When to Cut Back Perennials: Fall, Winter or Spring Dates by City

Cut back a perennial after several hard frosts have killed its top growth if it was diseased, pest-ridden or turns to mush, and leave healthy stems and seed heads standing until late winter or spring, before new growth starts. That is what Penn State, Purdue, New Hampshire and Iowa State extension say, and for the median US ZIP code it puts fall cutting no earlier than November 4, the median first 28°F hard freeze, and spring cutting from about April 4, the median last one, until the shoots show.

The sources disagree on two things: whether to cut in fall, winter or spring, and how low to cut. We turned each rule into dates with NOAA 1991–2020 normals¹ for 25 cities and about 39,000 ZIP codes, so you can see what “after a hard frost” and “before new growth” mean where you live. Dividing a clump is a different job with its own dates: see when to divide perennials.

Fall, winter or spring? What extension services say

There are three schools of advice. Penn State and Purdue say to cut diseased plants in fall and let most healthy ones stand;^2,3 New Hampshire and Iowa State say the same and add that fall clean-up is optional;^4,5 and NC State says the best time for stem-nesting bees is a winter trim to a tall stubble.¹⁰ Minnesota’s calendar lists November for fall cutting and March and April for spring cutting.¹³ The table lines them up.

Iowa State states the shared point plainly: it is not necessary to clear-cut the perennial garden after the first freeze, because leaving the stems protects the crown and feeds wildlife.⁵ The cost is a little more work in spring, which New Hampshire accepts in exchange for wildlife value, beauty and plant survival.⁴

Cut in fall: only the plants that need it

Four groups are worth cutting in fall. Plants with disease or insect trouble come first: Penn State names bee balm and garden phlox with powdery mildew and says to destroy the stems, not compost them, and Purdue adds peonies and black-eyed Susans with blackened foliage.^2,3 Hostas come next, because their dead leaves hold slug eggs that hatch in spring.² Plants with browning or blackened foliage and bare stalks, such as peonies, daylilies, brunnera and speedwell, can go too, and so can iris foliage, which Purdue says removes overwintering sites for the iris borer.^2,3 The fourth group is weedy seeders, which Iowa State says benefit from late-season deadheading.⁵

Timing is the other half of the rule. Penn State sets it this way:

wait until several hard frosts have killed back the tops

The reason is that roots pull energy back from dying tops, and Penn State says to give them time to do it. Purdue makes one exception: badly damaged or infested foliage can go at once.³ Iowa State’s peony advice, late October or early November in Iowa, sits on top of the Des Moines median first hard freeze, October 30.⁷

NOAA’s normals have a date for the first 28°F hard freeze but not for “several”, so the table below shows the earliest date the rule can be met. The median ZIP code has its first 32°F frost on October 25 and its first hard freeze on November 4. By October 22 the median hard freeze has already passed in 21.5% of ZIP codes, by November 1 in 44.8%, by November 15 in 71.4% and by December 1 in 88.2%. So in most of the country October is too early to cut a healthy plant under the Penn State rule.

Leave standing: seed heads, grasses and tender perennials

Most healthy perennials can stand until spring, and extension services give four reasons. Seed heads feed birds: Penn State names purple coneflower, black-eyed Susan and oxeye sunflower, and New Hampshire lists coneflower, coreopsis, liatris and switchgrass.^2,4 Dead tops collect leaves and snow around the crown, which Iowa State says gives an extra layer of protection and helps shallow-rooted plants that frost-heave.^5,6 Structure adds winter interest: ornamental grasses, sedum, Joe Pye weed and the pods of Baptisia and Siberian iris.² And standing material shelters beneficial insects, so Penn State says not to rake debris from around the plants.²

Marginally hardy perennials need the stems most. Purdue says that waiting until early spring to cut garden mums can improve their chance of surviving, and Penn State adds anise hyssop, red-hot poker and Montauk daisy.^3,2 Evergreen and semi-evergreen plants such as hellebores, heucheras, hardy geraniums, dianthus and moss phlox are only tidied in spring.² Ornamental grasses stand until spring and are cut before the new shoots appear; Iowa State says April, to 2 to 4 inches from the ground.^2,8 Late emergers such as butterfly weed and rose mallow should keep a stub so you do not dig into them in spring.²

The bee question: trim in winter or wait for warm nights

Illinois Extension says 30 percent of native bees nest above ground, and some species overwinter in the pith of stems.¹² The two best-sourced answers differ. NC State sampled nearly 3,000 stems from 20 North Carolina gardens and found no nests in stems in their first winter. Its publication says:^9,10

trim between the first fall frost and the last spring frost

Its advice is to trim to a 12- to 24-inch stubble in the first winter, leave it alone and let seed heads feed birds for a while.¹⁰ Illinois puts the emphasis on the spring side. It also counts butterflies and moths wintering in stems and litter, and says to hold off clearing until nights are consistently over 50°F, to cut no shorter than 15 inches if you must tidy early, and to delay mulch so ground-nesting bees can emerge.^11,12 Maryland’s extension says to wait until late spring and remove only unhealthy stems.¹⁴

The studies measure different things, so we do not call one right. We can show how much time each rule leaves. NC State’s window, first 32°F frost to last 32°F frost, is a median of 174 days across ZIP codes (middle half 140 to 198) and at least 150 days in 68.1% of them. In Minneapolis it runs 189 days, from October 16 to April 23, and in Chicago 156 days, from November 5 to April 10. Houston has 39 days and Charleston, South Carolina 36, and 8.7% of ZIP codes have under 90.

Spring: before new growth, and what that means in dates

New Hampshire says old stalks can be cut in spring before new growth begins, Penn State says the same for grasses, and Minnesota’s calendar says March and April.^4,2,13 Illinois gives a temperature instead:¹¹

hold off until temperatures are consistently over 50 degrees overnight

We computed that rule from NOAA’s monthly lows. The normal night first reaches 50°F in the median ZIP code on May 13, which is a median of 25 days after the last 32°F frost (middle half 22 to 30) and after the median last frost in 99.6% of ZIP codes. “Consistently” would be later still. That is a long wait: in 67.8% of ZIP codes the 50°F date is May 1 or later and in 18.8% June 1 or later, and the median last hard freeze is April 4, so a March-and-April calendar is much earlier than Illinois’s rule in most places.

Minneapolis shows the gap: the last 28°F freeze is April 14, the last 32°F frost April 23 and the 50°F date May 20. Chicago’s are April 1, April 10 and May 11. In Seattle the 50°F date is May 21, 69 days after the March 13 last frost, and in Portland, Oregon it is 85 days. Dallas, where lows reach 50°F on March 19, is the opposite case. We read the late date, the 1-in-10 date for the last 32°F frost, as a margin: it falls May 3 for the median ZIP code, and 50°F nights come after it in 88.9%.

A practical order follows. Cut grasses and sub-shrubs when the sources say, tidy stems to a tall stubble as the ground thaws if you want order, and clear the rest when nights warm. For a plant already damaged by frost, wait: our frost-cover guide quotes UF/IFAS on not pruning until new growth shows. Purdue says butterfly bush, bluebeard and Russian sage are cut to a few inches above the ground in the dormant season, best before new growth begins.¹⁵ That is a spring job; for true shrubs see when to prune roses and when to prune hydrangeas.

How far to cut

The height depends on the reason. Penn State and Purdue say to leave about 2 inches so you can find the plant, and Iowa State says 2 to 4 inches for grasses.^2,3,8 NC State recommends 12 to 24 inches in winter for bees and Illinois 12 to 18 inches in early spring, never below 15 if you cut early.^10,12,11 If you cut stems earlier in the season, NC State says do not cut them again in winter, because bees can move into cut stems.¹⁰ Whichever height you choose, loosely stack what you remove: Illinois says dense piles can trap insects, and its 2022 piece says to chop trimmed stems into large chunks and spread them in the garden instead of composting them.^11,12

Plant by plant

This table collects what the sources say for the plants that cause the most questions. “Either” means a source accepts both seasons.

Mild-winter ZIP codes

None of the sources we read gives cut-back timing for places without a regular freeze, and the frost-based rules do not apply there. NOAA gives no median frost date for Orlando, Phoenix or Los Angeles, and 1,549 lower-48 ZIP codes have none; 1,454 more have a first frost but no median hard freeze. Where a plant does not die back, trim it when it looks tired and ask your county extension office. Our frost vs. freeze guide explains the terms and the frost date lookup gives the dates for your ZIP code.

How we calculated these dates

Frost and freeze dates are medians from NOAA’s 1991–2020 normals for the station nearest each ZIP code: the first and last 32°F frost, the first and last 28°F hard freeze and the 1-in-10 late spring frost.¹ The 50°F date is the first spring day the daily minimum reaches 50°F, with monthly normals placed mid-month and interpolated, the same method as our other guides. National figures use 39,017 lower-48 ZIP codes with a median first frost (37,563 with a median hard freeze; 38,063 that reach 50°F nights). Individual years vary by weeks, and “several hard frosts” cannot be computed from the normals.

Next steps

Read what a last frost date means, see our leaf-raking guide for the lawn side of fall clean-up, and browse the other flower and bulb guides.