Gardening Articles: Flowers :: Bulbs
May's Flower: Bearded Iris (page 3 of 4)
by National Gardening Association Editors
Fertilizing and Long-Term Care
As growth begins in spring, apply low-nitrogen fertilizer around established plants. Compost or a low-nitrogen controlled-release fertilizer is ideal in spring, paired with a high-phosphorus fertilizer in fall. In the coldest climates (zones 3 through 5), don't fertilize after mid-July to give plants time to harden off before winter.
After blooms fade, cut flower stalks close to the ground, leaving healthy green leaves in place to nourish next year's growth. Remove dead leaves and debris in late autumn, then trim the foliage back by two-thirds after the leaves have died back.
In cold-winter regions, cut back the tops to about 6 inches after the first hard frost, and rake up any debris. After the soil freezes, mulch with 4 to 6 inches of loose compost or weed-free straw. Soon after the snow melts in spring, remove the top layer of mulch to allow air and sunlight to dry the surface. Wait a few days before removing the rest of the mulch, taking care to not to break the center sections of fans where flower buds are developing.
Dividing. Lift and divide the rhizomes every three to four years. Without division, both the quantity and quality of blooms will diminish. Do it after bloom in the optimum planting time for your area. Rhizomes are easy to pry up with a garden fork or shovel. Pull clumps apart, replanting the thickest, firmest rhizomes; these are usually the ones that have grown during the current season. Throw away the oldest, leafless rhizomes, usually at the clump's center. Trim leaves to about 6 inches long, then let all the cleaned and groomed rhizomes air-dry overnight. Before replanting, amend soil with lots of fresh compost or organic matter.
When plants don't bloom. Plants may not bloom for many reasons, and sometimes it's a combination of two or more. Most often the clumps are overcrowded. If bloom is increasingly sparse, it's time dig up, thin, and replant the rhizomes.
Another reason for poor bloom is nutrient-poor soil. Bearded iris have a remarkable ability to survive poor conditions, but they grow and bloom best in rich, well-drained soil. A thin mulch of compost is beneficial.
Other reasons for lack of bloom include inadequate sunlight and small or not yet established divisions. Recently planted divisions may not bloom their first year.
Pests. Iris borers are the most destructive pests. They are the larvae of moths that lay eggs in fall in dead flower stalks and debris. Eggs hatch early the following spring, and then, as tiny caterpillars, they bore through the leaves and eat their way down into the rhizome. As they feed, borers introduce bacteria that cause a soft rot that ultimately turns leaves brown and destroys the plants.
Control borers by first removing and disposing of infected rhizomes and leaves. Clean up dead leaves and debris after hard frost in fall to remove the eggs and reduce borer numbers the following year. Iris leaves are most susceptible to borers in spring when they are 5 to 6 inches tall. If you see any borer "mines," or tunnels, in the leaves, pinch them to kill the hidden larvae.