When and How to Divide Perennials in Fall

When to Divide Perennials in Fall

Divide perennials in fall at least 4 to 6 weeks before the ground freezes, and choose plants that bloom in spring or summer. Fall bloomers wait until spring. In Iowa the fall window is mid-August through September, according to Iowa State University Extension. In South Carolina, Clemson puts it between early September in the uppermost Piedmont and mid to late October on the coast. Wherever you garden, the 4 to 6 week margin is the part that carries over, because the divisions need that time to establish roots before winter.

The bloom-time rule comes from Clemson and University of Illinois Extension in almost the same words: divide spring and summer blooming perennials in the fall, and fall bloomers in the spring. Penn State Extension states the principle underneath it, which is to divide a perennial when it is not in bloom. A plant that has finished flowering can put its effort into roots.

Pick the day with some care too. Clemson’s advice is never to divide perennials on hot, sunny days, so a cool, overcast day in September is the one to wait for.

How to Tell a Clump Needs Dividing

Most perennials should be divided every three to five years. Clemson lists three signs that the time has come: flowers that are smaller than normal, centers of the clump that are hollow and dead, and bottom foliage that is sparse and poor. Penn State adds a fourth, a plant that is beginning to crowd out its neighbors.

The interval depends on how fast the plant grows. Iowa State separates vigorous growers such as bee balm and obedient plant, which benefit from division every 2 to 3 years to contain their spread, from perennials that need rejuvenating every 3 to 5 years. Some plants hardly need it at all. Illinois notes that hellebores, bleeding hearts and peonies rarely need dividing unless you want more plants, and Iowa State says peonies and hostas can be left undisturbed for many years.

Which Perennials to Divide in Fall

Iowa State University Extension publishes a plant-by-plant list. The entries below are for Iowa, so shift the months later in warmer regions while keeping the season.

Perennial	When to divide	Notes from Iowa State
Peony	Late summer to fall, September best	Can be left undisturbed for many years
Hosta	Spring or late summer to fall	Can be left undisturbed for years
Daylily	Spring or late summer to fall
Garden phlox	Spring or late summer to fall	Every 3 to 4 years
Bearded iris	Late summer, July or August	Every 3 to 4 years in the general list, 3 to 5 in the iris guide
Bleeding heart	Late summer to fall	Division is difficult
Siberian iris	Early spring, early summer after blooming, or late summer	Needs dividing less often than bearded iris
Astilbe	Spring	Every 3 or 4 years
Purple coneflower	Spring	Every 3 or 4 years
Aster	Spring	Every 2 or 3 years
Chrysanthemum	Spring	Every 2 or 3 years
Ornamental grasses	Spring or late summer to fall	Spring is better than fall

Early spring in Iowa means April, just as the foliage emerges. Any perennial marked for either season can be done then if you miss the fall window.

Some perennials should be left alone. Butterfly weed, false indigo and balloon flower have long taproots that make division difficult, and Iowa State points out that butterfly weed and false indigo can be started from seed instead. Penn State’s list of plants not to divide also includes lavender and other shrub-like perennials, euphorbia, lupine and eastern columbine. Clemson adds oriental poppies, baby’s breath, gas plant and Japanese anemones. If you are choosing plants that will not need this kind of attention often, our list of low-maintenance perennials is a place to start.

How to Divide a Perennial, Step by Step

The procedure is the same for most clumps. Penn State, Clemson and the Royal Horticultural Society describe it in closely matching terms, and our general guide to dividing perennials without killing them covers spring division and how often each plant needs it.

1. Water the plant thoroughly a day or two beforehand.
2. Cut the stems and foliage down to 6 inches from the ground so you can see what you are doing.
3. Dig down deep on all four sides of the plant, about 4 to 6 inches away from it, and lift the whole clump.
4. Split the clump into pieces, each with three to five vigorous shoots and its own roots.
5. Replant the divisions as soon as possible at the same depth they grew before, firm the soil, and water well.

How you split the clump depends on the roots. Penn State sorts perennials into four types:

• Spreading roots. Pull the smallest apart by hand or cut them into sections with shears.
• Clumping roots. Cut through the crown with a sharp knife, keeping at least one eye, or bud, with each division.
• Rhizomes. Cut sections that each hold a few inches of rhizome and a fan of leaves.
• Tuberous roots. Cut pieces that each contain at least one bud.

Large fibrous clumps such as daylilies are hard to cut. The Royal Horticultural Society’s method is to push two garden forks into the crown back to back and lever them apart. Throw away a dead center and replant the vigorous outer pieces. A sharp pair of pruners makes the cutting back in step 2 quick, and our guide to choosing pruning shears covers the types.

Peonies and Bearded Iris

These two have their own rules, and both are sensitive to planting depth.

Peonies. September is the best time to divide peonies in Iowa. Cut the stems off near ground level, dig the clump, and cut it so that each division has at least 3 to 5 buds, called eyes, and a good root system. Iowa State warns that divisions with fewer than 3 buds will likely need several years to flower well. Set the division so the buds sit 1 to 2 inches below the soil surface. Clemson, writing for South Carolina, says no deeper than 1 inch, so stay at the shallow end in warm climates.

Bearded iris. These are a late summer job. July or August is the best time to dig, divide and transplant them, and they need it every three to five years. Cut the leaves back to one third of their original height. Replant so the rhizome is just at or below the soil surface, with the top just showing in Clemson’s wording, and space the rhizomes 12 to 24 inches apart. Siberian iris is handled differently. Iowa State’s iris guide says it can be divided in early spring or late summer, while its general perennial list gives early summer, after blooming. For a late summer division the guide cuts the foliage back to 6 to 8 inches.

Aftercare: Water and Winter Mulch

Water the divisions in well on planting day and keep going. Iowa State’s instruction is to keep newly divided perennials well watered through the entire first growing season, which for a fall division means the weeks until the ground freezes and then all of the following year.

Winter mulch reduces the chance of winter injury to divisions made late in the season. Iowa State recommends mulching late divisions in November with a 4 to 6 inch layer of straw placed over the plants, and removing it in early April. For peonies the timing is mid to late November with the same 4 to 6 inches. In South Carolina, Clemson suggests pine straw or leaves for fall-divided perennials.

Label each division before the mulch goes on, since a bed of cut-back crowns under straw gives no clue in spring about what is planted where. If the newly opened gaps in the bed need filling for a season, our comparison of annuals and perennials covers quick options.