It’s trees and shrubs we typically think of when we reach for our pruning shears, but according to Louis Bauer, we ought to include perennial and annual flowers as well.
Bauer is the director of Horticulture at New York City’s great public garden, Wave Hill, and, among other things, one of the most technically accomplished gardeners I have ever met. As mentor to the younger generation of Wave Hill staff, Bauer has been intent on passing along skills, and it’s his high standards that are reflected in Wave Hill’s meticulous beauty. Accordingly, when I had questions recently about pruning flowers, I turned to Louis.
Bauer offered a number of reasons for this practice. One obvious effect of pruning flowers is to direct their growth and control their form. Pruning back the flowers, for example, can encourage more compact, denser growth. This can greatly reduce the need for staking. Another potential benefit of timely pruning is increasing the plants’ bloom. Cutting back plants’ stems encourages suppressed buds below the cut to sprout and form a number of new shoots where formerly there had only been one. This, in turn, increases the number of flower-bearing branches. An example of how this can work is the cushion-shaped chrysanthemums you’ll find in garden centers in the fall; it’s shearing that produces both their form and the fact that they are covered with blossoms.
Pruning back flowers also sets back their growth and delays flowering for up to a couple of weeks. This, too, can be an advantage, for if you prune back some perennials and not their fellows, you’ll have flowers blooming both early and late, and overall a longer season of bloom. Bauer suggested pruning back selected perennials at the front of a border, where the more compact growth won’t block the view, and leaving their relatives inside the border or at its back unpruned, so that their greater height will help their flowers to emerge from the surrounding foliage.
Pruning enhances a number of late-blooming perennials, not only chrysanthemums but also asters, heleniums, summer phloxes, and Joe Pye weeds, which have a tendency to grow tall and flop over. It can also benefit a number of annual flowers, such as cosmos, coleus, petunias, snapdragons, zinnias, impatiens, and salvias. When applied to annuals, the process is best performed early, when the plants are only six to eight inches tall. At that time, you can use the tips of your thumb and forefinger to pinch off the topmost pair of leaves. You may wish to treat the new shoots that emerge in the same fashion a few weeks later to further boost the bushiness and floriferousness of the plants.
For the perennials, Bauer recommends what is called “the Chelsea chop.” That’s the more drastic process of shearing back the shoots by a third or so at about the same time that Britain’s famous Chelsea Flower Show takes place, typically in mid- to late May (the show is taking place in September this year thanks to COVID-19, but the timing of the chop shouldn’t change). Because our spring runs a little later in the North Country, early June is a good time for us to impose this discipline on our perennials. It can be done even later, though Bauer warns that a shearing after July 4 will not give the new growth time to produce flower buds, so that the bloom will be decreased.
If you’ve got lots of perennials to cut back, you can use a sharp-bladed pair of manual hedge-trimming shears. For just a few flowers, a pair of flower snips or a good pair of by-pass pruning shears, such as Felcos, will do the job just fine.
This treatment is not for every flower. Some, such as daylilies, won’t tolerate it. Cutting back their flower stems or “scapes” simply prevents them from blooming. In the case of some others, such as delphiniums, pruning will destroy the stately form that lends the plants their special beauty. Flower arrangers may decide that they would prefer to harvest fewer flowers, but ones with longer stems.