Cut Back Perennials This Fall or Leave Them for Winter? Cut the Sick Ones, Leave the Rest

The mums are done and the coneflower heads have gone brown. The peony leaves are spotted, and each time you walk past the bed you wonder if this is the weekend you have to cut it all down. It isn't. Cut the plants that carry disease or pests in fall, and leave the healthy plants standing until spring. Below is a short list of what to cut now and what to leave, with the timing and cutting height for each. Most of the bed can wait.

The short answer

Cutting back in fall is about sanitation, not looks. Purdue Extension says most healthy perennials can stand all winter. The seed heads and foliage give you something to look at, feed birds and shelter overwintering butterflies. Plants with disease or pest problems are different. Those should come down in fall. Chicago Botanic Garden agrees that many perennials need to be cut to the ground in fall for sanitation reasons, and it also lists plenty that can stay up through winter.

So the job splits in two:

• Problem plants: cut in fall, after the tops are dead, and get the debris out of the bed.
• Everything else: leave it. Seed heads feed birds, hollow stems shelter insects, and dead foliage helps protect the crowns underneath.

On a golf course you don't work on every bed every week. You set things up so they look right for the people using them without constant labor. A perennial bed works the same way. A few targeted cuts in fall and one cleanup in spring will beat a full scorched-earth weekend in October. The full cut-back takes more work, and for most of the bed it's also the worse choice.

Cut these in fall

Time it right. Purdue says to wait until several hard frosts have killed the tops. Cutting before then just throws away good green leaves. Purdue's general cutting height is about 2 inches above the soil. Use hand pruners for a few plants. A power hedge trimmer will save your back on big plantings.

• Peony. Purdue and Chicago Botanic Garden both put peonies on the fall list. Chicago Botanic Garden says peonies with fungal disease should be cut back in fall "to prevent the overwintering of fungal spores." Garden Gate says to cut them 2 to 3 inches above the soil.
• Bee balm and phlox. Both often get powdery mildew, and Purdue recommends cutting them back in fall.
• Rudbeckia with blackened leaves. If the foliage turned black and ugly this summer, Purdue says to cut it in fall. Healthy black-eyed Susans belong in the next section.
• Iris. Purdue says to remove iris foliage in fall to cut down on places for iris borer to overwinter.
• Hosta. Wait until frost has collapsed the leaves. Slugs and snails hide and lay eggs in that debris, so removing it takes away their winter cover. Garden Gate suggests leaving 2 to 3 inches of each stem to protect the crown. If your hostas are still green and clean, there's no rush. Several sources say healthy foliage can wait until late winter or early spring.

Don't compost any of this. Garden Gate's advice is to never compost suspicious-looking or infected leaves, stems or flowers. A home compost pile usually doesn't run hot enough to be sure it kills spores or pests. That last point is my opinion, not a lab result. Bag the debris or put it out with the yard waste.

Leave these standing until spring

These plants do more for the yard standing up than lying in a yard-waste bag:

• Coneflower and other seedhead plants. Chicago Botanic Garden says not to deadhead plants that draw migrating songbirds or reseed themselves, such as coneflower. It also names black-eyed Susan, Baptisia and clematis as plants with seedheads worth keeping through winter.
• Ornamental grasses. Purdue points out that grasses look their best in fall and winter. Leave them up until spring, then use the hedge trimmer on the big clumps.
• Garden mums. Purdue calls mums marginally hardy and says to wait until early spring to cut off the dead tops, which can improve survival. Chicago Botanic Garden says chrysanthemum foliage insulates the crown. If the mums on your porch are perennial types planted in the ground, leave them alone.
• Lamb's ear (Stachys). Chicago Botanic Garden groups it with mums. The foliage insulates the crown, so leave it.
• Yarrow, Shasta daisy and globe thistle. These form a low basal rosette, a flat clump of leaves at the base, in fall. If the bare flower stems bother you, you can snip them, but don't disturb that rosette. It's the start of next year's plant.

Across the northern US, that dead top growth works like a cheap blanket. Garden Gate notes that leaving foliage standing through winter can help protect crowns. It holds snow, it breaks the wind, and it's free.

Why standing stems matter: native bees and wildlife

Besides the crown protection, standing stems have a second job. According to Illinois Extension, about 30% of native bees nest above ground in cavities such as plant stems. The rest overwinter in the ground. Pulling everything out early in spring disrupts both groups. The seed heads you leave also feed birds through the winter, which Purdue and Chicago Botanic Garden both point out.

That changes when you do the spring work, not just whether you do it. Illinois Extension gives a clear rule: wait until nights are consistently above 50°F before clearing or cutting back plant material. When you do cut, leave stems 12 to 18 inches tall. That height gives bees opened stems to move into and encourages spring egg-laying.

The sources don't all agree on height, because they're solving different problems. Purdue's 2 inches is a sanitation and tidiness number for fall. NC State Extension, writing for a warmer state, suggests trimming in early winter to 1 to 2 feet so bees can move in during January and February. Across the northern US, the Illinois spring approach fits better: leave the stems up over winter and cut them high once nights warm up.

A simple fall and spring routine

Here's the whole program for a homeowner with a budget and limited weekends:

Fall (after several hard frosts)

1. Walk the bed and find the problem plants: peony, bee balm, phlox, iris, mildewed or blackened rudbeckia, and hosta once frost has flattened it.
2. Cut those to about 2 inches. Leave 2 to 3 inches on peony and hosta.
3. Rake their fallen leaves out of the bed and bag the debris. Don't compost it.
4. Leave everything else and put the pruners away. That's an hour, not a weekend, and it can be done well before Thanksgiving.

Spring

1. Leave the bed alone until nights stay above 50°F.
2. Cut the standing stems to 12 to 18 inches. Use the hedge trimmer on grasses and large clumps.
3. Don't compost the stems right away, because bees may still be nesting in them. Illinois Extension suggests chopping them into large chunks and spreading them around the garden.
4. Wait a bit longer than usual before you mulch. Illinois Extension recommends that too, since many bees overwinter in the ground.
5. Leave the short stubs. New growth will cover them, or you can cut them to the ground later in spring.

The healthy plants get left alone, the sick ones get cut, and you get your fall weekends back.

Sources (10)

• Purdue University Extension, Yard and Garden: Fall cleanup of perennials
• Chicago Botanic Garden: Winter perennial preparation
• Chicago Botanic Garden FAQ: Winter perennial preparation
• Garden Gate: How to clean up your garden in fall
• Illinois Extension: Delay spring garden cleanup to encourage native insects
• NC State Extension
• The Old Farmer's Almanac: How to cut back perennials in fall
• Gardener's Path: How to overwinter hosta plants
• Gardenia.net: When to cut back hostas, fall or spring
• Epic Gardening: Fall hosta care

Photo: Echinacea purpurea 7/2021 Coneflower- by F. D. Richards, BY-SA 2.0.