Most Iris Need Thinning Every Three To Five Years

Q: When is the best time to divide and move spring-blooming bearded iris? Also, my iris is planted in a barrel. It's growing good foliage, but not too many flowers. Why?

A: Several of you have asked whether spring-blooming iris benefit from being divided or moved in July or August when they're starting to go dormant. Let's begin with a little background. All bearded iris like an open, sunny site. Watering deeply at least twice a month after bloom will help the plants develop. These bearded iris have "rhizomes," flattened, bulbous root segments that grow parallel to the surface of the soil. Bearded iris do well if the rhizomes are close enough to the soil surface to receive some direct warmth from the sun.

Every three to five years, clumps become crowded. Lift the entire plant by gently using a garden fork to loosen the soil at the edges of the clump, then raise the whole clump at once. Shake off the soil. You'll have a mass of interconnected rhizomes that cross over and under each other. Break off and discard any withered, shrunken sections (these will be in the center of the mass.)

Separate and replant the plump, sturdy, and solid segments, setting them out at least three at a time, in a triangle about six inches apart. You'll notice that the rhizome has leaves at one end. Point the leafless ends into the middle of the planting area, and set the leafy ends at the edges so that the rhizomes resemble the spokes of a wheel and the leaves are the outer rim. Water them in well when replanting. Long floppy leaves can be cut back to about six inches to make them easier to handle. They will fill in again. The plants may not bloom heavily the first year after division, but if settled in a sunny spot, they'll bloom prolifically in a year or so.

As to the question about iris rhizomes in a barrel producing lots of leaves but not blooming well, here are a few thoughts. Is the barrel in sun? Were the plants recently divided? Plants in shade, or recently divided, will bloom poorly. Are the plants crowded? Being crowded and overgrown will also reduce bearded iris bloom.

Irises are a huge and intriguing floral family. There are bulbous iris, planted in fall with tulips and daffodils, that bloom in May. Some iris don't form rhizomes, but have moisture-retentive roots, such as the lovely Pacific Coast iris, hybridized from plants native to the coast north of California.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by Mary Robson, area horticulture agent; Holly Kennell, Washington State University/King County Cooperative Extension agent, Susan Miller, integrated-pest-management specialist, and volunteer Master Gardeners.