From the Ground Up: Iris blooms struggle during wet weather

iris not blooming

Irises are lovely additions to any garden.

The plant takes its name from the Greek word for a rainbow, referring to the wide variety of flower colors found among the many species. Linn County Master Gardener Judy Stevens provides some answers to those who are wondering why their irises have disappointed them by not blooming this year.

Q :My iris did not bloom this year. What can I do?

A : One potential reason for lack of blooms may be lack of sunlight. Irises need at least six hours of sunlight to ensure good blooms. Bearded irises perform best in fertile, well-drained soil in full sun. While they tolerate light shade, maximum flower production occurs in full sun. Another reason for lack of blooms is that irises benefit from regular division every three to five years. If not divided on a regular basis, the plants become overcrowded and flower production often decreases. Crowded plants are also more susceptible to disease problems. The ideal time to divide and replant irises is July and August. To divide irises, carefully dig up iris clumps with a spade. Cut the foliage back to one-third of the original height. Cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome, and several large roots coming off the rhizome. Check closely for any signs of disease or insect damaged rhizomes and discard those.

To obtain a good flower display, plant three or more rhizomes of one variety in a group. Space the rhizomes about 12 to 24 inches apart. Point each fan of leaves away from other irises in the group. Newly planted or transplanted bearded irises are susceptible to injury their first winter from repeat freezing and thawing of the soil so make sure you cover plants with several inches of mulch such as straw or pine needles in late fall. Remove the mulch in early spring. The transplanted irises will bloom sparsely the first spring but should be in full bloom in their second and third years.

The Iris Borer is the most prevalent and devastating pest of the iris. The cycle starts with the adult grayish moth that lays eggs on iris foliage and nearby plants in late summer and early fall. The eggs hatch in the spring and the small caterpillars bore into iris foliage and feed. Signs of feeding are brown streaks on the leaves as the caterpillars grow and tunnel their way to the rhizome. If these brown streaks are detected, pinching the terminal end of the brown streak will destroy the caterpillar. Another method is to cut off the leaf below the terminal end of the brown streak. Once the caterpillar gets to the rhizome it feeds inside the rhizome and destroys it. At this point soft rot sets in and creates a soft, foul smelling mess. If you've encountered Iris Borer damage to this point, you won't likely forget it.

Occasionally, there may be enough healthy rhizomes left so you can save your favorite iris. Do this by cutting away the infected part and replanting the healthy rhizome.

To prevent Iris Borer, remove and destroy dead foliage in late fall or early spring which will eliminate the eggs. Also, carefully examine irises in late summer or early fall for soft rhizomes by gently probing rhizomes with a stick. Remove any soft rhizomes before the larvae move into the soil and start the entire process over again.

 

Questions on gardening? Contact Elizabeth Ward, eward@iastate.edu.

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