About the SFMGA

Saturday, January 26, 2008

Dividing the Rhizomes: What, How and Why

By Cindy Bellinger

Now is the time to divide plants that grow from rhizomes such as the Papaver Poppy, the Bearded Irises and Daylilies. Rhizomes are fleshy, underground rootstalks that send out shoots from its many nodules or eyes as they're sometimes called.

Why Divide

Dividing improves the vigor of plants. In some cases, like the irises, if they get overgrown they may develop a kind of soft, mushy rot that can be taken care of by separating parts of the root. If left undivided, the root systems become enmeshed resulting in:

  • the reduction of size and numbers of flowers
  • many shoots that remain undeveloped
  • new foliage grows poorly

When to Divide

Most can be divided as soon as they are through blooming for the year, and it all depends on the growing conditions. If your plants are happy, they may need dividing sooner because that means they're growing quickly. But here's the rule of thumb:

Papaver Poppies
every 5 years
Bearded Iris
every 2-3 years
every 3-4 years

Late summer is the best time to divide these plants because they've finished blooming and the plants are resting before going into dormancy. When poppies are growing, they become sensitive to disturbance around their roots; so it's best to wait until they've finished their blooms. Irises and daylilies aren't so particular.

How to Divide

The best tools to use are garden forks or rounded spade shovels.

  • water the plant the day before to keep the soil intact
  • dig around and under the entire plant; lifting it carefully
  • prune the blades to about 6 inches; daylilies about 12 inches
  • separate nodules by breaking or using a sharp knife

Planting New Divisions


These plants don't like to have their soil disturbed so try not to knock off the dirt. Set immediately into new holes about 12 to 15 inches apart. They like improved soil. Cover with ½ inch of soil.


Dig hole deep enough to accommodate all those gangly roots, spreading them out the best you can. Iris are tough little guys and don't require much fussing, especially in the Santa Fe area. Our soil dries out enough to discourage rot. But they still like to have portions of the root exposed a bit to catch the air.


Dig a wide shallow hole, making the depth slightly less than the height of the rootball and 6 to 9 inches wider. Place in the hole and backfill with soil, lightly tamping into place.