The first time I heard of the Chelsea Chop, I pictured one of those slightly fey and faded eateries off the King’s Road that had miraculously survived from the Biba era. The macramé lampshades and ratatouille only dissipated when I realised that people were talking about a horticultural manoeuvre. But there was still something of the closely guarded mystery about it.

The Chelsea Chop, I imagined, was a martial-arts stroke, perhaps administered with an umbrella, and invaluable in beating other buyers to the great flower-show sell-off. Finally, a nurseryman let me in on the secret: the chop was aimed at plants, not people, and was more aggressive than I’d suspected. It was a decapitation, no less. A decade later, the phrase has become smart, a shibboleth even. Everybody who’s anybody seems to be doing the Chelsea Chop. As herbaceous perennials have surged in popularity, so has this method of stopping them from surging quite as much as they’d like.

The idea is this. By the time they flower, some perennials have grown too tall for aesthetic and practical purposes. Confined to the summits of leggy stems, their blooms look ungainly and have only one shot at glory. The Chelsea Chop creates shorter, stouter plants and more prolonged and prolific flowering. In theory, the technique couldn’t be simpler: take a perennial with a good crown of thrusting new shoots and cut off the uppermost third. What remains will continue to grow, but it won’t reach the same height as its unchopped equivalent. Its stems will be sturdier, obviating the need for staking. They will also branch out, making bushier plants that are clothed, rather than merely capped, with flowers. So far, so good. There are problems, however.

The first is that word ‘Chelsea’. The chop acquired its name from growers who were producing plants for the flower show. By late May, their produce is already acting as if it were midsummer; the chop itself will have been administered long before their plants see SW3, and often under cover. It seems that some gardeners interpret the Chelsea Chop as meaning that they should lay about their perennials around the time of the show. In the open garden, this may be far too early for such surgery, and especially after a long hard winter. One should let the plants attain at least half their final (unchopped) height before going in with the shears. For many, this means that an Ascot Axe
or a Henley Hew will be more the ticket than a Chelsea Chop.

Cut around then, some perennials benefit brilliantly from the trick. Most prominent among them are late-summer-flowering members of the daisy family. But these illustrate another consideration with chopping-the habit of the plant itself. Helian-thus Lemon Queen, soaring, bushy and with flowers all over its upper branches, is an ideal candidate; likewise, the taller Michaelmas daisies and Helenium cultivars. Chop Echinacea and Rud-beckia, however, and you’ll have a dull autumn ahead. The critical difference is that the latter two bear flowerheads on separate stalks that arise from or near the base of the plant: there is nothing to be gained by reducing them, and a great deal to be lost.

Only perennials that bear flowers on the upper branches of their main stems are suitable for shearing. In other words, chop Phlox paniculata, but leave Phlomis tuberosa well alone. To take two earlier-flowering examples, Campanula lactiflora Loddon Anna repays a pinching-out (if not quite the full one-third chop) around Chelsea time; whereas Campanula punctata, with inflorescences that arise from the basal leaf clump, will simply sulk and not try to flower again until next year. Equally unsuitable are plants with spire-like inflorescences, such as Aconitum, which are meant to be tall and tapering, not truncated and tortured. Still less suitable are perennials with reedy growth, such as most ornamental grasses.

Forgive me if I’m teaching you to suck eggs, but I remember a query from a dear friend who was carried away by the sudden vogue for this technique and all the expert brouhaha around it. Having given her newly sprouting Crocosmia and Miscanthus a well-meant Chelsea tonsure, she was wondering why her garden looked so bare all summer. However fashionable the recipe may be, not every perennial is for the chop.