Yard and Garden: Manage Bearded Irises, Peonies, Daylilies

Mid-to-late summer is an excellent time to manage the flowers growing around the house, yard and garden. But what are the specifics? When is the best time to manage particular flowers? Iowa State University Extension and Outreach horticulturists offer tips on maximizing the summer floral experience. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

When and how do I divide bearded irises?

While bearded irises are easy-to-grow perennials, they need to be divided every three to five years.  If not divided, plants become overcrowded and flower production decreases. Crowded plants also are more prone to disease problems. In Iowa, July or August is the best time to dig, divide and transplant bearded irises.

Bearded irises grow from thick, underground stems called rhizomes. Carefully dig up the iris clumps with a spade. Cut back the leaves to one-third of their original height.  Wash the soil from the rhizomes and roots with a steady stream of water. Then cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome, and several large roots. Discard the old, leafless rhizomes in the center of each clump. Also, discard all diseased or insect damaged rhizomes.  

Bearded irises perform best in fertile, well-drained soils and full sun. In clay soils, incorporate compost, peat, or well-rotted manure into the soil prior to planting. When planting bearded irises, dig a hole large enough to accommodate the rhizome and roots. Build a mound in the center of the hole. Place a rhizome on top of the mound and spread the roots in the surrounding trench, then cover with soil. When planted, the rhizome should be just below the soil surface. Finally, water each plant thoroughly.  

To obtain a good flower display, plant at least three rhizomes of one cultivar in a group. Space the rhizomes 12 to 24 inches apart. Point each fan of leaves away from the other irises in the group.

When and how do I divide peonies?

September is the best time to divide peonies. By September, peony plants have been able to store adequate food reserves in their roots. Also, the replanted divisions have several weeks to get reestablished at their new sites before the onset of winter.  

Begin by cutting off the peony stems near ground level. Carefully dig up the plants and wash or gently shake off the soil. Using a sharp knife, divide the clump into sections. Each section should have three to five buds (eyes) and a good root system. Divisions with fewer than three buds may take two or more years to flower. Plant the divisions in a sunny, well-drained site.  

When planting, dig a hole large enough to accommodate the root system of the peony. Position the peony in the hole so the buds are  one to two inches below the soil surface. (Plants may not bloom well if the buds are more than two inches deep.) Fill the hole with soil, firming the soil around the plant while backfilling. Then water thoroughly. Space peonies 3 to 4 feet apart.  

Mulch newly planted peonies with several inches of straw or pine needles in late fall. Mulching prevents repeated freezing and thawing of the soil that may heave and damage young plants. Remove the mulch as growth resumes in spring.

When and how do I divide daylilies?

Daylilies can be divided in early spring (as new growth begins to emerge) or in late summer/early fall (September). Dig up the entire clump with a spade. Shake or wash off the soil. Then carefully pull the clump apart. Often, a sharp knife is necessary to divide large, dense clumps. Each division should have two or three fans of leaves and a good root system. When dividing daylilies in September, cut back the foliage to a height of 6 to 8 inches.  

Replant the divisions as soon as possible. When planting, the daylily’s crown (the area where the shoots and roots meet) should be approximately 1 inch below the soil surface. Water thoroughly. Divided plants usually don’t bloom well for one or two years.
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