainage holes and anchoring holes

Wiring the tree into the pot:

The simplest way to ensure a newly root-pruned bonsai sits securely in its pot is by wiring it into place. The roots of a properly stabilised tree will not be damaged when the tree moves in the wind, or when we are working on it.

Look at the underside of your pot. Some pots have drainage holes only (as in the top two pots in photograph 7); in this case, you will need to pass the anchoring wires(s) through them. The wire holding the mesh in place can be used to anchor the tie wires.

If your pot has four holes, you can secure the tree with two wires in an X formation as shown in photograph 8.

In April 2015, Wirrabara reported on Grant’s method of wiring a tree into its pot. In this case, the pot has four holes. Grant created a ‘cage’ using 4 wires as in photographs 9 and 10.

Twist the two wires in each corner together. The twisting should continue to a point just below the lip of the pot, although the amount of twisting can be varied to suit the footprint of the root ball in the pot.

More twisting will bring the ‘cage’ created by the wiring closer to the centre of the pot; whereas less twisting will leave the ‘cage’ closer to the edge of the pot.

Use the cage method where greater stability is important, e.g. for large or top-heavy trees, windy areas, or where trees need to be transported.

If the pot has three holes, use the same ‘cage’ technique as shown by Photograph 11. If the pot has two holes, you have little option but to pass one or two wires through the available holes, also shown in photograph 11.

If the pot has only one hole, the anchor-wire will need to use the mesh ‘butterfly’, as shown in photograph 12, to hold it in place. Alternatively, you could use a short piece of large gauge bonsai wire, as shown in photograph 13.

Potting the tree

Cover the base of the pot with potting mix, mounding the mix where the tree is to be placed. Sit the tree in place, paying careful attention to its height and location (front, back, left, right) in the pot.

Wiggle the tree and press down firmly to make good contact with the soil under the centre of the root system. If the tree is too high or too low, adjust the amount of potting mix where the base of the tree is to sit.

Remember, at this point you are making some design decisions. You can also add some slow release fertiliser at this point, if not already added to your potting mix.

When you are happy, wire the tree securely in place and fill the pot with potting mix. Use a chopstick to help make sure you fill any gaps. Push the chopstick into the mix and wriggle it back and forth, adding more mix as necessary. Repeat this, working around the pot, until the mix is firm to touch and no more mix can be ‘absorbed’.

This process is critical to re-establishing good contact between roots and particles in the soil mix. If the soil is quite damp, be careful not to compact too much as you will lose too much air spaces.

For information on potting mix recipes, see Soil and bonsai soil mixes in the CBS Knowledgebase.

Re-potting aftercare

Don’t forget your after-care: give your newly potted tree a good drink – if possible, dunk it for 10 minutes. Seasol is good for helping the tree recover. Place the tree in a sheltered spot for the next few weeks – filtered sun is good – and manage its water carefully. Aim to keep damp, but not wet. Newly re-potted trees take up less water than before their root-prune, and the new potting mix will almost always hold more moisture than the old mix.

Canberra Bonsai Society Newsletter
Thank you to Ruth McL and Roger H for their help by checking over the draft of this article.
Phil R

Guide to Repotting