September is the month when a perennial border can look finished long before the gardening season actually is. Spent flower stalks lean across paths, mildew turns leaves gray, and once-full clumps collapse into their neighbors. These 11 perennials can take a decisive cutback now—when the plant gives you the right cue.
1. Peonies
Herbaceous peonies are one of the strongest reasons to keep pruners handy in September. Once their foliage is yellowing, spotted, or collapsing—and flowering has been over for months—the top growth has finished its job. Removing it also takes away places where botrytis blight and peony leaf blotch can overwinter.
How hard to cut: Take every herbaceous stem down to about 1–2 inches above the soil. Collect the leaves and stems instead of leaving them as mulch, and trash any diseased material. Wipe the blades with disinfectant before moving to the next peony.
Timing cue: Cut when the leaves are clearly declining or after the first light frost. If your peony is still solid green in September, let it keep feeding the roots and wait. This advice is for herbaceous peonies—not woody tree peonies, which should not be chopped to the ground.
2. Hostas
Hosta foliage can go from handsome to soggy almost overnight after a frost. Once the leaves turn yellow or collapse, they no longer help the crown. Removing them now makes spring cleanup easier and reduces the damp shelter favored by slugs, snails, and their eggs.
How hard to cut: Gather each clump and cut the leaf stems to 1–2 inches above the crown. Do not slice into the crown itself. Rake away the loose foliage and old flower stalks, then check the soil surface for slug eggs.
Timing cue: A frost-blackened or uniformly yellow clump is ready. Healthy green leaves are still making energy, so leave those alone until they fade. In warm climates where hostas decline later, September may be too early.
3. Daylilies
By September, many daylily clumps are a mix of tired leaves, dry tips, and empty flower scapes. A hard cleanup restores order and removes foliage that can harbor leaf streak and other debris-borne problems.
How hard to cut: Pull or clip the spent flower scapes first. Then shear dormant and fully yellowing types to a neat fan 4–6 inches tall. Comb out loose brown leaves with gloved hands and clear the debris from the crown.
Timing cue: Wait until most foliage has yellowed or flopped. Evergreen and semi-evergreen daylilies in mild regions may keep useful green leaves; on those, remove only brown or damaged growth rather than scalping the whole clump.
4. Bee Balm
Bee balm often reaches September looking gray with powdery mildew and bare around the lower stems. Once its main bloom is finished, there is little benefit in keeping diseased top growth. A clean cut improves air movement around the crown and removes a major source of spores.
How hard to cut: Cut stems to 2–3 inches above the ground. Bag or trash mildew-covered leaves instead of composting them in a cool home pile. Thin overcrowded clumps in spring, and avoid piling mulch against the crown.
Timing cue: Cut after the flowers are finished and the foliage is declining. Healthy hollow stems can shelter overwintering insects, so wildlife-minded gardeners can leave a few clean stems standing in an out-of-the-way section and clear them in spring.
5. Garden Phlox
Tall garden phlox is another late-season magnet for powdery mildew. When bloom is over and the lower half of the plant is yellow, spotted, or bare, cutting it down removes tired growth and keeps infected leaves from blowing around the bed.
How hard to cut: Clip every stem to 2–3 inches above the crown. Gather fallen leaves from beneath the plant, especially if mildew or leaf spot was present. Disinfect pruners before working on another clump.
Timing cue: Wait for the final flowers to finish and the foliage to decline. If a late-blooming cultivar is still flowering for pollinators, deadhead the finished trusses and postpone the hard cut until bloom ends or frost arrives.
6. Catmint
Catmint can sprawl into neighboring plants after its summer flush. In places with a reasonably long fall, an early-September shear removes the wiry bloom stems and exposes the tidy mound of basal foliage underneath. It may even produce a small fresh flush before cold weather.
How hard to cut: Shear exhausted stems to about 3–4 inches, but keep any fresh basal leaves intact. Remove the cut material so the crown stays open and dry.
Timing cue: Make this cut early in September where several weeks of growing weather remain. In cold zones facing an imminent hard freeze, skip the regrowth haircut and simply remove dead top growth after frost. The same technique works for common ornamental Nepeta, although the pictured species is Nepeta cataria.
7. Perennial Salvia
Hardy perennial salvias such as Salvia nemorosa often finish their second flush by September. Their old flower spikes turn brown, lean outward, and hide the low rosette that will protect the plant through winter.
How hard to cut: Follow the spent stems down and cut to 2–4 inches, stopping above healthy basal growth. Clear away the trimmings and keep mulch from covering the crown.
Timing cue: Cut after the final flush is spent or when frost has blackened the stems. This applies to herbaceous perennial salvia—not woody culinary sage or tender tropical salvia, which follow different pruning rules.
8. Yarrow
Yarrow stems tend to lodge after wind and rain, leaving a loose nest over neighboring plants. Once the flower heads are brown and the stems are no longer attractive, a hard cut tidies the clump and encourages attention back to the low basal leaves.
How hard to cut: Cut spent stems to 2–3 inches, preserving any green basal rosette. Remove matted or diseased foliage, but do not bury the crown in heavy mulch; yarrow prefers good drainage.
Timing cue: September is appropriate after flowering has ended. If the seed heads are upright and you like their winter silhouette, leaving them is also valid—this is a “can cut,” not a “must cut.”
9. Coreopsis
Many perennial coreopsis plants look thin and exhausted after weeks of blooming. A September shear removes seed-heavy stems, limits unwanted self-sowing, and leaves the bed looking intentional instead of abandoned.
How hard to cut: Shear large-flowered and threadleaf perennial types to roughly 2–3 inches, or just above a healthy basal tuft. Clear the clippings rather than letting them mat over the crown.
Timing cue: Cut after the final major bloom. Stop if the plant is still pushing buds or if a very late cut would force tender growth just before a hard freeze. Check the label first: annual coreopsis species will not return simply because they were cut back.
10. Black-Eyed Susans
Black-eyed Susans can be cut hard once their stems brown, especially when leaf spot, rust, or mildew has made the patch unattractive. Removing infected foliage can lower the amount of disease carried into next season.
How hard to cut: Cut stems to 2–4 inches above the soil and remove all spotted leaves from the bed. Mark the clump if its crown will be hard to see during spring cultivation.
Timing cue: Wait until flowering is over and the plant is declining. Healthy seed heads feed finches and add winter structure, so consider leaving a portion standing and cutting only the diseased or badly flopped stems. Also remember that Rudbeckia hirta is often short-lived, while other black-eyed Susan species are longer-lived perennials.
11. Shasta Daisies
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Shasta daisies rarely improve after their last flowers fade. The stalks lean, leaves yellow from the bottom up, and a dense clump can trap moisture around the crown. September cleanup keeps the plant compact and removes old foliage that may carry leaf spot.
How hard to cut: Cut finished flowering stems and declining foliage to 2–3 inches above the crown. Keep any clean green basal leaves that remain, and pull away loose debris without damaging new buds at soil level.
Timing cue: Make the hard cut when flowering is finished and most top growth is yellowing, or wait until frost. If the clump is still green and vigorous, deadhead now and delay the full cleanup.
A Fast September Cutback Checklist
- Cut now: foliage that is yellow, frost-blackened, mildewed, spotted, or completely finished.
- Wait: healthy green foliage, unopened buds, and plants still flowering heavily.
- Preserve the crown: leave the recommended short stubs and keep mulch from smothering the plant’s center.
- Control disease: trash badly infected material and sanitize blades with 70% isopropyl alcohol between plants.
- Make room for wildlife: where stems are healthy, leave a few seed heads or hollow stems for birds and overwintering insects.
A hard September cutback is not about making every bed bare. It is selective cleanup: remove what is finished, keep what is still working, and let each crown head into winter clean, visible, and unburied.
Image Credits
All images below are hosted by Wikimedia Commons and displayed at a responsive size without altering the source files. Each title links to its Commons file page.
- Peonies: Pæonia lactiflora.jpg by Kelvinsong (CC0 1.0); Paeonia lactiflora.jpg by Andrei Stroe (CC BY-SA 3.0).
- Hostas: Hosta leaves.jpg by W.carter (CC BY-SA 4.0); Hosta Plant In Garden Hampshire UK.jpg by Si Griffiths (CC BY-SA 3.0).
- Daylilies: Blooming Hemerocallis fulva.jpg by Miron A. Stratan (CC BY 4.0); Hemerocallis fulva FLC.jpg by Freak-Line-Community (CC BY-SA 4.0).
- Bee Balm: Monarda didyma.jpg by uwbotanicgardens (CC BY 2.0); Monarda didyma flower.jpg by Susanne Nilsson (CC BY-SA 2.0).
- Garden Phlox: Phlox paniculata.jpg by Jerzy Opioła (CC BY-SA 3.0); Phlox Paniculata.jpg by Atilin (Public domain).
- Catmint: Catnip flowers.jpg by D. Gordon E. Robertson (CC BY-SA 3.0); Nepeta cataria (5201973078).jpg by Matt Lavin (CC BY-SA 2.0).
- Perennial Salvia: Woodland sage-Salvia nemorosa.jpg by 3268zauber (CC BY-SA 3.0); Salvia nemorosa pm.JPG by Peter A. Mansfeld (CC BY 3.0).
- Yarrow: Achillea millefolium 104218314.jpg by #Mondsee #Irrsee (CC BY 4.0); Achillea millefolium 139228692.jpg by Allen Browne (CC BY 4.0).
- Coreopsis: Coreopsis grandiflora.jpg by Rob Duval (CC BY-SA 3.0); Coreopsisgrandiflora.jpg by Stickpen (Public domain).
- Black-Eyed Susans: Bunga Rudbeckia hirta.jpg by Antya Anantari (CC BY-SA 4.0); Rudbeckia hirta (1046464129).jpg by Jason Hollinger (CC BY 2.0).
- Shasta Daisies: Leucanthemummaximum.jpg by xcaballe (CC BY-SA 2.0); Leucanthemum × superbum (2560634670).jpg by Dinesh Valke (CC BY-SA 2.0).