It’s time to be courageous and cut back your perennials to extend their bloom time and encourage repeat blooming. Because we have had such a warm winter, and an even warmer spring, perennials in this area are about three weeks ahead in growth than normal. Therefore, traditional June tasks need to be done in the next few days of May.
Pruning back summer- and autumn-flowering perennials before flowering is essential to maintaining their health and to extending bloom time into late fall. Perennials such as blackeyed Susan (rudbeckia), bee balm (monarda), tall-growing asters, sedum (particularly "Autumn Joy"), coneflower (echinacea), Shasta daisies (leucanthemum) and tall-growing forms of goldenrod (solidago) are good subjects for this pruning.
For many years, my gardening friends expressed amazement that black-eyed Susans and bee balm bloomed in my garden well into late September and early October, and that my sedum "Autumn Joy" was still upright with bright buds well into November. I had noticed that cutting the blooms from my overeager perennials and bringing them in to use in flower arrangements seemed to encourage them to bloom more vigorously and last longer, but it wasn’t until I read "The Well Tended Perennial Garden" by Tracy DiSabato-Aust (2006) that I truly understood why. This book is a “must have” for any garden library.
Early pruning can strengthen perennials, prevent them from becoming “leggy," and encourage perennials to increase branching and achieve a fuller silhouette. Pruning sections of an area planted with perennials such as blackeyed Susan and bee balm will create height variations as well as a staggered bloom time.
Here are some guidelines for a number of commonly grown perennials:
- Bee balm (monarda) seems to have grown the most quickly this spring. Ideally, the plant should be cut back to 6 inches when 12 inches tall, but my monarda are already 24 inches in height. If you just can’t bear the thought of losing 12 inches of plant material, prune those in the front of the garden bed to 6 inches and work towards the back of the bed in stages, leaving taller plants in the back.
- Stonecrop (sedum "Autumn Joy") should be cut back when the plant is between 8 inches and 12 inches tall. Prune or “pinch” about 4 inches from each stem, shaping the plant as you go. Don’t be alarmed at the misshapen look — those lovely buds will grow back quickly. And, you will have prevented your sedum from flopping over in the fall.
- Coneflower (echinacea) should be cut back to 12 inches when the plant reaches about 24 inches in height. This will cause the plant to flower about two weeks later than usual (early July rather than late June), but it will also keep the plant flowering well into September.
- Shasta daisies (leucanthemum) can be cut back by half when they reach 12 inches; they also benefit from deadheading (pruning off the dead flowers) to encourage repeat bloom through the summer.
- Aster: Tall-growing forms of aster should be cut in in half by late May; this will prevent them from becoming lanky and losing leaves towards the base of the plant.
- Blackeyed Susan (rudbeckia): Both the hirta and the "Goldstrum" varieties of rudbeckia will benefit from cutting back to 6 inches when the plants reach 12 inches in height. As with bee balm, gardeners can add variety to the flower bed by cutting some plants back and leaving others in the same bed to bloom taller and earlier.
- Goldenrod (solidago): Tall-growing forms of solidago such as "Golden Fleece" or solidago rugosa tend to flop and get bedraggled by the end of the season. Cutting back by one-half when the plants reach 12 to 16 inches will result in a more compact growth that will flower in August and last through September.
If you cut back your perennials now instead of waiting until after they begin flowering, bloom time may be slightly delayed, but you will be enjoying healthy plants and blooms well into the fall.
This spring pruning is not to be confused with the technique known as “pinching,” in which the tip and first set of leaves are removed or “pinched off." Rather, I am advocating the removal of several inches of plant material when the plant reaches 8 inches or so in height.
This technique can seem drastic and somewhat traumatic for a gardener, but the end result of a blooming autumn garden will be well worth the temporary pain and sacrifice.
Eleni Silverman is a certified Master Gardener, Vice President of the Belle Haven Garden Club, Chair of the Landscape Committee at Thomas Jefferson High School for Science and Technology and author of the garden blog "Belle Haven Garden Maven." She admits to a fascination with all things gardening, and will eagerly discuss the relative merits of leaf mold versus hardwood mulch. Please let her know if you have any comments or questions about this column or gardening in general. She would love to hear from you.