Repotting, Potting on and Root pruning


This section is largely taken from my Bonsai Primer, as it seems daft to re-invent the wheel.

Most gardeners, or those who keep plants in containers, would never consider repotting a plant. The reasons for doing so, together with an illustration of a re-potting follow, and all you have to do is (mentally) delete the word 'tree' and substitute 'plant'.

I've added a section on Potting on, as it applies to most plants until they reach maturity, however bonsai are usually returned to the same container.

Repotting is another of those mysterious tricks that it will take you many minutes to master. It can I will admit be a daunting prospect, the thought of ripping your (expensive?) tree out of the pot, hacking large bits off the roots, and forcing back into it's pot, it's not like that at all, and it will only benefit the tree.

So why do we repot our trees, is it to keep them small?. No! limiting their space in a pot, and pruning the top will achieve this, repotting is if anything carried out to increase the tree's vigour, and maintain its health.

As the tree grows the roots extend, now, not all of the root is capable of absorbing the nutrients the tree needs to grow, in fact this absorbtion mainly takes place in the growing tips of the roots. A fair analogy would be if you tried to drink a carton of milk through a 10 Meter long straw. you would need to suck hard to get any benefit. So does the tree!.

Just as pruning the top of the tree produces side buds, root pruning produces side roots, each capable of feeding, and watering the tree.

When to Repot

It is not essential that a tree is repotted every year, but you would be unwise to leave a vigorous tree more than a few years without a repotting. A good indicator that the tree needs repotting is that the volume of root in the pot causes the tree to rise up in the pot.


Root growth causing the tree on the right to rise up in the pot

Those of you who keep houseplants may wonder why you have never been advised to repot those plants, this is I would suggest down to the fact that most suppliers of houseplants are not aware of the benefits of repotting.

Repotting Time

The best time to repot deciduous trees is in the Spring before the buds break open, however flowering trees are different. Flowering trees fall into two categories when it comes to repotting time, those that flower before the leaves open, and those that flower after the leaves have opened, Apricot, japanese quince, and Plum fall into the first category, Apple, Cherry, Pear, and of course the Azalea family all fall into the latter.

The Pre-leaf flowerers should be repotted after flowering but before the leaves are open, while the After-leaf flowerers should be repotted prior to leaf opening.

Conifers are best repotted in the late winter/early spring, although i've taken to repotting my pines at the end of August, that's the end of summer here in the UK, and have found fresh young roots breaking through the surface in November. A sure sign of a healthy tree.

How to Repot

The actual mechanics of repotting are simple.
You will need as a basis.

A work area
enough potting compost to replace what you will remove.
Sharp cutters.
Enough small pieces of mesh (preferably Plastic) to cover the pots drainage holes, and some suitable method of securing (tying) the tree into the pot, Bonsai wire is commonly used, however copper wire is better, as it is less prone to break when twisted.

Place the tree on the work surface, and ease it out of the pot, placing the pot to one side. If the tree has a mass of roots curled around the pot tease them out so they radiate away from the trunk like spokes on a wheel.

I use the hook to the left for teasing out the root ball when repotting. I'm not sure of its origins, but have been told it's for cleaning out horses hooves.

Remove (cut away) about 33% of the total root mass including the soil, removing some from the underside of the tree. While doing this you should have a small hand sprayer near to hand, to mist the roots with water, stopping them from getting to dry, killing the root hairs

If the tree has an uneven root structure, with some roots smaller than others, remove less root from the thinner roots. Over time the root sizes will come into balance.

When repotting your tree, you should remember that you are trying to get as much new root close to the tree as possible. One way of achieving this is to remove 'V' shaped sections, between the major roots, allowing space for new roots to develop. This has the added benefit of removing old, compacted soil, close to the tree.

This pot is now ready for the tree. It has the plastic hole covers in place, and long wires ready to tie the tree into the pot.

Put a layer of compost in the bottom of the pot, to about 1/3 of the depth of the pot. Hold the tree over the pot, and push the tie wires up through the root mass, and lower the tree into place.

Now twist the wires together until they hold the tree firm in the pot. Ensure the wires are positioned so that they will not be visible when the remaining soil is placed. You might consider using some aquarists 'air line', threaded over the wire, and positioned to protect the roots.

Now add extra soil, and work the compost into any spaces in, and around the root mass, chopsticks are often used to work the soil in between the roots.

Water the tree with a fungicide added to the water, and stand it in a sheltered spot for a few days.

When the soil is in place, after repotting, try to ensure that it does not come up to the rim of the pot. Allow somewhere for water to 'pool', and seep into the compost. This space will also allow somewhere for soil to go if you have used a hose with to fast a jet.


Positioning the tree in the pot

The tree should be planted at such a height in the pot as to allow the roots as they leave the trunk to be visible above the pot.

Place the tree in the pot so that the trunk is neither the front, or back third of the pot, nor should it be planted far away from the centre line.

If the first (lowest) branch leaves the tree in the direction if the arrow then the tree will usually look best if planted offset from the centre line.

This will not apply to round pots, where you should always plant dead centre.

When repotting try to arrange the two trunks, of a two trunk bonsai, so that the smaller is positioned further back in the pot than the larger. this adds an extra 'depth' to the planting.

This is particularly important to remember if the two trunks are similar in size.

Creating a balance between the roots, and foliage

When you repot your tree, try to create a balance between the volume of root, and the amount of foliage present. This may mean reducing the foliage, but a tree that is 'out of balance' like the one in the middle may find it difficult to survive.

This applies more to conifers than deciduous trees, but leaving to many buds on a deciduous tree may stress it, so carry out the spring pruning when the tree has settled.

Moss, and other plants

Having just repotted your tree, you will notice that the whole thing looks a little unnatural, all that fresh soil will not help the impression of an old tree, dressing the surface with moss will add to the impression of age.

There are lots of plants that will, one way or another try to grow on the soil surface of your newly repotted bonsai. Of these only moss should be tolerated, and then only in reasonable amounts, no more than a third of the soil area should be covered. Moss while adding to the looks of the Bonsai will use any fertiliser intended for the tree, and acts as a waterproofing, making it difficult to water the tree.

Moss is a bit of a two edged sword, on one hand it adds maturity to the planting, on the other it acts as a 'raincoat' when watering.


Moss is easily transplanted, and I usually recycle mine at repotting time, or collect new stock from damp patches in the garden.

I do feel that you are going to find it difficult to get moss to thrive in an indoor environment, however the best advice I can give is that you keep the soil constantly damp, make sure the moss is firmly (and I mean it!) pressed into the compost. Its shape may be distorted while you do this, but it will soon come back. I would also advise that you cover the surface of the pot with clear plastic for a couple of weeks, providing a moist microclimate, that will help it settle.

As to reproduction, moss is a resiliant plant, and can be propogated easily. I normally allow it to get really dry then reduce it, by rubbing it to a powder in the palms of my hands. This powder is sprinkled over the areas I want moss to grow, and it soon flourishes.

It is not a good idea to allow moss to grow up, or on the tree trunk. This spoils the look of the root flare. I use a plastic brush of the type sold for washing dishes, to scrub moss off the trunk, and indeed branches should they need it. Spraying the tree after scrubbing will wash away any residue, and the tree will look much better.

Liverwort is indicative of poor drainage, so with a good compost it should not be a problem.

If liverwort is present, either remove it with a small tool (knife, tweezers, screwdriver, anything!). Or paint it with malt vinegar, which I feel has the added benefit of killing any spoors in the area. Bear in mind that vinegar is acid, so don't overdo it.

Any other plants growing can be removed by hand, when big enough.

Repotting in action

Here we see a Forsythia I was given, on inspection I decided that the tree would make a good semi-cascade, if on repotting its angle to the soil was changed. This would also require it to be put into a cascade pot.
The tree was in need of a repot, as you can see the roots were compacted.
The roots, and soil have now been raked out. That has broken a bit of the fine fiberous roots away, so there's no need to cut any more off.
The pot is prepared. Fixing wires have been inserted through the drainage hole, and as with all single hole pots the wire will need to be anchored, a small piece of strong wire will do this.

The tree has been positioned in the pot, and some soil has been put around the roots. The compost has been worked in around the roots using a chopstick, as more soil is added the fixing wires will be tied.

The repot is finished, with a good watering, and some moss placed around the trunk to give the impression that the tree, and pot have been together for some time.

Allen. C. Roffey Wednesday, August 8, 2018 8:42