What is the Chelsea chop and should I do it?

The name comes from the timing rather than the technique — it is done around the time of the Chelsea Flower Show in late May. What it actually does is exploit the same hormone effect as pinching out a seedling, applied to a mature plant: removing the growing tips releases the buds below them, so the plant branches instead of extending, and ends up shorter and denser.

What you'll need

• Shears or secateurs
• Gloves
• Watering can, if the weather is dry

Step by step

1. Do it in the last week of May, or thereabouts.Late enough that the plants have made substantial growth, early enough that they have time to regrow and flower. In a cold spring or a northern garden, a week or two later is right — judge by the plant, which should be roughly two thirds of its eventual height.
2. Choose plants that flower from midsummer onwards.Sedum and Hylotelephium, phlox, helenium, echinacea, rudbeckia, aster and Symphyotrichum, nepeta, achillea, solidago, campanula and heleniums all respond well. Anything that flowers after about July is a candidate.
3. Cut back by about a third.Shears are quicker than secateurs on a big clump. Cut above a leaf joint, and aim for an even dome rather than a flat top.
4. Or cut only half the clump for a longer season.Chop the front half, or every other stem, and leave the rest. The uncut stems flower on time, the cut ones flower two or three weeks later, and the plant is in flower for far longer overall. This variation is usually the better choice in a mixed border.
5. Use it on sedum specifically.Sedum is the textbook case. Left alone, almost every mature clump splits open in the middle by August under the weight of its own flowers. A Chelsea chop produces a compact plant that stands up unaided, and it is more reliable than staking.
6. Water afterwards if the weather is dry.You have just removed a third of the plant's leaf area and asked it to regrow. A dry fortnight after chopping produces a stunted result.
7. Never chop anything that flowers before midsummer.Peonies, oriental poppies, irises, aquilegias, early geraniums and anything already in bud have their flowers formed. Cutting them removes the entire display.
8. Do not chop plants that make a single stem or spike.Delphiniums, lupins, hollyhocks, verbascum and anything else that produces one flower stem will not branch usefully. They are staked, not chopped.
9. Do not chop a plant in its first year.A newly planted perennial needs all its leaf area to establish. Wait until it is a proper clump.
10. Expect smaller flowers.More of them, on a sturdier plant, but individually smaller. On a helenium or a sedum this is a clear gain; on something grown for large individual blooms it may not be.

Tips

• The same technique done after the first flush of flowers — shearing hardy geraniums, nepeta and alchemilla back hard in July — produces fresh foliage and often a second flowering. That one is sometimes called the Hampton hack.
• If you are unsure whether a plant will take it, chop half the clump. You get an answer and you have not risked the whole plant.
• In a very wet or a very cold season, delay by a week or two rather than sticking to the calendar.
• Chopped material from healthy plants goes straight on the compost heap and is excellent green material.

Common mistakes

• Chopping a peony, poppy or iris — They flower on growth that already carries buds. Cutting in late May removes the whole year's flowers.
• Chopping too late, into June or July — The plant has not enough season left to regrow and flower, so you lose the display rather than delaying it.
• Chopping newly planted perennials — They need all their foliage to establish a root system in the first year.
• Leaving the plant dry afterwards — Regrowth needs water. A dry spell after chopping produces a stunted, poorly flowered plant.

Questions people ask