On my drive to work each morning, I pass a stand of three-foot-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. “Dead, ” as some might say. But I love them, and am grateful to the homeowners who leave them on display. Birds relish the dried seed heads when other garden pickings are slim; and to my eye, there is a certain beauty in the structure and muted colors of many hardy perennials, including yarrow, in fall and winter.
So, I have mostly chucked the gardening aesthetic that says anything that looks dead should be immediately whacked to the ground or pulled up. While there are some perennials that should be cut back in fall (more about that later), many are better left standing until spring. Here are some reasons why:
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. Native 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 beautyberry bush has gorgeous rosy-purple berries that hang in clusters well into snow season.
Winter protection for the plant. 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. Another aspect of winter protection is the fact that, generally speaking, cutting back often stimulates new, tender growth that definitely cannot withstand our winters; and this dieback can sometimes endanger the whole plant.
Shelter for wildlife. Many butterflies overwinter in plant “debris,” and even birds can take cover under substantial overwintering perennials.
A flag for you. Ever forget where you planted a prized perennial the previous spring? If you leave the plant standing through winter, come spring you’ll have a reminder. The other 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.
While many hardy perennials are indifferent to the issue of cutting back — they will survive either way — there are definitely some plants that prefer not to be cut back in fall. A few examples of popular perennials that appreciate being left alone until spring 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.
All of that said, there are perennials and certain situations that call for fall cutback. First and foremost, 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 definitely be cut back in order to reduce the innoculum that can reinfest the plant 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, as it reduces overwintering sites for iris borer caterpillars. And plants that are prone to mildew — think phlox, hydrangea and zinna — are usually better off without leaves during the wet season.
If you have to cut back a perennial in fall because of disease or its overwintering preference, or because you just can’t be talked into seeing the beautiful architecture of a dormant, snow-covered yarrow, here are a few tips for doing it right:
When: 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.
How: Cut the entire plant back to within two to three inches of the ground — but no more. The reason is, 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.
Whether for the sake of wildlife or a new appreciation for the beauty of dormant plants, think twice about cutting back perennials in your garden.
Rachel Oppedahl is a UCCE Tuolumne County Master Gardener. This article first appeared in the Union Democrat.