Time for the 'Chelsea Chop': The garden job to do as the Chelsea Flower Show ends which will give you more, better, healthier flowers for the rest of the summer

Herbaceous perennials are now bolting out of the ground, looking vigorous and healthy. It is precisely at this moment that some gardeners, to the bewilderment of many people, will ruthlessly cut their plants back. Applied at about the same time as the flower show, the Chelsea chop, as it is known, is simply a method of pruning some perennial plants so that they produce more blooms later in the season and generally look neater.

Pruning is a job usually confined to trees and shrubs, but there are ways in which perennials can be pruned and this, the Chelsea chop, is the most popular. The Royal Horticultural Society Dictionary of Gardening explains succinctly that pruning is a mechanism for keeping plants neat and tidy, to bring them to the desired shape and to ‘induce the plants to produce to the maximum those qualities for which they are grown’. This is exactly what the Chelsea chop does.

Two decades ago, American author Tracy DiSabato-Aust dedicated a large part of her book The Well-Tended Perennial Garden to the pruning of herbaceous perennials and did much to popularise the ‘chop’, as it is referred to by its champions. However, although the technique appears to be the ideal way to achieve tidy, floriferous plants, it cannot always be used. Perennials that are pruned in this way will never reach their true stature and their flowers are often smaller than if the plant had been left alone.

Before deciding if the approach is suitable for your plants or for the look of your garden, it is useful to consider the science of what is happening to perennials as they grow. A little bit of botany can go a long way in helping us garden. The tip of a shoot, the apical bud, is where new plant growth happens. As well as spurring on the main stem, the bud produces a hormone, auxin, that flows down the stem and inhibits auxiliary buds from developing. Without this hormone, these buds would produce new side shoots. The effect of the hormone is strongest close to the stem’s tip. Further down, towards the base of the shoot, the hormone is weak and lateral buds are able to develop.

Cutting off the tip of the primary shoot removes the apical bud and the hormone is no longer produced, causing the stem to lose its apical dominance and allowing side shoots to develop. These side shoots can now grow at the same rate as the main stem, resulting in bushier growth with numerous stems producing more flowers later in the season than they normally would.

It is not easy to dare to cut down plants that appear to be flourishing, but, having decided to go ahead with the chop, take courage — and shears — in both hands and remove the top part of the plant. Most gardeners will reduce the plant by one-third, whereas fearless ones cut away the whole top half. A light touch is advisable until you are familiar with both the technique and its effect on your plants. Gardeners with time and patience could use secateurs to cut each individual stem just above a leaf joint, but most of us have too much else to do in late May and shearing works fine. However the cuts are made, the plant will look sad for a couple of weeks, but new shoots appear quickly.

Cutting back a section of a plant and allowing some stems to grow on will produce a two-tier effect, with tall stems and short. The taller ones will flower before the shorter and thus lengthen the flowering period. For some gardeners, increasing the time a plant is in flower is an important reason for performing the Chelsea chop. Do you prefer uniform flowers at a similar height or are you happy with differing heights and flower sizes over a longer period? For many gardeners, the reason to grow, for example, Helianthus ‘Lemon Queen’ is to have the 5ft-tall wands of sunshine-yellow flowers swaying in late-summer light. Reducing its height diminishes its impact. As with so many aspects of gardening, it is a matter of personal choice.

Plants that are cut back obviously need time to regrow, so the chop is only suitable for late-flowering perennials, such as asters (now mainly known as Symphyotrichum), rudbeckia, hardy chrysanthemums, nepeta, heleniums and Phlox paniculata. Upright sedums, such as ‘Purple Emperor’, ‘Herbstfreude’, ’Xenox’ and Sedum spectabile cultivars, are, perhaps, the plants that benefit most from being cut down. Allowed to develop naturally, by the end of August the stems have flopped over and are sprawling across the ground, so that the centre of the plant is bare. Staking can help some plants, but sedums invariably seem to appear to be trussed up, however discreet the stakes. When they have been chopped in May, they remain upright and sturdy.

It is important not to be gung-ho with the technique. Ensure that a plant has time to produce its flowers after having been cut down and keep the shears away from early-flowering plants. The chop should not be confused with the practice of removing stems of perennials that have already flowered. Delphiniums, poppies and hardy geraniums will often respond with a second small crop of weaker flowers after the first flush is removed, but chopping them down before flowering could result in no flowers at all. If in doubt, resist the chop and enjoy the plants as they are naturally meant to be.

John Hoyland is gardens advisor at Glyndebourne, East Sussex

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