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RHS Journals

The Garden
August 2005

Off with their heads

Phil Clayton looks with renewed interest at gardening techniques that can be used to prolong border interest and manipulate flowering time

Images: Simon Garbutt, with thanks to Rasell’s Nursery, Lincolnshire

Pruning is a task some gardeners, rather irrationally, develop a fear of, especially with herbaceous plants. While many shrubs have a ‘shear without fear’ reputation, cutting back and pinching perennials, especially while in growth, can make many of us feel distinctly uneasy. However, give them credit: many of our border favourites are more forgiving than we realise and can be manipulated with pruning to suit our needs.

Without attention, by midsummer, gardens can lose their freshness, with some plants appearing overgrown or simply spent. Other plants flower late to extend the season but, by this stage, can appear out of scale or self-conscious after earlier bedfellows have faded.

Plants that were splendid in spring often start to cause problems by summer. Most objectionable to my eye are Pulmonaria with flowers that have run to seed and the coarse foliage now shabby and grey-white with mildew in dry weather. I simply crop the lot off back to the ground in June, spread a handful of pelleted chicken manure over the crowns and soak the clumps. In about two weeks, large, fresh healthy leaves emerge and the dishevelled stage is forgotten. Related Brunnera responds similarly, as does Lamium.

Cutting back for fresh foliage

Many early-summer-flowering plants are treated similarly. Papaver orientale is one of the first really spectacular herbaceous plants of the season, but is short lived in flower and descends into a particularly untidy decline. The seedheads do little to redeem it. If you have nothing in place to mask it in a border (there is an old trick of growing clematis over the corpses), it is best cut down promptly then mulch it with garden compost. Soft new leaves will soon fill your gap.

Much the same is Alchemilla mollis. Once the green flowers have turned brown (before, if you do not want seedlings) in early July, shear off flowers and older foliage above the crown when new leaves appear. Nepeta (catmint) is also inclined to be ragged after flowering, especially if it has received unwanted feline attention. A chop to about 7.5cm (3in) results in fresh shoots that may flower in a warm summer. If you feel a touch hesitant, try staggered pruning, which will avoid a large hole. Simply cut half the plant, wait for new shoots, then shear the other half a couple of weeks later.

A favourite plant in spring becomes an irritation in my borders by summer. Clematis recta ‘Purpurea’, grown for its dark purple non-climbing shoots, needs support. Mine is on a wigwam, but soon gets too high and flops about hopelessly. Give it a taller support, you may rightly say, but another solution is to cut it down in late June after the white flowers and enjoy the shoots again in August.

Some plants will not only produce fresh foliage but also repeat flower. Delphiniums can be swiftly deadheaded and given a feed to ensure a second display; Astrantia, Anthemis and later Phlox will respond similarly.

Pruning before or during flowering for later blooms

American plantswoman Tracy DiSabato-Aust has experimented with herbaceous plants to manipulate flowering; similar methods are possible in the UK. Reducing Helenium to 30cm (1ft) when flowering in July (left and right) results in compact, blooming plants by September (middle, foreground), by which time those allowed to grow naturally (middle, background) have faded. Alternatively, pinch between May and June for smaller, more floriferous plants.

Popular Echinacea purpurea (above) will also respond to a midsummer trim. Shortening top growth with flower buds by about 20cm (8in) (left) resulted in later, more profuse flowering on more compact plants (right, foreground). It is also possible to cut plants back by half in early June for flowers a couple of weeks later on plants 30cm (1ft) shorter. The aim of these techniques is to remove the apical dominance of stems and encourage sideshoots that will bloom. The resulting flowers, though later and more numerous, will be slightly smaller.

With arresting foliage and mauve flowers, Phlox paniculata ‘Norah Leigh’ (above) is a fine plant. Many gardeners know pinching phlox results in a more compact plant, but reducing plants by about half in June - or removing about 15cm (6in) when in tight bud in early July (left) - results in flowers at a lower height (right, foreground) compared to those plants left to develop naturally (right, rear). It also improves the variegation of the plant’s foliage, but green-leaved cultivars can be treated similarly. Once again, the size of the flowerheads is reduced.

Tracy DiSabato-Aust also recommends cutting back Veronicastrum to produce more compact, self-supporting plants, ideally by early June. However, I found that even when cut back as late as July they manage to flower rather later in the season on far shorter stems (foreground).

If cut back mid-season, Helianthus ‘Lemon Queen’ (in bloom) produces flowers on a more compact plant, and H. salicifolius grown for foliage effect, produces a more manageable plant (those in foreground).

The ‘Chelsea chop’

Canny nurserymen have capitalised cutting plants back for years. Herbaceous plants that did not sell in spring quickly look shabby in containers, and are unlikely to be sold in summer as demand for plants falls. However trade picks up in autumn. Nurserymen know some plants can be sheared off or heavily pinched, fed and put in a polytunnel to flower later, just when customers are looking for some late colour. This technique is known as the ‘Chelsea chop’ as traditionally it is carried out just after the Chelsea Flower Show. Many later-flowering daisies such as Coreopsis, Rudbeckia and perennial Helianthus, together with Veronicastrum and Sedum, are candidates.

Similar techniques can be employed in the garden. Geranium endressii can be a menace, seeding alarmingly, but few deny its use for dry soil in shade where mounds of ground- covering foliage are topped by vivid pink flowers on tall stems. As summer heats up, however, plants becomes straggly and flop, producing fewer flowers and plenty of dying leaves. Reduced to about 7.5cm (3in), fed and watered, plants will start to bloom again.

These easy tricks, however useful, can be taken a stage further and used to manipulate plant growth and even alter flowering times. In the US, plantswoman Tracy DiSabato-Aust has been experimenting with cutting back herbaceous plants at her garden in Columbus, Ohio, to get them to flower later, more profusely and produce compact and ultimately more useful plants. Her book The Well Tended Perennial Garden makes quite fascinating reading, but I wondered how much difference the American continental climate with its long, hot growing season makes. UK gardeners are aware that an Indian summer can induce some plants to repeat flower, but how do her ideas stand up in a typical British summer?

Last year (not generally one of the warmest or sunniest), with the help of a local nursery, I tried out some of Tracy’s ideas. I was rather late in setting about my job; it was late June before I got to grips with the shears so I concentrated on later-flowering plants in the stock beds, cutting some specimens and leaving others for later comparison. Many were already in bud or flower and cutting them down seemed horticultural butchery. My misgivings deepened in the following weeks. Mostly wet, cool overcast weather: hardly ideal conditions for success.

Any fears were misplaced. I returned to the nursery stock beds in September, prepared for disappointment, only to be pleased with the performance of many plants I cut two months before. While my experiments went well (for results see photos and captions), they only scrape the surface of what may be possible, and a prompt start could have produced rather more dramatic effects. Even so, many plants flowered later than those left unpruned, and at a reduced height.

Tracy’s work confirms that the closer to flowering time you prune, the greater the delay in flowering and less resulting regrowth. This means that, with planning, plants that have usually faded can be used with others that flower later, such as Tricyrtis, colchicums and nerines. Some of these will also make fine associations with shrubs displaying early autumn colour such as Rhus, some Euonymus and various Japanese maples. Importantly, compact growth is more self-supporting so reduces the need for staking, saving valuable time in spring, and, in addition, cultivation in more exposed locations is possible.

While decapitating your favourite border perennials is, at first, alarming, for spectacular, later displays it would seem fortune may favour the braver gardener.

Other perennials to cut

The following plants also suit pruning before flowering for reduced size and later flowers:
Anthemis: cut back by 15cm (6in) in both May and then June for delayed flowering.
Aster novi-belgii: pinch in late July to delay flowering - any later and display is reduced.
Eupatorium: try reducing by half when 60cm (2ft) tall for later-flowering, compact plants.
Macleaya cordata: reduce by half in May.
Monarda: in May stems can be cut by half and again in late May for flowers three weeks late.
Rudbeckia lacinata ‘Herbstonne’: cut by half in June for compact, later-flowering plants.
Sedum ‘Herbstfrude’: cut to 10cm (4in) in June for more compact plants at flowering.

Phil Clayton is Garden Writer’s Guild Horticultural Journalist of the Year

 

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