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The Chelsea chop explained

Val Bourne / 11 May 2022

Find out why gardeners chop back their summer-flowering plants in May.

Phlox paniculata
Border phlox such as phlox paniculata benefit from having a third of their stems Chelsea Chopped to encourage a longer flowering season

What is the Chelsea chop?

The Chelsea chop coincides with the Chelsea Flower Show in the third week in May, hence the name. It either reduces all the shoots, or just the front stems, by a third and it’s traditionally used with summer-flowering herbaceous perennials.

Why do the Chelsea chop?

The Chelsea chop is a simple technique designed to reduce apical dominance – a plant’s habit of producing one single flower per stem. Following the Chelsea chop you should get more flowers per stem on a shorter, bushier plant. Flowering time is also delayed, by a month or more, which can be useful because summer-flowering perennials can be encouraged to follow the first flush of roses.

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A word of caution regarding later flowering perennials

In the past the Chelsea chop has been recommended across the board, for all perennials, even late-summer flowering herbaceous.

However, the UK climate is changing and we can no longer rely on regular rainfall. Springs have become much drier in recent years and we no longer feel able to pour water onto our borders, because water is in shorter supply than it was. Later flowering perennials, such as asters, heleniums, eupatoriums and veronicastrums, are thirsty plants and they struggle in our changing climate and often can fail following a dry spring.

Performing a Chelsea chop on these later flowering plants may stop them flowering altogether. Gardeners who live in wetter areas, in the western half of the UK, are more fortunate than those on the eastern side. They are likely to get much more rainfall. The message is, proceed with caution when it comes to taller later flowering perennials, if the rainfall in your area is light and patchy.

The Chelsea chop does not suit everything!

When plant trials are held at RHS Wisley, three plants are assessed for the AGM – or Award of Garden merit. The front one is often subjected to a Chelsea chop, so that the effects can be noted. Sometimes it works well and the perennials produce later flowers.

However, sometimes, the chopped plant produces wonderful foliage, but no flowers. This happens with summer-flowering sedums such as ‘José Aubergine', ‘Matrona’, ‘Xenox’ and ‘Karfunkelstein’.

The Chelsea chop isn’t suited to many half-tender and borderline tender plants with a Mediterranean or South Hemisphere provenance either, because the stems have to be left intact overwinter in order to protect the plant’s crown.

Plants like these, including salvias, penstemons, fuchsias and anthemis, are generally cut back hard when spring arrives and then they reshoot from base level. It doesn’t work with peonies either, because they start into life very early in the year and, by May, they are already in bud. Grey-leaved nepetas, or catmints, also start for ground level and flower early.

Give nepetas a Hampton Court haircut instead and shear them back to nothing in July so that they flower again.

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What plants to Chelsea chop

English lavender (Lavandula angustifolia)

This hardier lavender flowers in June, when left to its own devices, and plants are compact, with narrow foliage and non-tapering flower heads. Its compact habit and hardy nature lend itself to forming low hedges and this lavender can be cut back in late-August, after flowering is finished. You can reduce all the growth in the second half of May and this delays flowering until mid-July, when many roses have finished.

English lavender can live an average of ten to twenty years and excellent, compact varieties include ‘Folgate’, ‘Hidcote’, ‘Loddon Blue’, ‘Melissa Lilac’ and ‘Beechwood Blue’. Downderry Nursery in Kent offer an excellent mail order service and they sell the widest range of hardy, frost-hardy and tender lavenders.

Top tip – Pick English lavender for drying during Wimbledon.

Border Phloxes (Phlox paniculate and P. maculate)

These summer-flowering perennials respond well to a partial Chelsea chop, especially the taller ones. Just cut back a third of the stems at the front of the clump, but leave two thirds of the stems untouched and then you’ll get a longer flowering season. This works better with border phloxes than an all-over chop.

Most border phloxes need moisture-retentive soil in order to avoid mildew, which is a water-stress disease. There are three I’d recommend for drier gardens though. The first is an airy, ethereal white named P. paniculate ‘Alba Grandiflora’, because this seems to do well anywhere. The second is a pale-pink called ‘Monica Lynden-Bell’ and this was found growing in a garden in Hampshire on chalky soil. The tastefully variegated ‘Norah Leigh’ is also drought-tolerant, although you must take care not to interfere with the roots when weeding. If you do so, green shoots will pop up! ‘Becky Towe’ is a vulgar variegated phlox – that you’ll love or hate.

The other consideration is vigour, because this varies from variety to variety and some phloxes are rather wimpish in growth habit. Tough, very persistent varieties that return well include the pink-eyed blush-white ‘Bright Eyes’, the deeper pink ‘Eva Cullen’, the lavender-blue ‘Franz Schubert’, the cherry-red ‘Starfire’ and two oranges – ‘Flamingo’ and ‘Prince of Orange’. Look out for the Flame and Peacock series too. There are also taller, slightly later flowering hybrid phloxes with smaller flowerheads and they include P. x arendsii ‘Hesperis’ and ‘Luc’s Lilac’. All phloxes are scented and insect-friendly.

Golden Rod (Solidago)

The name Golden Rod strikes fear into the heart of gardeners, because they conjure up some coarse brassy yellow thug. However, there are some good golden rods with arching sherbet-yellow flowers and they include S. rugosa ‘Fireworks’ and ‘Loydser Crown’. Both are tough enough to be given the Chelsea chop, but bear in mind that normal height, of between 4 – 5 ft (1.5m) makes them an ideal companion for taller asters such as the rich-purple, September-flowering Symphyotrichum novae-angliae ‘ Helen Picton’. The decision to chop, or not, is yours.

‘Achillea’

Achilleas vary in their treatment. ‘Moonshine’, for instance, produces acrid-yellow heads of flower from May onwards with finely-cut silvery filigree foliage, so this is best left alone to produce its succession of flowers. However, later flowering greyer-leaved rust-brown and salmon-pink achilleas, such as ‘Terracotta’, 'Lachsschönheit' and 'Walther Funcke', could be cut back in May, if they have made sufficient growth.

Taller border campanulas

The classic candidate for the Chelsea Chop is Campanula lactiflora ‘Prichard’s Variety’. It can be reduced by a third, to promote later flowers, and thereafter it can be deadheaded to encourage further flushes of flower. The globular heads of mid-blue flowers, reminiscent of Cadbury’s milk chocolate foil wrapping, go particularly well with roses. You could also cut back ‘Sarastro’ and forms of Campanula latiloba, should you wish.

Later flowering hardy geraniums

Perhaps the very best hardy geranium, ‘Rozanne’, can be induced to flower in autumn after a rather drastic Chelsea chop. This shifts this sprawling, pale-blue mound maker into a fresh autumn display rather than a summer one. Left to its own devices, it flowers in summer and looks ragged by late August. When happy ‘Rozanne’ will cover a metre of ground. Cutting it back makes it later and more manageable.

Read more of our gardening articles, and find out what else needs doing in the spring garden.

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Disclaimer

The opinions expressed are those of the author and are not held by Saga unless specifically stated. The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.