May 2014 | The Garden 75

Pruning perennialsMany herbaceous perennials benefit from a range of pruning regimes, promoting

later, longer or more compact flowering, repeat displays, or to improve appearance

Though it remains a much under-used technique, you can easily trim many herbaceous perennials at strategic points in their growing cycle to manipulate growth patterns and flowering times.

In small gardens, selective trimming helps restrict growth and overcrowding, while on windy sites an early chop can limit the height of tall perennials reducing the need for staking. You can also prune to extend flowering times. Cutting back individuals in a group planting of a specific perennial delays flowering, extends the season and brings colour to fading borders. This is particularly helpful during a long, mild autumn.

Hard prune a herbaceous plant after the first flush of growth and flowers and it will often produce a new,

fresh crop of leaves (for example, Alchemilla mollis) and even occasionally a second floral display (some Geranium can be relied on). This neat trick can provide much-needed space and fresh growth in border displays. Removing faded flowers also encourages perennials to continue flowering: deadheading pays dividends if carried out regularly in the growing season.

There has been a trend in recent years for leaving the winter skeletons of perennial border plants to stand, which adds a structural element to winter gardens, especially when sparkling with frost. Many stems and seedheads also provide a valuable habitat for over win-tering insects, but those that disintegrate at the first frost (see p79) will still need an autumn trim.

After flowering, Nepeta can be cut back for a fresh flush of growth and a second show of flowers.

Author: Paul Cook, Curator, RHS Garden Harlow Carr. Photography: Tim Sandall

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May 2014 | The Garden 7776 The Garden | May 2014

Pruning perennials

Cutting back stems by one third as flower buds appear will keep plants more compact and delay flowering. This technique is also known as the ‘Chelsea chop’ because it is traditionally done around the time of the famous flower show (mid- to late May).

What to do: take a large clump of a suitable herbaceous plant and selectively prune growing stems to form a layered effect. Flowering can be delayed by about six weeks; pruned sections are shorter when they flower, so a bushier plant is created.

When to do it: from mid-May onwards, just as flower buds first begin to form. If you prune later, plants may take longer to recover and flowering may not avoid the first frosts.

How to do it: cut outer stems by up to half of their length using secateurs and making clean cuts above a growing point (node). Mulch around the clump with garden compost and keep it well watered. Remove any yellowing leaves to keep the plant tidy. Avoid using this technique on the same plant in consecutive years, as it may eventually weaken it.

Suitable plants include: Campanula lactiflora, Echinacea purpurea, Eupatorium maculatum, Heliopsis, Monarda didyma, Pholx paniculata, Rudbeckia ‘Herbstonne’ and Solidago.

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The back of this clump has been left to grow naturally, the taller stems flowering at the normal time.

The front section has been reduced in height by around half in mid-May. Later-flowering shoots will form lower down.

Reduce stems for lower, later flowers

Alternative method: cut one in three stems to stagger floweringRather than cutting down sections of the perennial, reducing stems selectively (thinning) throughout the whole clump can provide a softer, more natural effect. Try cutting back one stem in three; cut stems should grow back without being masked by the earlier-flowering, uncut stems and thus will prolong the overall flowering period.

Phlox paniculataCultivars of this perennial prove to be well suited to the ‘Chelsea Chop’.

The clump seven weeks after pruning, showing the

delayed, lower flowering.

Cut back perennials to maintain bushy growthShortening developing stems of perennials that tend to collapse and leave an open centre later in the year helps to keep plants tidy and compact.

What to do: cut back the vigorous early growth of some perennials (such as border Sedum) by half to prevent long stems flopping over later in the year, leaving an open centre. These heavy stems can be difficult to support without unsightly staking; by reducing stems, a more compact bushy plant is produced.

When to do it: in late spring as the developing stems are growing.

How to do it: shorten stems by half with sharp secateurs. The soft growing points can also be pinched out later as they develop.

Suitable plants include: Amsonia, Aster, Helenium, Leucanthemum, Sedum telephium, S. spectabile, S. Herbstfreude Group and other similar border sedum cultivars.

Collapsed stems of Sedum ‘Gooseberry Fool’ leave an

unsightly open centre.

Late-spring trim for late-summer effectIn May or early June cut back stems of sedum to around half their height. The cut stems develop sideshoots that go on to produce flowers on bushier plants.

Remove old flowers to prolong blooming Cutting off faded blooms will improve plant appearance and encourages further flower production, extending the season.

What to do: deadhead unsightly faded flowers to improve and freshen the look of plants, and to prevent seedheads or seed pods forming. While often attractive, these can limit further growth and flower production. Removing old blooms causes the plant to produce more flowers.

When to do it: as soon as flowers fade, remove them as part of regular garden maintenance. Little and often is preferable.

How to do it: softer flower stems can be removed with a pinch but cut stringy stems with secateurs. Long flower stalks should be cut to the base of the main stem.

Suitable plants include: Anthemis, Cephalaria, Dahlia, Delphinium, Dianthus, Lupin, Tanacetum and Scabious. Cut the long stalks of Lupin and Delphinium low down once all the flowers have faded; smaller spikes may appear later in the year.

Diligent deadheadingCut long flower stems of plants such as Tanacetum coccineum as low as possible to hide unsightly stalks.

Remove old dahlia flowers to prolong displays into autumn.

Thin Monarda stems by cutting back one in three.

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May 2014 | The Garden 7978 The Garden | May 2014

Pruning perennials

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Gardeners often anticipate how plants respond to pruning, but what are the

physiological mechanisms triggered by our actions, and how do they work?

Trimming growth tips (apical buds) of perennials such as Sedum and Helenium results in sideshoot growth (from axillary buds) and stockier, bushy plants. An interaction of mainly two plant hormones – auxins and cytokinins – regulates this process.

Auxins, synthesized in the apical bud, suppress growth of axillary buds below. When the apical bud is removed (by gardeners or by grazing herbivores) it changes the balance of hormones in the shoot. Cytokinins determine growth of axillary buds, so reducing auxin levels stimulates sideshoot activity.

Why some perennials suit the ‘Chelsea chop’ (see p76) better than others is not clearly understood but may relate to the distribution of axillary buds or to the control of buds after apical dominance is removed.

Pruning influences other processes. By removing photosynthesizing leaves, trimming initially reduces photosynthetic rate and the plant undergoes a process of wound healing. But this ‘shock’ effect is temporary and, after a day or two, photosynthetic rate actually increases by 20–40 percent as resources within the plant are directed into replacing the lost parts.✤ With thanks to Tijana Blanusa, RHS Senior Horticultural Scientist.

Science of plants and pruning

Helen Bostock, RHS Senior Horticultural Advisor

What to do: some early-flowering perennials can be revived by cutting them hard back after displays of flowers start to fade. If the pruning is done promptly, a second flush of growth and in some cases flowers add to late-summer displays.

When to do it: as soon as flowers fade and growth begins to flop or look tatty then it is time to cut back. Plants will need time to recover from their trim, so act promptly.

How to do it: shears are best to reduce the tangle of stems as close to the ground as possible. Tidy up with secateurs afterwards, if necessary, but the new growth will soon cover any cut stubs. It also pays to mulch and water around plants to give them a kick start.

Suitable plants include: Nepeta, some Geranium, Pulmonaria and Alchemilla mollis.

Shear back untidy growth after floweringCutting plants back hard after flowering encourages a flush of fresh growth and even a further display of flowers, if done early enough.

June: before cutting backNepeta ‘Six Hills Giant’ in flower with stems beginning to fall open from the clump. Flowering is starting to decrease and faded flowers are appearing.

August: a week after cutting New growth is appearing from the base. The short stems continue to wither and will soon be covered by a mass of fresh, emerging shoots.

Sept: six weeks after cuttingNew shoots clothe the now neat, bushy plants and a second (if rather reduced) flush of flowers starts to appear on the fresh growth.

Rejuvenating Geranium

1 By August growth is becoming straggly.2 Crop back top growth to just above soil level.3 After five weeks top

growth has regrown into a neat mound of foliage. Some selections such as G. endressii or popular G. x oxonianum ‘Claridge Druce’ may produce more flowers.1

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Removing apical shoots often stimulates axillary bud (sideshoot) growth.

Pulmonarias can be cut to the ground in early summer. Fresh, healthy foliage will arise.

Cut back dead growth before winterWhat to do: while some plants with ornamental seedheads or skeletons can be left to stand for winter (see below), those that collapse after frost need clearing away before wet weather makes the messy task of cutting back more difficult. Some grasses from cooler climates restart growth in late winter and are best tidied in autumn, along with some herbaceous perennials.

When to do it: once plants flop in late autumn, usually after frosts have started.

How to do it: cut stems with a knife or secateurs as close to the ground around the ‘crown’ of the plant as possible.

Suitable plants include: Aconitum, Geranium, Hosta and Phalaris. Grasses from cooler climates including Calamagrostis, Deschampsia, Festuca, Hakonechloa, Holcus, Melica, Milium, Phalaris and Stipa. These typically restart growth from late winter, flower early and are best planted or divided from autumn until late spring.

Prune in spring after winter displaysRemove old stems, seedheads and foliage to expose new growth.

What to do: many perennials are sturdy enough to retain their structure after the first frosts strip their leaves. Dead stems and seed heads can be left over winter as an attractive addition to the garden but will need removing before the new growth starts in spring.

When to do it: in spring before the emergence of new shoots, usually from March onwards.

How to do it: use a knife or shears to cut close to the crown of the plant.

Suitable plants include: Achillea filipendulina, Aconitum, Eryngium, Foeniculum, Miscanthus, Pennisetum, Phlomis russelliana and Sedum.

End of season tidy upRemove soft stems and foliage from beds before winter to clear the way for the resumption of growth in spring.

Before winter, remove stems of perennials likely to disintegrate in frosts.

Cut faded stems as close to the ground as possible but keep any early basal shoots.

Crop old growth of grasses close to the ground before new shoots arise in spring.

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80 The Garden | May 2014

Pruning perennials

Spring pruningSome perennials need a spring tidy to remove tatty, overwintered foliage or to promote new shoots from woody stems.

Find out moreFor more information, search ‘Cutting back perennials’ at www. rhs.org.uk

Further reading: ✤ RHS Encyclopedia of Perennials, Editor-in-Chief, Graham Rice, Dorling Kindersley, 2006, isbn 9781405306003 rrp £25

✤ The Well-Tended Perennial Garden, by Tracy DiSabato-Aust, Timber Press, 2006, isbn 9780881928037 rrp £25

Revealing early bloomsCut old foliage to the ground to expose new flowers.

What to do: foliage that has lasted throughout winter may help to protect emerging flowering shoots but it can look unsightly, mask the flowers, harbour fungal disease or even provide cover for rodents that often nibble emerging flower buds. Remove old leaves early in the New Year to allow the flowers to be enjoyed.

When to do it: late winter or early spring, just before the new flush of growth emerges.

How to do it: remove tatty leaves as close to the ground as possible. If new growth is appearing take care not to damage the new shoots.

Suitable plants include: Epimedium, Helleborus x hybridus (above) and Pulmonaria.

Renewing frameworkReduce stems of perennials which grow from a framework of overwintering woody stems to help keep them compact.

What to do: woody perennials such as penstemons and salvias that produce a woody crown with stems that survive winter in mild areas can shoot from the base or from branching points higher up on the stems. Prune to keep plants bushy and to prevent them becoming too woody and leggy.

When to do it: after winter and the risk of hard frost has passed, usually late April onwards, and as strong shoots start to appear from the branches.

How to do it: shorten woody stems selectively to a healthy shoot. If there is plenty of healthy growth at the base of the plant, stems can be cut down further to allow new shoots to arise.

Suitable plants include: Pentstemon (below) and some Salvia.

Calendar of pruning perennialsSpRing

✤ April: reduce stems of woody perennials to a framework (Salvia).✤ Late May / early June: cut back upright foliage stems of perennials by a third to delay

flowering (such as Phlox) or to encourage bushy growth (Amsonia, Sedum).

SummeR✤ June to August: shear back untidy foliage after flowering (Pulmonaria, Geranium).✤ June to September: deadhead flowerheads as necessary (such as lupin, dahlias,

Gaillardia, Helenium, Knautia and Penstemon).

winteR ✤ Late Jan / early Feb: remove foliage from hellebores and epimediums.

✤ February: cut back grasses and perennials with attractive seedheads left standing for winter interest as they collapse.

Autumn✤ October: cut back soft foliage and stems as they die back (such as Amsonia,

Hemerocallis, Hosta, Monarda, Paeonia and Rodgersia).rh

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