Signs that your bearded irises need dividing and transplanting:
1. Rhizomes have heaved out of the ground. When an iris is overgrown, crowded rhizomes shift against each other like tectonic plates until they erupt out of the soil. It’s not exactly an earthquake, but it’s ugly and a sure sign it’s time to lift that clump and start dividing.
2. You notice fewer and fewer flowers each season. A scant scattering of flowers makes a dismal display for beardies, whose glorious colors and beauty are meant to grace the garden. Or you may get foliage and no flowers.
Bearded irises should be divided every three to five years, says Kelly D Norris, author of one of my favorite — and most useful books — on the subject, “A Guide to Bearded Irises: Cultivating the Rainbow for Beginners and Enthusiasts.”
In Iowa, mid-July through September is recommended for dividing and transplanting irises because the plants are dormant in the hot days of high summer. Plus it’s a cheap way to increase the number of irises. If you need to tackle this garden chore, there’s no time like the present. Transplants need to get established before a hard freeze or killing frost.
Irises need a well-drained site and 6 to 8 hours of sun. Shade usually results in foliage, not flowers.
Rhizomes are not bulbs. They are modified stems that grow at or just below the soil surface and are the main stem of the plant. It’s also where the plant stores nutrients.
Here’s how to divide and transplant an iris:
1. Cut back foliage to 6 or 7 inches so energy is put into roots, not supporting foliage.
2. Use a sharp spade to loosen soil the soil around the clump and lift it out. Leave roots intact.
3. Use the garden hose to wash away soil from the clump.
4. I like to use a sharp transplanting trowel to divide the rhizomes. A knife will work. Discard soft rhizomes. Each division should have a leaf fan, a 3 to 4 inches rhizome and a few roots. Check for signs of iris borers.
5. Re-plant the rhizomes. Make a mound of soil and set the rhizome on top, feathering out roots. The rhizome should be slightly covered with soil and allowed to shallowly anchor itself in the ground.
6. Rather than plant irises in a row, create groupings on one variety by planting three or five rhizomes spaced 12 to 24 inches apart. Point the rhizomes away from each other when planting groups.
7. Water deeply after planting.
If bearded irises have a mortal enemy, it’s the iris borer. A nondescript grayish brown moth/miller emerges in August and September. After mating, the female lays eggs on dead iris leaves.
The larvae overwinter and wake up in spring to dine on the foliage and rhizome until they slink away to pupate. Damaged rhizomes can develop bacterial soft rot. It adds up to one big “ick.”
If you have a problem with borers, remove the problem by removing foliage in fall or early in spring. You can also apply an insecticide when iris shoots are 4 to 6 inches long.
This will destroy larvae before they can bore into foliage and rhizome.Look for acephate, permethrin or spinosad on the label and use according to directions.