Dahlia

Pruning flowers: how to prune dahlias, grasses and more

Pruning both perennial flowers and grasses can help the plant. John Hoyland sets out a seasonal strategy for revitalising plants that includes cutting back and deadheading in summer. Photographs Gavin Kingcome

The need to prune plants in order to improve their flower and fruit production, or their shape, is not limited to trees and shrubs. Perennials and grasses also benefit from a pruning regime. “I’m off to prune the catmint” may not be a phrase you will ever hear, but cutting down the plant’s straggly stems after it has flowered is pruning by another name. Many vigorous perennials, such as hardy geraniums, nepetas, salvias and alchemillas, look bedraggled after they have flowered. Cutting the plant down to soil level will make the garden look tidier and produce a fresh crop of foliage and flowers later in the season.

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Pruning asters
Pruning asters
© Gavin Kingcome

This is not a job that calls for precision and delicacy. Use your shears to cut the whole plant down as close to the base of the stems as you can. Feeling apprehensive the first time that you do this is normal. Hold your nerve, remove the whole plant, and in a week or so young foliage and flower-buds will be replacing the mess you have removed. Last year I cut down the faded spires of the purple stemmed, violet-flowered Salvia nemorosa ‘Caradonna’ twice: once after it had flowered in June and again after its second-flowering in August. It was still producing flowers in October. Even those plants that will not re-flower significantly after being cut back, such as Pulmonaria and oriental poppies, will produce fresh, healthy foliage.

How to cut when you prune
© Gavin Kingcome

Deceiving plants into flowering for longer than they would in the wild means that they consume a great deal of energy, so make sure that plants treated in this way are growing in rich soil and do not dry out after the trauma of being cut down.

The Chelsea Chop

For many years British nurserymen and gardeners have cut down late-flowering perennials before they flower in order to produce a more compact plant. Sedums, Helianthus and Rudbeckia are notorious for flopping over in late summer. Cutting back their stems by two thirds in May produces plants that are more compact and do not slump. This tried-and-tested technique has become known as the Chelsea Chop.

The American garden writer Tracy DiSabato-Aust has described pruning a wide range of perennials in a similar way, to produce shorter or later-flowering plants. Many British gardeners are experimenting with her ideas to see how they adapt to our conditions.

Pruning asters
© Gavin Kingcome

Deadheading

Removing the dead or dying flowers from annuals and herbaceous perennials, as with roses, prolongs the plant’s flowering period. The function of a flower is to attract pollinators that will enable the plant to set seed. As soon as they have produced seed most plants will stop expending energy on creating more flowers. Taking off dead flowers before seed has formed encourages the plant to carry on producing more flowers. Deadheading is one of gardening’s easier jobs – the sort of thing you can do during a stroll around the garden on a summer’s evening with a glass of chilled Chablis. With plants that flower close to the stem, such as daylilies, Campanula and Coreopsis, you can simply pinch out the dead flowers between your finger and thumb.

Flowers that grow in panicles, or on the end of short stems, should be cut off individually. Follow the stem of the dead flower down to where it joins a main stem, and cut it off. I use lightweight snips with pointed blades, sold as florist’s snips, as these are easier to manoeuvre right down to the base of the stem you are removing. Flowers that grow on long, single stems, such as Cirsium, red-hot pokers and delphiniums, should be cut to the base of the stem. Use secateurs to cut the stem off as close to the ground as possible. Deadheading will prevent plants from seeding about, so if you want your aquilegias, for example, to sow themselves around the garden you will need to leave some flowers to set seed. Seed is also a useful source of food for wildlife, particularly in the autumn, and seeing a family of chaffinches gorge on the seed of echinaceas is a far more inspiring sight than that of a few extra flowers. Read our full guide on pruning roses. 

Pruning perennials for foliage

Not all perennial plants will re-flower after deadheading or cutting back, but removing the dead flowers and foliage will improve the plant’s appearance. These include: iris, peonies, lilies, Acanthus, Actaea, Aruncus, Bergenia, Epimedium, Ligularia, Melittis, Rodgersia.

Dahlias

Occasionally a bud and a dead flower can appear remarkably similar. The first time I was given my own dahlias to grow I carefully removed lots of perfectly formed buds, thinking that they had already flowered. In general, buds are fat and firm. On dahlias the buds tend to be rounded and hard, while the flowered ‘shell’ is pointed and squashy.

Controlling Grasses

Leaving the dead stems of grasses through the winter provides structure in the garden during the bleak season. The moment when a hoar frost sparkles on dead grasses in winter sunlight is a magical, if fleeting, moment, although in my experience it is more often seen in moody photographs of winter gardens than in real-life gardening. The time to cut down panicums, Miscanthus and other tall grasses is late February and early March, as the new shoots are beginning to push through the base of the grass. I prefer to do this with secateurs, removing each of last year’s stems individually, and being careful not to cut off the new shoots. If the clump is too large for this treatment, shears will be necessary, but leave about 10cm of the old stem to avoid cutting through new growth.

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In mild winters the bamboo-like Arundo donax may stay evergreen, and it is tempting to leave the stems alone. However good they are looking in March, cut them down to the ground as this will encourage lots more fresh, clean and healthy growth from the base. By the end of winter the giant oat, Stipa gigantea, is looking untidy, with dead leaves among the mainly evergreen foliage. When I first started to grow this plant a head gardener advised that the best way to deal with it is to cut down the dead flower spikes and then to comb through the grassy leaves to remove the dead ones. Bear in mind that this is very laborious and time-consuming. It is far easier to cut the whole plant down with shears or a mechanical hedge trimmer, in late February or early March, to form a dome about 25cm high. Within a few weeks new leaves will push through to create a lush base for the golden panicles that are the plant’s main attraction.

Stipa gigantea

Miscanthus