Daylillies and Iris Perennials Best Divided in August

Two Easy-Care Perennials That Need Periodic Dividing in August

daylily needs division in August

This year’s hot and often humid summer weather doesn’t seem like the best time to garden… unless gardeners love irises or daylilies.

In early to mid-August, both long-time perennial favorites are in a kind of dormant period. So, that’s when gardeners are most likely to succeed in dividing the plants’ clumps. The clumps tend to become overcrowded and less productive every three to five years, said Dennis Patton, horticulturist with Kansas State University Research and Extension.

“For the most part, we’re talking about extremely easy-care plants. To keep them healthy and promote optimum flowering, however, you simply have to divide them periodically,” Patton said.

During dry weather, the horticulturist recommends watering overcrowded clumps a day or two before digging, just to make the process easier. Some gardeners also cut back the leaves by half to make handling the plants easier, too.

Is there an easy way to divide iris or daylillies?

“After that, there is no simple approach. You’ve simply got to jump in and tackle digging up the entire clump,” Patton said. “Fortunately, though, irises have a fairly shallow root system. Daylilies’ roots can be harder to dig up, because they’re a little deeper and get more tangled with each other, but they’re also better able to handle some injury.”

If gardeners plan to move some divisions to other parts of their landscape, they should select sites that receive at least six hours of sunlight, he said. One reason irises and daylilies have maintained their popularity since pioneer days is that they tolerate most garden soils. Even so, the plants also prefer well-drained soil that is neutral to slightly alkaline.

Their differing root systems require different methods of dividing.

Irises have fat food-storage structures called rhizomes that produce leaves and flowers above and roots below ground level. Gardeners must be careful with the leaves and roots, Patton said. But, they can assume the rhizomes are fairly tough and just knock away any excess soil, so they can see where to snap or cut the rhizomes apart.

The best iris divisions are sound rhizomes with two or more leaf-growing points.

“You’re always going to throw some rhizomes away. And, the first ones to discard are those with holes and worm-like insects called iris borers – which can seriously injure or destroy the plant. Then cut off any remaining affected tissue” he said.

Rhizomes spread outward, forming a new “knee” or bump every year. The only necessary bump is the one that’s attached to the leaves. The others are no longer productive.

Some iris lovers do leave one or two others to help stabilize the division in the soil more quickly. To discourage borers, however, they cut away the oldest rhizomes and discard them far from the iris bed.

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Iris plants like well-cultivated soil, improved with organic matter. The traditional way to plant them there is the “hill in the hole” method, Patton said.

“Dig a hole that’s deep enough for the longest roots. Mound up a hill in the middle of that hole and place the rhizome on it, with the roots dangling down the hill’s sides. A rhizome must never be more than an inch below the soil line. In fact, it will be fine if its top surface stays a bit exposed to air and sunlight,” he said. “After that, fill the remainder of the hole and water. Wait until spring to fertilize.”

Patton prefers planting iris divisions in groups of three, with their rhizomes nearly touching and the leaves on the outside of the circle. As they grow, the divisions will widen and thicken the circle.

Daylily lovers who regularly divide their plantings can sometimes use a spading fork to “peel off” a fan or two while an overgrown clump is still in the ground, the horticulturist said. But, the first step in dividing large, tightly packed clumps is to dig them up. After that, gardeners have three options for creating divisions:

  • Wash away the clump’s soil. Roll the clump back and forth until divisions separate off.
  • Use two spading forks, back-to-back, to divide the clump into sections.
  • Use a sharp spade to cut the clump into pieces, thereby losing quite a few roots.
  • Patton said the best size for a daylily division depends on how fast the gardener wants an area to fill in. But, he doesn’t recommend replanting one that’s much larger than a head of cauliflower.

The divisions go 24 to 20 inches apart, unless they’re miniature varieties.

Written by by Kathleen W. Ward More of Patton’s gardening advice is available on the Web
at the Extension Infonet, a joint project of K-State Research and Extension and University of Missouri Extension.
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