How to root-prune pot plants and bonsai trees

Root pruning is a technique traditionally used by Chinese and Japanese gardeners when growing bonsai. The same rules apply when growing shrubs and trees in pots a well as bonsai trees – the aim of root pruning is to reduce the vigour of the tree so it will grow in a confined space, such as a pot.

Using an acer, David demonstrates how to tease out the plant's roots and prune them, and suggests the best time of year to root prune trees and shrubs.

Find out how to root prune plants and bonsai trees in this No Fuss video guide.

Root pruning: transcript

Us gardeners, we're really familiar with wielding the secateurs to prune the top growth of plants to keep them in control. But actually, there is a technique that you may not be so familiar with, which is called root pruning, and that involves cutting back some of the roots to reduce the area of rooting, to constrain the growth of plants.

Now, this is a technique that's been used a lot by the Japanese and the Chinese to produce bonsai trees, where they are growing small, miniature versions of the plant in a small restricted pot. If you prune the top growth, you'll end up getting stubs and stumps and unnatural formations, whereas pruning the roots will help
control the growth, slow it right down and will make the leaves small as well, in proportion to the rest of the plant.

This acer, for example, I'm going to knock it carefully out of the pot and you can see that this is one that I've just bought at the garden centre and it's got a mass of roots, growing around the outside of the pot. It's not quite root bound, but it's getting that way. And if we just potted this straight up into our large container, you'd find that it would soon fill that pot with roots and the plant would carry on growing quite strongly. But actually, if we prune those roots back before we pot it up, it will restrict the growth of the plant and it won't outgrow its pot too quickly.

So, the key thing to do is to start by teasing some of this compost away from the rootball of the plant. It's fine to do this in the summer or any time during the growing season. If you do it in the dormant season, then the roots don't get the chance to heal themselves, so it's actually quite good to do it when the plant's growing strongly.

So, the key thing to do is to start teasing out the compost from around those roots. You can be quite brutal really with it. The plant has the ability to regrow roots quite easily, this fibrous root, any of the really thick roots really in a pot. It doesn't really matter too much, but the fibrous roots will regrow very quickly when we've
cut them back. So, having teased those roots apart, we can now get in there with the secateurs. And again, don't be afraid of this. Look and find some really thick roots like that and pull them out and then cut them back really quite seriously. A long way back. And that will produce a mass of really fibrous growth that will grow very successfully. And, as I say, limit the top growth of the plant, but still keep it healthy.

So it looks a bit drastic, but by the time you've taken so much of that root out, you can see that there's still plenty in there. And you've worked all the way around the rootball and reduced the amount of root and then you can go in and tease a little bit more of that compost apart. So you loosen it and allow the roots to go out into the new compost and start feeding the plant. Give it a quick tap and there we are. We're ready to put it into a new pot. So, don't be alarmed, give it a good water - your plant will soon recover.

Part fill your container with a good quality compost. Now, this needs to be a good quality peat or peat based compost. You can also use a John Innes compost for this very successfully. A John Innes number two or number three. Offer your new rootball of your plant into the pot and it should actually then be possible, once you've popped it in the right level - make sure that the rootball is just sitting slightly below the level of the pot. And then fill in around those chopped up roots with your compost, teasing it down the side of the pot. So, firming that compost in around the edge will help to support that chopped up rootball. And then, once you've got it roughly level the top of the compost, don't worry about getting it exactly level. Give the sides of the pot a tap and that will help to settle the compost around the roots.

And then we need to give it a good water. Make sure you give it a thorough soak to get the water into that new compost and to get it in contact with the chopped up roots. And then that plant will start to grow its new roots out into that surrounding compost. If it starts to wilt, just take a few of the branches out just to reduce
the leaf area of the plant, so it doesn't lose so much moisture, move it into perhaps a sheltered, shady position again, so it's not in full sun. In a couple of weeks time, it will be ready to support itself again and then that plant will be ready to grow in that pot for years to come.