Cutting Back Perennials
On my drive to work each morning, I pass a large stand of tall yarrow, a nearly indestructible perennial with long-blooming, flat-topped yellow flower clusters and feathery foliage. For months now, the flower heads, leaves and stalks have been brown and withered. I am grateful to the homeowners who leave them on display. To my eye, there is a certain beauty in the structure and muted coloring of many hardy perennials, including yarrow, in fall and winter.
Food for the birds. Some of the loveliest yet toughest flowering perennials, such as coneflower, globe thistle and black-eyed Susan, offer dried seed heads birds love. Vines like pink honeysuckle and Virginia creeper produce, respectively, red and dark blue berries in fall that help birds fatten up for winter. The American beauty berry bush has gorgeous rosy-purple berries that hang in clusters well into snow season.
Winter protection for the plant. In colder areas, the dried flowers, seed heads, leaves and even stalks collect snow and other loose leaves that help insulate the plant's crown, thereby increasing its chances of surviving the winter. This is especially true of some marginally hardy perennials, like garden mums. Cutting back can stimulate new, tender growth that may not withstand our winters; and this dieback can sometimes endanger the whole plant.
Shelter for wildlife. Many valuable insects overwinter in plant "debris," and even birds can take cover under substantial overwintering perennials.
A flag for you. The benefit of leaving the dormant plant as a marker is that you won't inadvertently dig up a beloved perennial in a spring planting craze.
Some plants that prefer not to be cut back in fall. A few examples are artemisia, asters, butterfly bush, campanula, coral bells and upright sedums. Coral bells (heuchera), for example, have a tendency to heave in soils that freeze and thaw, so leaving foliage intact adds a protective mulch through winter.
Sperennials do need fall cutback. If a plant has diseased or insect-infested foliage, cut back and dispose of the leaves (but not in your compost pile). Plants with blackened foliage - often a sign of fungal disease - should be cut back in order to reduce the innoculum that can infest the plant again next year. (Peonies, for example, are prone to fungal diseases such as boytritis blight in overcast, wet weather.) Removing most bearded iris foliage is a good practice, to reduce overwintering sites for iris borer caterpillars. And plants that are prone to mildew (phlox and hydrangea ) are usually better off without leaves during the wet season.
In most cases, it is best to wait until after the first or second hard frost, when most perennials begin to go dormant, to cut back severely. Cut the entire plant back to within two to three inches of the ground - but no more, because many perennials will form next year's new growth at bud and shoot sites located just above the soil. If you cut back too close to the ground, you might prevent the plant from reemerging in spring or summer.
Now that fall has arrived, and winter is almost upon us, think twice before you cut back the perennials in your garden.
Rachel Oppedahl is a Tuolumne County Master Gardener.