How to root prune your plants in pots – and why it matters

March 24th, 2019
Posted In: Container gardening, Gardening know how

‘Give it a root prune every two or three years.’ I was given this advice when I bought my beloved topiary spiral.

It is the most significant plant in my garden, and also the most expensive.

So if I’ve been told to root prune, I will root prune. Cautiously.

If you have evergreen plants in pots, then you will need to root prune them after a few years. As evergreen plants make a brilliant long-term display, it’s really worth taking this extra trouble.

My beloved box topiary spiral was dug up from a field in Dorset three years ago. It’s been in this pot for nearly three years. Now it needs a root prune.

Why not just add a few inches of fresh compost?

If you have shrubs in a big pot, you are usually advised to replace the top few inches of compost every year, as well as feeding with liquid fertiliser. The compost in a pot no longer has any nutrition after a couple of months, and plants in pots can’t access nutrition from the soil.

But my box topiary spiral is a big plant. It’s also very dense.

There is absolutely no chance of carrying out any advice to ‘remove the top few inches of compost and replace it with fresh.’ You can barely get your hand in between the tightly packed branches. I would not be able to get either any old compost out or new compost in.

Does it need replanting in a bigger pot?

I’m sure it would love to be in a bigger pot, but I don’t have one. It’s not always practical to keep putting plants into bigger and bigger pots.

At some stage you will run out of pots or out of garden…and definitely out of money, as nice large pots are expensive. I bought this one as a ‘second’ from Hode Pottery near Canterbury, because it had a firing crack.

Topiary in pots is great for winter interest. – as are grasses. They too, will soon need re-potting and if they don’t go into larger pots, I will need to reduce the root ball by dividing them.

While your box topiary is growing to the size you want it, you can pot it up into a bigger pot. After that, the answer is to root prune it, taking out as much compost as possible. Then replace it with fresh.

When and how to root prune your evergreen plants

March is a good time to root prune. My topiary spiral is certainly looking a bit sad. The tips of some leaves are yellowing and there are a few bald patches.

I hope that this isn’t either box tree moth caterpillar or box  blight, but I think it’s unlikely. No new box plants have entered this garden since the topiary spiral arrived. And yellowing tips are one of the signs that the plant lacks nutrients and needs fresh compost and fertiliser.

So we slowly and carefully tipped the big pot over on its side. It took two of us. Then I hauled the plant in one direction while Mr Middlesize carefully pulled the pot the other way. The spiral reluctantly emerged from its pot.

The root ball was a densely packed mass of tiny roots, fiercely holding the spent compost in place.  It took me about half an hour to trim it with a combination of secateurs, loppers and shears, using my hand to rub the soil away from the root ball as I went.

There was a dense mass of tiny roots, all woven tightly together and holding the compost in place.

It’s hard to judge what ‘reduce by one third’ adds up to. But I just kept going until the pile on the ground seemed to have reduced the rootball to about two thirds.

What compost to add?

I used a multi-purpose compost and added a handful of slow-release plant food.

And if you’re thinking about reducing single-use plastics in your garden, you may be interested to know that my local nursery, Edible Culture in Faversham, are now selling compost in re-usable bags. You buy the bag and the compost for £8, but when you bring the bag back, re-filling it only costs £6.50. There may be a nursery near you doing this, or you may be able to suggest it to them.

Re-usable compost bags from Edible Culture in Faversham. A great initiative to reduce single-use plastics in the garden.

See how on video

I’ve included the root prune in the March garden video tour on the Middlesized Garden YouTube channel. If you just want to see how the root pruning was done, skip to 4 minutes 10 seconds. Though you will miss some lovely daffodils and cherry blossom….

How to get an enormous plant back into the pot

It’s not as easy as getting it out. You need a strong person.

But first measure how much fresh compost has to go back into the pot. I measured the remaining root ball roughly with my arm, and worked out that we did indeed need to fill the pot about one third full of compost. Result!

Then there’s the heave-ho and the swearing. I wish I could tell you how to circumvent that part, but I can’t. If you have any suggestions, do say (in the comments below)!

After which, you spend about ten to fifteen minutes shoving handfuls of compost all around down the sides, taking care to keep the plant central in the pot.

You can also add fistfuls of compost to the top. Water it to help it settle and add more. In the end we added a total of two bags of fresh compost to Mr Topiary Spiral’s pot, so I think he’ll feel much better later on in the summer. I’ll also add a liquid feed from about mid-summer onwards as the slow-release fertiliser will only help for about three months.

When firming the plant into the pot with more fresh compost, keep checking that it’s central in the pot. Look at it from a distance as well as close up.

It’s best to wait till June before pruning it.

Do join us on the Middlesized Garden blog and YouTube channel

If your garden is less than an acre, the Middlesized Garden blog and YouTube channel aims to find tips, ideas and inspiration that will make your gardening easier, more fun and more successful.

You can find more tips on growing pots successfully. For example, there are 6 top tips on growing successful container plants here. And if you want to grow colourful flowers, such as dahlias, then don’t miss Claus Dalby’s how to display dahlias in pots here. And finally, find out why choosing plants for winter pots is a little different from choosing plants for summer pots.

So do join us either on YouTube (there’s a new upload every Saturday) or on the blog (which will pop into your inbox every weekend). And do let me know if there’s anything you’d like me to find out more about, either in the comments below or via Twitter. Thank you!

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