Anybody who grows perennials in their garden is familiar with the idea of cutting off the dead tops of their plants in late fall or early spring. By nature, most flowering perennials will die back to ground level in the fall, leaving behind their dried-up tops; cutting these back at the appropriate time is a familiar and easily understood task.
There are many other pruning techniques that gardeners can make use of at other times of the season, in an effort to coax the best possible performance and garden worthiness from their perennials. These are less familiar tasks to most gardeners, but worth exploring at this point in the season when a quick and timely trim can produce some amazing results. I’ve broken the techniques into a few basic groups.
This idea is a simple one and fairly familiar: by trimming off the faded flowers, many perennials can be coaxed into producing more buds and flowers, rather than wasting their energy forming seeds. For certain plants (peonies, for instance), although no amount of deadheading will trick them into repeat bloom, plants look so much better after deadheading that it becomes part of the regular list of summer chores.
New gardeners often ask us how far down to cut below the flowers, but unfortunately it’s not always an easy thing to explain, each type of plant responding in a slightly different way. Experimentation is the best way to learn this; after playing around with it for awhile, most gardeners sort of develop an instinct about where exactly the cut should be made. A few general tips:
Light Shearing Back
With many of the spring and early-summer flowering rock garden perennials, a light shearing after they finish flowering will keep them in top form for many years. This helps to maintain a dense and bushy habit, keeps them from dying out in the middle too quickly, and also prevents them from self seeding. Shear these types of plants back by half after flowering: Wall Cress (Arabis) Rock Cress (Aubrieta), Perennial Alyssum (Alyssum and Aurinia), Evergreen Candytuft (Iberis), and Moss Phlox (Phlox subulata).
Cutting Back Hard
With just a few exceptions, the vast majority of late spring and early summer-blooming perennials take a fast nosedive after flowering, and can look fairly hideous later in the summer. The mounding types of Cranesbill Geraniums and Silver Mound Artemisia are just two examples of plants that are notorious for looking terrible by July. With both of these a hard shearing back will encourage a new round of fresh, healthy and compact foliage to be produced, causing the plants to actually be an attractive addition to the border during the heat of summer. Cut Cranesbill Geraniums back more or less to just above the ground after blooming, taking care to leave the already-forming new foliage in the centre of the clump untouched. With Silver Mound, I like to shear the whole works back to 2 inches in height as soon as the tiny round silver ball-shaped flowers appear in mid to late June. This hard-pruning technique also works well with any of these: Lady’s Mantle (Alchemilla), Old-fashioned Bleedingheart (Dicentra spectabilis), Chrome Spurge (Euphorbia polychroma), Catmint (Nepeta), Blue Salvia, Meadow-rue (Thalictrum), Spiderwort (Tradescantia).
Pinching for Height Control
Pinching of fall-blooming Garden Mums (Chrysanthemum) is a familiar technique for most perennial gardeners. The same basic concept works for these late summer and fall bloomers: Monkshood (Aconitum), Michaelmas Daisies (Aster), Joe-pye Weed (Eupatorium), Helen’s Flower (Helenium), Beebalm (Monarda), Summer Phlox (Phlox paniculata), Autumn Stonecrop (Sedum) and many, many others.
June is the ideal time for cutting these back. Pruning off one half to two thirds of the growth will result in an especially bushy plant, with reduced height and often considerably more flowers, although smaller in size. This technique is especially useful to reduce the need for tedious staking. Pinching too late in the season can cause some of the autumn bloomers to flower so late that the frost gets the blooms before they have a chance to open. In general, pinching fall bloomers no later than the beginning to middle of July is wise, across Canada and in the northern States.
The Ultimate Pruning Reference
An indispensable reference book for the addicted perennial gardener is The Well-Tended Perennial Garden by Tracy DiSabato-Aust (1998, Timber Press ISBN 0-88192-414-8). Tracy’s colleagues refer to her as “the deadhead Queen”, and she offers an extremely thorough look at every aspect of perennial garden maintenance. Chapters on site and soil preparation, pests and diseases, plant selection, pruning methods, staking and everything else a gardener needs to do are both detailed and entertaining to read. Especially useful is the A to Z encyclopedia of perennials, with all the maintenance requirements for each plant laid out in a quick and concise format.
— John Valleau, Corporate Horticulturist