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 and alchemillas, look sad and bedraggled after they have flowered. Cutting the whole plant down to soil level will not only make the garden look tidier but will also produce a fresh crop of foliage and flowers later in the season.
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 Salvia ‘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 new, fresh, healthy foliage.
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 that are treated in this way are growing in rich soil and that they do not dry-out after the trauma of being cut down.
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 under the weight of their flowers. Cutting back their stems by two thirds during 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 in order to produce shorter or later-flowering plants. Many British gardeners are experimenting with her ideas to see how they adapt to British growing conditions.
Leaving the dead stems of grasses though the winter provides some 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 secatuers, 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 then shears will be necessary, but leave about 10cm of the old stem to avoid cutting through the new growth.
By the end of winter the giant oat, Stipa gigantea, is looking very untidy, with lots of 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. This is good advice if you have an under-gardener to do it for you, but it’s 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.