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Phormiums are one of my favourite shrubs and are very popular at my family’s nursery. Over the years, I’ve written a few guides, including this guide on phormium care and phormium mealybugs, which can be an issue, and so are worth looking out for.
Over the years, as a professional gardener, one question I’ve often been asked is, ‘How (and when) can you prune phormiums?‘ The answer depends on whether you need to give your phormium a tidy-up and remove winter damage and dead and diseased leaves.
The other reason is that you have a size issue, and it has outgrown its original space and needs to be made more manageable. The same goes for phormiums grown in pots, too. For most shrubs, this would mean pruning them back hard; however, with phormiums, it’s better to lift and divide the plant rather than hack it back. Definitely don’t do this; they don’t respond very well, more on this further down the road.
In terms of when to prune, I used to do this in Autumn; however, these days, I’ve had a much better response to pruning them in Spring, as the foliage helps protect the crown (centre) of the phormium during harsh winters.
Below, I have a more detailed guide on when and how to prune phormiums as well as how to divide phormiums.
When to prune phorium
As mentioned earlier, I have found that the best time to prune a New Zealand flax (Phormium) and remove damaged and dead leaves is in spring. Once the worst of the winter weather is over, it’s time to look at removing any dead, damaged or diseased leaves so new growth can thrive.
Most of the damage is usually just part of the growing process during winter, and even though the plant is technically evergreen, the leaves themselves may be damaged by the intense cold they experience in the winter. I have found this to be fairly common if you live in an area with a very harsh UK winter climate where you live.
How to prune New Zealand flax
Phormiums, also known as New England Flax, are very hardy shrubs that are much loved for their large arching elongated foliage, which is sometimes mistaken for cordylines. The first thing you need to do is remove any damaged leaves. This could be winter damage, holes in leaves from pests, or just any sickly-looking leaves, of which there are usually plenty after the winter.
Removing damaged leaves is easy enough. Use a decent pair of gardening gloves and tear off any damaged leaves by pulling them downwards; they should just come away easily at the base of the plant. For any leaves that are a little stubborn, use a sharp pair of scissors to remove them as close to the base as possible.
This is a fairly simple process that, when done every spring, will help to keep new growth appearing every spring and improve the overall appearance of your plants.
Reducing the size of an overgrown phorium
With most shrubs, when you want to tackle an overgrown plant to make it more manageable, you can simply prune it back hard in autumn or early spring. However, with phormiums, you don’t want to do this. They can take a very long time to recover and look very unsightly.
In this situation, you can also consider dividing your Phormiums to make new plants or to help reduce their overall size. This is best done in spring, so give them a good tidy-up first, removing damaged foliage, as explained earlier. Now, I have a detailed guide on dividing phormiums here, but basically, you can lift the whole plant and then divide the plant using a spade into two or more sections. Replant the small section and plant or dispose of the spare sections. I usually plant them in new pots and give them to other gardeners if I don’t have space for them myself.
If you have a very large phormium and it’s not realistic to lift the entire plant, I start by removing sections around the edges using a spade and garden fork to reduce the size.
After pruning or dividing, I always put a thick layer of mulch around the base to help with moisture retention, suppress weeds, and add a little winter protection for the roots as they are shallow-rooted.
