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If your irises don't bloom, they're either growing in too much shade or they need dividing.

After they have been in the ground for three or four years, they become crowded. Their roots intertwine, the clumps rise ever higher out of the ground, and if they're not divided they'll stop blooming. In cooler areas divide irises as soon as flowering is finished. In very warm areas, wait until October. Sometimes, in old gardens, irises have been neglected for years and it's a big job to divide them, so do the ones in the worst shape first, in case you can't get to them all at once.

How to Divide Irises

·With a garden fork dig up an entire clump, shake off excess soil from the roots, then squirt it with the hose to wash all soil from the rhizomes. (A rhizome is a thickened stem that grows horizontally underground or on the surface of the ground.)

·Working with a sharp knife from the outside of the clump, cut vigorous, healthy divisions. Each division for planting should have one fan of leaves, a section of young, healthy rhizome approximately 2 to 6 inches long, and some roots coming out the bottom. It may also have one or two new growth buds, or eyes, bulging out on the sides.

·Discard the old woody center of the clump that has no leaves; anything that is diseased, rotted or has been attacked by pests; any thin or spindly growth; and all immature rhizomes with no leaves.

·Cut off the tops of the fans at a neat right angle, with the center point 4 inches higher and the sides 2 to 3 inches higher than the rhizome.

·Cut back the roots by about one-third, dip the cut ends of the rhizomes in a fungicide, such as captan, and allow them to dry in the sun for two or three hours.

·Dig up the bed or prepare individual planting areas, in full sun in cooler areas or where there's six hours of sun in warmer areas. Work in compost and bone meal.

·Replant the rhizomes on the same day, three to a clump, with the leaves pointing out from the center. Irises keep growing in the direction of each fan of leaves. On hillsides plant them with the bare rhizomes pointing downhill and the part with the leaves pointing uphill.

·For each rhizome use a trowel or a small spade to dig a hole approximately 4 inches deep and 8 inches wide. Make an elongated mound in the planting hole. Arrange the roots over the mound with the rhizome resting on top so that the top of the rhizome is level with the surrounding soil. If the roots bend on the bottom dig the hole deeper. Cover the roots with soil and press it down firmly with your hands. When you're finished the top of the rhizome should still be level with the surface of the soil.

·Water the bed thoroughly after planting and keep it damp, but not soggy, until the plants are rooted.