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Your First Garden: What You Need to Know About Cutting Back Perennials in the Fall

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Your First Garden: What You Need to Know About Cutting Back Perennials in the Fall

November 15, 2018

The notion of putting a garden to bed for the winter really resonates me. I’m not someone who has sleep issues, but the quality of my slumber is hugely affected by whether I’ve adequately prepared for bedtime. On the nights I’m able to follow through on my normal routine—brush teeth, shower, put on crisp cotton pjs, and, most important, slip into a well-made bed—I find I have a far more serene night’s rest than had I just stumbled into bed without any preparation.

So I get that it’s important to give your garden a good foundation for its wintertime hibernation—to clean it up a bit, weed one last time, protect delicate new plants, and cut back perennials. I really do. So what’s stopped me from doing it? The last part. Which plants exactly need to be cut back? Why? And how? I asked San Francisco-based garden designer Sarah Madeline Stuckey Coates for advice on cutting back perennials in the autumn.

Featured photograph by Christin Geall, from Flower Design: A Week at the Cambo Estate in Scotland.

Ornamental grasses are one group of perennials that shouldn&#8
Above: Ornamental grasses are one group of perennials that shouldn’t be cut back till late winter or early spring, as they provide continued visual interest during the colder months. Photograph via Hummelo: A Journey Through a Plantsman’s Life.

Q: How much of the plant should you cut back?

A: Sarah says there are two types of pruning: “ongoing light pruning (also known as deadheading), multiple times a year when you shear off the spent blooms before they go to seed, and an annual ‘hard prune’ when you cut the plant all the way down to the ground or severely reduce it once or twice a year so the plant looks fresh and clean when the new foliage emerges.” When gardeners talk about cutting back in the fall, they’re talking about a hard prune.

Q: Why should you cut back perennials?

A: “The short answer is the aesthetics and health of all perennial grasses and flowering plants benefit from being cut back,” says Sarah. On the aesthetics front, “perennials are often reinvigorated and perform and look better when they get cut back. The plant naturally dies back annually when it goes dormant, and you want to remove the unsightly material so that when the spring emergence happens, the new foliage comes up all fresh with no scraggly, dead stuff marring its beauty.” On the health front, getting rid of dead or dying foliage on plants discourages fungal growth, disease, and infestations. Another reason for cutting back: “Keeping plants in their lane,” says Sarah. “Prune to keep plants from overtaking other plants. If you have a more wild, Monet type garden, you’ve planted a bunch of different species in drifts that blend together. If one of them is being thuggish and taking over another, prune it away so the less aggressive plant can shine and play it’s role in the garden, too.”

Q: So, cut back all perennials in the fall?

Many seedheads provide food to wildlife and are worth keeping around through winter. Photograph by Marie Viljoen, from Putting a Garden to Bed: My Autumn Check List.
Above: Many seedheads provide food to wildlife and are worth keeping around through winter. Photograph by Marie Viljoen, from Putting a Garden to Bed: My Autumn Check List.

A: This is the tricky part. Some perennials should be left alone till spring to prune; for instance, hostas, asters, and heucheras need their foliage for protection over the winter. “And many plants look fascinating and gorgeous in their winter dormant form,” she notes, not to mention passing birds and beneficial insects “may depend on the dried flowers, fruit, seed pods, and foliage for sustenance or habitat.” Unfortunately, there is no easy way to figure out which plants are best cut back in the fall and which are best cut back in the spring (though, in general, leave woody plants like lavender and Russian sage for the spring). Here are some popular perennials that fare well with a fall cutback: bearded iris, columbine, salvia, yarrow, peonies, and day lilies.

Q: How should you deal with diseased plants or infestations?

A: “Any outbreaks of the various maladies can not only kill your perennial, they also can spread to others,” she warns. “Look closely because sometimes the culprits are quite small or on the underside of the leaves. Remove the infested parts, dispose of thoughtfully by keeping them contained so they don’t spread and put them in the trash rather than the compost.” Last, clean your pruners after each use with isopropyl alcohol or a bleach-water solution (with a 1:9 ratio of bleach to water).

Q: When is the best time to cut back in the fall?

A: “When they start to look too ratty for you and before the fresh new growth begins,” says Sarah. For plants that are frost-sensitive, wait until after the plants have gone through several hard frosts to ensure they’re dormant before cutting back.

For more on putting your garden to bed, see Putting a Garden to Bed: My Autumn Check List and Expert Advice: 7 Tips to Put Your Garden to Bed for the Winter.

For more Your First Garden posts, see:

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