Recommendations for when to divide irises depend on the species. Follow these tips to maintain a colorful, attractive iris planting.
Bearded Iris | Siberian Iris | More Information
Bearded Iris
Bearded irises are one of the most popular and widely grown perennials in the home landscape. While bearded irises are beautiful, they do require moderate levels of maintenance. One important chore is to divide bearded irises. If not divided, the plants become overcrowded and flower production decreases. Crowded plants are also more prone to foliar diseases.
When to Divide Bearded Iris
Bearded irises should be divided every three to five years, as the plants quickly become overcrowded and don’t bloom well. July or August is the best time to dig, divide and transplant bearded irises.
How to Divide Bearded Iris
Bearded irises grow from thick, fleshy, underground stems called rhizomes. Carefully dig up the iris clumps with a spade. Cut the leaves back to 1/3 their original height. Wash the soil from the rhizomes with a forceful stream of water from the garden hose. Then cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome, and several roots. Discard the old, leafless rhizomes in the center of each clump. Also, discard all diseased and insect damaged rhizomes.
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. Point each fan of leaves away from the other irises in the group. Then cover with soil. When planted, the large fleshy rhizome should be just at or below the soil surface. Finally, water each plant thoroughly.
Divided irises typically bloom sparsely the following spring. However, plants should be in full bloom by their second year.
Best Growing Conditions for Bearded Iris
Bearded irises perform best in fertile, well-drained soils and full sun. The planting site should receive at least six hours of direct sun per day. Plants that don't receive sufficient sunlight will not bloom well. Bacterial soft rot is often a problem in wet, poorly drained sites. Wet, poorly drained sites can often be improved by incorporating organic matter, such as sphagnum peat moss or compost, into the soil prior to planting. Raised beds are another option for gardeners with poorly drained soils.
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.
Siberian Iris
Siberian iris are great additions to the garden. Their flowers resemble those of bearded irises but are smaller and lack fuzzy beards. Brilliant blue, white, lavender or yellow flowers appear 1 to 3 weeks later than bearded iris. Leavea are more grass-like and dark green. They are a great alternative to bearded iris in areas with heavy soils or poor drainage as Siberian iris prefers wet conditions.
When to Divide Siberian Iris
Siberian irises don’t have to be divided as often as bearded irises. It’s advisable to divide Siberian irises when clumps become crowded, plant vigor declines or clumps have formed solid rings with bare centers. Siberian irises can be divided in early spring or late summer.
How to Divide Siberian Iris
When dividing Siberian irises in early spring, dig up the entire clump when new growth has just begun to appear. Divide the clump into sections with a soil knife or spade. Each section should have several growing points and a good root system. Replant immediately.
When dividing plants in late summer (August), dig up the entire clump, cut back the foliage to 6 to 8 inches and divide the clump into sections with each division containing several fans of leaves and a good root system. Promptly replant the divisions.
Best Growing Conditions for Siberian Iris
Siberian irises grow and bloom best in full to part sun. Plants prefer moist, fertile soils and are suited to edge of ponds and streams. While they don't mind wetter conditions, they also grow well in average moist, well-drained soils.