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How Do I Thin Iris Bulbs?

By Bobbi Keffer ; Updated September 21, 2017

With over 200 different varieties, iris has long been a gardening favorite all over the world. Iris fall in two categories--those that grow from bulbs, and those that have a rhizome root system. Bulbous iris should be dug and stored for the winter each year in cold climates while rhizomes can overwinter with a mulch blanket. Digging and inspecting iris bulbs each year is important because it is susceptible to disease and pests such as iris borers, and a quick yearly inspection will help keep your plant healthy. Plan to divide iris--regardless of bulbous or rhizome--every three to five years.

Dig around the base of your iris plants with a spade in late summer after the bloom has faded. Know if your plant has a bulbous or rhizome system because division is different for each. Bulbs have an onion-shape mass with some roots growing out of the bottom. One plant grows from each bulb. Rhizomes are tubular masses with roots that spread along just under the ground and can grow more than one bloom.

Trim fauna with garden shears down to a couple of inches above the ground. Leaving a few inches protects the bulb from anything damaging the next year's growth.

Lift bulbs, inspecting for signs of iris borers or mushy spots. Borers will leave holes and may be present. Discard diseased or rotten bulbs. Healthy bulbs will feel hard but slightly give with some pressure to the hard outer leaves.

Wiggle any new bulb growth until it separates from the adult bulb.

Replant healthy bulbs with the crown upwards 8 to 10 inches apart immediately. Bulbs' active growing season is fall to winter, so bulbs need to be replanted by Sept. 1 to ensure blooming in the next growing season.


Things You Will Need

  • Spade
  • Garden shears or clean, sharp scissors
  • Bucket


  • When dividing rhizomes, lift rhizome from the ground, inspect in the same manner for signs of disease and pest, and cut apart with a sharp knife. Make sure each piece of the new rhizome has at least one fan of leaves. Replant rhizomes by creating a cone shape in the soil and laying the rhizome across the top, leaving the fan above the ground. Fill in.

About the Author


Bobbi Keffer attended Kent State University to study education but soon found her true love to be in the garden. She prides herself on her frugal skills reusing, recycling and reinventing her whimsical style in her home and garden.