How, When, And Which Plants To Chelsea Chop

How, When, and Which Plants to Chelsea Chop

By the end of May, at about the same time as the Chelsea Flower Show, your early flowering bulbs have done their bit and are fading around the edges while your herbaceous perennials are bursting into life with gusts of foliage and new flower buds. 

This may be counter-intuitive for new gardeners, but now is the ideal time to cut back (at least some of) that new growth.
You sacrifice some early flowering in exchange for an even better display a bit later in the season, which is incredibly useful as many borders begin to look a little tired by late July!

What is the "Chelsea Chop"?

The Chelsea Chop - so called because you do it around the time of the Chelsea Flower show in late May - means to cut back certain tall herbaceous perennial plants in mid to late Spring.

Of course, some plants get off to a racing start and need chopping earlier, while after a cool spring, you may still be Chelsea Chopping well into June.
The date is not fixed, but the last week of May is a pretty good guide; let common sense prevail, and only you know your garden best.

This practice is as old as gardening, calling it the Chelsea Chop is just a handy reminder!

Why Should I Cut Back My Herbaceous Perennials in Spring?

You don't have to cut anything back, it's only an option in your gardener's toolkit.

You might want to cut back your herbaceous perennials in Spring for one or both of two reasons:

1. To produce a stockier plant that does not need staking or other support, and flowers a bit later, and/or
2. To prolong the flowering period

In all cases, you should end up with more total flowers over the season, because cutting the plants back early encourages them to produce more flowering shoots. The individual flowers may be a little smaller, but that's no problem because it's the mass effect of all of them together that matters.
The principle here is essentially the same as cutting plants back after they flower for the first time in a season, to encourage a second flush.
However, the difference is that you cut them back before they flower, so the first flush is stronger than it would have been. 

The other reason to Chelsea Chop is simply to control the size of large perennials: keeping things relatively compact means you can squeeze even more plants into your borders!

How do I Chelsea Chop My Herbaceous Perennials?

There is no one right way to cut back your perennial plants: it's a matter of personal preference.
There are 3 main variants on the basic method that you commonly see.

How much height you cut down will vary mostly according to the effect you want to achieve, and the plant in question: take off at least a third of the height, but you can take off more if you want, down to about 20cm high for some plants.
Much the same goes for how many stems you cut back: between a third and all of them. 

You won't kill the plant by cutting too much, and the worst case is that may lose some flowers, so some trial and error won't hurt!

All methods should give you more flowers over the growing season compared to not cutting the plants back.

If you have multiple clumps of the same plant, you could, for example, Chelsea Chop the ones at the front of your border, Half Chop the ones in the middle, and leave the ones at the back alone, thus greatly extending the total flowering season & quantity of flowers.

Basic Method - The Chelsea Chop

Cut the entire plant back in one go using secateurs or shears (a hedge trimmer works well enough too).
Use this method to produce a stockier plant that does not need staking, and flowers a bit later.

Variant 1 - The Half Chelsea Chop

Cut down stems of the front half of the plant, leaving the ones at the back.
Use this quick and easy method to prolong the flowering period: the stems you did not cut will flower early (but may still need staking), the rest will flower later. 

Variant 2 - The Chelsea Thin

Cut about half the stems down, removing them pretty evenly from all over the plant, but perhaps taking a few more from the edge than the centre.
Use this method to prolong the flowering period: the stems you did not cut will flower early, the rest will flower later.
With certain plants, this method will also prevent the need for staking, at least in sheltered gardens, because the shorter, stronger new stems help to support the taller, weaker stems.

Variant 3 - The Staggered Chelsea Thin

Cut about a third of the stems down hard, removing them evenly from the around the clump.
Two or three weeks later, cut down half the remaining tall stems (i.e. another third of the original number). Be more gentle than with the first chop
Use this method to prolong the flowering period (even more than in variant 2 above): the stems you did not cut will flower early, the rest will flower later.
As with the Chelsea Thin, for some plants, this method may also prevent the need for staking, at least in sheltered gardens, because the shorter, stronger new stems help to support the taller, weaker stems.

Note: Britain is not famous for having dry Spring weather, but it happens occasionally. When it does, water your plants well the day before cutting them back: really soak the soil properly. This will reduce the stress of being chopped back. 

Which Plants Should I Chelsea Chop?

Not all herbaceous perennials can be Chelsea chopped, because they won't flower at all if you do! We have put the most common perennial plants into 3 groups:

1. Yes: Plants that are typically cut back for best results, and to avoid staking,
2. No: Plants that should not be cut back

These lists are not exhaustive, only some of the most widely grown perennial plants in British gardens.

Group 1: Yes, Chelsea Chop is Recommended for these Plants

• Achillea - Yarrow
• Anthemis - Chamomile 
• Argyranthemum - Marguerite Daisy
• Asters & Symphyotrichum (which used to be called Asters!) over about 60cm tall
• Campanula - Bellflower
• Coreopsis - tickseed
• Echinacea
• Eupatorium 
• Helenium - Sneezeweed. Shorter varieties like Short and Sassy don't need support, so cutting back is optional.
• Helianthus - Perennial Sunflowers (not the annual ones, which are much more common)
• Leucanthemum superbum - Shasta Daisy
• Monarda - Bergamot (not the Citrus fruit which flavours Earl Grey tea!)
• Nepeta - Catmint 
• Penstemon
• Phlox paniculata - Tall varieties (not short ones like Phlox subulata Candy Stripe)
• Rudbeckia - Coneflower
• Sedum/Hylotelephium
• Solidago - Goldenrod
• Veronicastrum

Special mention goes to Lavender, which is a woody shrub that should be cut back quite hard every year to keep it nice and bushy.
In warmer countries, it is common to cut it back in Spring, but in the UK late Summer is the ideal time: still, cutting Lavender back hard in Spring is always an option if you need it. 
The Chelsea Chop method applied to Lavender would be to trim off most of the new green growth (not cutting back at all into last year's wood). 

Group 2: No, Chelsea Chop is not Suitable for these Plants

Generally, plants that are only capable of flowering once per season are not suitable for cutting back in Spring. If you did, you would be removing the only stems that could flower that year.

Similarly, plants like Hollyhocks or Lupins produce one big flower display on a tall central stalk, and then usually manage a few more small ones on stubby side-stalks: if you chopped these plants back early, you would simply lose that main display, and only get the smaller blooms.
Instead, you should cut back the stems that have flowered as soon as the petals begin to fade, giving the plant as much time as possible to flower again before the season ends.

• Alcaea - Hollyhock
• Aquilegia - Columbine
• Digitalis - Foxglove
• Irises
• Lupinus - Lupins
• Lysimachia - Loosestrife
• Paeonia - Peonies