September Garden Cleanup: Cut Back These Perennials Now

September marks the start of fall, and while some of us are experiencing lingering summer conditions, others are moving into cooler days and chilly nights. Our warm-season plants may be enjoying a fall flush of late-season growth and color before heavy frost (the average first frost in the U.S. ranges between mid-September and early November).

This month brings prime planting opportunities, allowing roots to develop before winter conditions. Trees, shrubs, and perennials benefit from the mild seasonal conditions ahead of winter dormancy, making transplanting less stressful. It’s also time to divide many of our favorite perennials to reduce crowding and to expand the collection. Seeding quick cover crops brings soil enrichment to bare beds this season.

With September’s seasonal transition, new additions, and changeouts, September garden cleanup is a good measure to promote garden health. A little maintenance now goes a long way toward preparing plants and beds for a winter rest. It’s also a good time to relax in the garden and breathe in the casual, cooler days as we turn to fall.

September Garden Cleanup

While regular maintenance is ongoing in September, it doesn’t need to be all-consuming this month. It’s time to harvest, save seeds, and sow fast-growing cool-season crops to enjoy before a heavy freeze. Garden tasks this month revolve around watering, weeding, and trimming crispy, end-of-season stems.

To make cleanup easier, we love tools of the trade that are lasting investments. As September garden cleanup involves multiple trips to the compost bin, mulch pile, and hauling out amendments (fall is optimal for amending soils), a wheelbarrow or garden cart makes the lift less labor-intensive. Check out Gorilla carts in heavy-duty poly or steel to withstand years of hard work.

Selective Trimming and Deadheading

September garden cleanup isn’t the time for severe pruning, but summer-worn or overgrown plants benefit from light trimming and shaping. Clip away spent flower stalks, diseased leaves, and brown, crispy stems from annuals and perennials. Allow six to eight weeks before fall’s first anticipated frost when pruning any healthy green growth, as this stimulates new growth that’s susceptible to cold damage.

Stop deadheading by early to mid-September to let blooms go to seed in the fall. Seeds left on the stem sprout for next year’s display and provide valuable forage for birds and small mammals heading into winter. But, if you want ot prevent unintended volunteers from vigorous reseeders, or to collect seeds for spring planting, clip off fading blooms as they dry.

Trim sprawling perennial herbs to get them back in shape, encouraging bushy forms. Use the clippings fresh, or dry them for easy flavor and herbal teas throughout the cold season. Favorites like thyme, oregano, rosemary, sage, and mint are prime for drying.

Debris Clearing and Bed Tidying

Our beds become a little worn as leaves and blooms drop and mulch fades late in the season. The dropped debris also harbors pests and diseases, even through the winter. Removing fallen leaves and petals around plants during September garden cleanup improves growing conditions for the following season.

A light rake to clear debris also creates fresh, tidy-looking beds. Leave the fall leaves with autumn’s natural leaf drop later in the season. The renewable resource is garden gold.

By late summer, mulch may be sun-bleached and worn. Plan to add a fresh layer after planting. Topdress with compost, leaves, straw, or bark to refresh the look while improving moisture retention, weed suppression, and temperature regulation.

Weeding

Weeds not only compete with our garden plants for resources like nutrients, moisture, and sunlight, but they also host pests and diseases over the winter. This time of year, they’re often setting seeds to disperse for next season’s population. Pulling weeds before they go to seed helps reduce future numbers.

If weeds are flowering, pluck them at the roots. If seeding, take care when pulling to limit seed dispersal.

Perennials to Cut Back in September

A gentle cutback during your September garden cleanup sets a healthy foundation for the following seasons, in improving form and in removing disease-affected portions. It also increases air circulation and limits overcrowding. 

While many perennials do well with cutback after a few heavy frosts, others are best left standing through winter. We’ll explore perennials prone to end-of-season diseases and overgrowth that benefit from selective cutback this month.

Bee Balm

	botanical name Monarda didyma
	sun requirements Full sun
	height 2-4’
	hardiness zones 4-9

Bee balm is a durable eastern U.S. native wildflower. Its flared, scarlet blooms cluster along stems and are beacons for hummingbirds. The herby foliage is aromatic with minty essential oils. Monarda boasts a long bloom season from early summer until frost.

With its vigorous growth, bee balm is prone to flopping over late in the season. It’s also susceptible to powdery mildew and other fungal problems from crowded stems. To improve airflow, thin the stems to open the crown. Cut back those that show signs of disease, yellowing, or browning. Trim flopping branches to just above a healthy node.

Monarda spreads through rhizomes and reseeds. Deadhead to prevent unwanted volunteers, or let seeds remain and selectively pull any unwanted sprouts in spring. Bee balm grows best in well-draining, organic soils with even moisture.

Garden Phlox

	botanical name Phlox paniculata
	sun requirements Full sun to partial shade
	height 2-4’
	hardiness zones 4-8

Tall garden phlox is a staple of the perennial border. The old garden favorite shines with domed flower clusters in saturated hues, from soft lilac to bold magenta. The attractive, fragrant native attracts butterflies and other pollinators with its extended flowering season.

Phlox is susceptible to a number of pests and diseases, like powdery mildew and root rot. To deter issues heading into fall and winter (and beyond), thin and remove diseased portions during your September garden cleanup. Make sure to keep beds clean and debris-free for overwintering. Prune away nearby plants encroaching on the perennial to improve air circulation and reduce damp conditions.

Phlox performs best in organically rich, well-draining soil. Provide supplemental water during dry spells and through frost. Mulch to keep roots cool in the summer and to prevent fungal spores from splashing up onto leaves when watering or during rain.

Bearded Iris

	botanical name Iris x germanica
	sun requirements Full sun
	height 1-4’
	hardiness zones 3-9

Bearded iris shows large, ruffled, colorful blooms in spring and early summer. If we’re lucky, we may see a repeat bloom in the fall. By late summer, deadheading and cutting spent stalks to the soil level are good practices. 

Clip off any brown foliage, too. Irises can get floppy as their leaves lengthen, and while you can cut fans to the base, they continue to photosynthesize for the plant. Leave them in place until they turn yellow and brown for the most root support.

Irises are Mediterranean natives that grow best in moist, well-drained situations but are also quite drought-tolerant. Aim for even moisture during the growing season, but avoid overwatering, as they are accustomed to arid summers in their native climate.  Adaptable, they tolerate varying soil types from acidic to clay to sand.

Hosta

	botanical name Hosta spp.
	sun requirements Partial to full shade
	height 6-36″
	hardiness zones 3-9

Hostas bring lush leaves from broad to strappy to curly in hues from jade to gold. The textural contrast offers visual interest along a bed edge with other perennials and beneath shrubs.

By late summer, hostas may show yellowing, browning, or dead leaves. Those that experience leaf scorch may show bleaching, with disease damage from fungal problems. Pest damage from snails and slugs, too, can cause ragged foliage. During your September garden cleanup, cut off damaged leaves at their base. Leave any that still show green to continue photosynthesizing before winter dormancy. 

After dieback from heavy frost, cut back remaining portions. Remove the debris for a disease and pest-free bed.

Peony

	botanical name Paeonia officinalis
	sun requirements Full sun
	height 2-4’
	hardiness zones 3-9

Peonies bear sweetly fragrant, fully-petaled blooms. They bloom once a season from late spring to early summer and devote the rest of the summer to developing leafy growth and sturdy roots. From the common garden peony to tree peonies to woodland and Itoh types, there’s a peony to meet varying exposures and situations.

Peonies are sometimes prone to fungal issues. Common fungal diseases and viruses impact overall health and relate to cultural practices that harbor spores and promote their spread. Peonies rely on ample air circulation through proper spacing and pruning, as well as irrigation that doesn’t cause damp conditions to the crown.

When herbaceous types die back after heavy frost, remove the dry stems and leaves. For tree peonies, whose woody stems persist through winter, remove fallen leaves. This cutback and debris removal prevents fungal spores from developing or overwintering to affect the next season’s growth. Bulk up the mulch layer for seasonal insulation.

Rose

	botanical name Rosa spp.
	sun requirements Full sun to partial shade
	height 3-50’
	hardiness zones 4-11

Late summer and early fall aren’t the time for heavy rose pruning, but targeted cutback promotes healthy stems. Along with deadheading, stop any trimming four to six weeks before the first heavy frost to prevent cold damage.

Preventative rose pruning now lessens winter damage, reduces dieback and disease spread, and increases airflow. Cut away dead, damaged, and crossing branches. Crossing stems may rub and expose healthy tissues to cold damage and disease issues.

To remove declining or dead canes, cut them back to the first section of healthy, green growth. Dead canes are brown and brittle, while healthy stems are green or white and pithy.

Cut off weak or spindly shoots and any suckers, especially in the interior sections. And, cut back long, whippy stems from climbers and tall specimens. The wayward stems may break or crack in winter winds.

American Wisteria

	botanical name Wisteria frutescens
	sun requirements Full sun to partial shade
	height 15-40’
	hardiness zones 5-9

American wisteria is a native, vigorous, and high-climbing vine. It’s less aggressive than the invasive Chinese wisteria, with similar blooms and foliage. Fragrant, pendulous purple blossoms and pinnate leaves intertwine on long, sprawling woody stems.