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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Division of Herbaceous Perennials

This article was published originally on 8/13/1999

Herbaceous perennials are commonly divided for three reasons: to control size, to retain vigor, and to propagate a prized perennial. Vigorous perennials may grow so rapidly that they choke out neighboring plants in the flower bed. Other perennials decline in vigor if not divided at the appropriate time. One of the easiest ways to propagate a prized perennial is to divide the plant into two or more smaller plants.

The best time to divide perennials varies with the different plant species. The appropriate time to divide widely grown perennials is presented below.

PlantWhen to Divide
Aster (Aster species) divide every 2 or 3 years in spring.
Astilbe (Astilbe species) divide every 3 or 4 years in spring.
Baby's Breath (Gypsophila paniculata) division is difficult, carefully divide in spring or late summer/fall.
Basket-of-Gold (Aurinia saxatilis) divide in late summer/fall. Basket-of-gold can also be propagated by stem cuttings in spring or fall.
Bee Balm (Monarda didyma) divide approximately every third year in spring.
Blanket Flower (Gaillardia x grandiflora) divide in spring.
Blazing Star (Liatris species) divide in spring.
Bleeding Heart (Dicentra species) division is difficult, best done in late summer/fall.
Butterfly Weed (Asclepias tuberosa) a taproot makes division difficult. However, butterfly weed is easily propagated by seeds.
Chrysanthemum (Dendranthema x grandiflorum) divide mums every 2 or 3 years in spring.
Columbine (Aquilegia species) many species and varieties are short-lived. Division is difficult, carefully divide in late summer.
Coral Bells (Heuchera species) divide in spring or late summer/fall.
Coreopsis (Coreopsis species) divide in spring or late summer/fall.
Cornflower (Centaurea species) requires frequent division every 2 or 3 years. Divide in spring.
Daylily (Hemerocallis species) divide in spring or late summer/fall.
Delphinium (Delphinium species) usually short-lived, division is seldom necessary.
False Indigo (Baptisia australis) division is difficult because of its long taproot. Plants can be started from seeds.
Gooseneck Loosestrife (Lysimachia clethroides) plants spread aggressively. Divide every 2 or 3 years in spring.
Hardy Geranium (Geranium species) divide in spring or late summer/fall.
Hardy Zinnia (Heliopsis helianthoides) divide every other year in spring or late summer/fall.
Hosta (Hosta species) plants can be left undisturbed for years. If additional plants are desired, divide clumps in spring or late summer/fall.
Iris, Bearded (Iris hybrids) divide every 3 to 4 years in July or August.
Iris, Siberian (Iris sibirica) divide after blooming in early summer.
Lily-of-the-Valley (Convallaria majalis) a rapidly spreading groundcover, divide in spring.
Lungwort (Pulmonaria species) divide in late summer/fall.
Obedient Plant (Physostegia virginiana) plants spread rapidly. Divide plants every 2 or 3 years in spring.
Oriental Poppy (Papaver orientale) divide when plants die back in mid to late summer.
Orange Coneflower (Rudbeckia fulgida) divide every 3 or 4 years in spring.
Ornamental Grasses the best time to divide ornamental grasses is spring. Can also be divided in late summer/fall.
Peony (Paeonia hybrids) peonies are long-lived and can be left undisturbed for many years. If additional plants are desired, divide clumps in September.
Perennial Salvia (Salvia hybrids) divide plants in spring.
Phlox, Garden (Phlox paniculata) divide every 3 to 4 years in spring or late summer/fall.
Phlox, Moss (Phlox subulata) divide plants in spring immediately after blooming.
Purple Coneflower (Echinacea purpurea) divide every 3 or 4 years in spring.
Shasta Daisy (Leucanthemum x superbum) tends to be short-lived. Divide plants in spring.
Speedwell (Veronica species) divide in spring or late summer/fall.
Spiderwort (Tradescantia virginiana) spiderworts are rapidly speading plants. Divide every 2 or 3 years in spring.
Stonecrop (Sedum species) divide in spring.
Yarrow (Achillea species) many of the yarrows spread rapidly. Divide every 3 or 4 years in spring.

Most perennials divided in late summer/fall (mid-August through September) should be mulched in November. A 4 to 6 inch layer of straw placed over the plants should reduce the possibility of winter injury. Remove the mulch in early April.



This article originally appeared in the August 13, 1999 issue, p. 112.

Year of Publication: 
1999
Issue: 
IC-481(21) -- August 13, 1999