CO-Horts Blog

Monday, July 13, 2015

Iris Rejuvenation

Jane Rozum, Horticulture Agent, Douglas County
Now that summer is in full swing and the garden has been planted, you can rest easy, right? Well apart from pulling a few weeds, there is a task that you can accomplish in July and early August. Irises are some of our toughest and most durable landscape perennials and are best divided after bloom. July and early August are just fine for refreshing your iris clumps. For best bloom, irises usually need dividing every 3-4 years.

Iris bed in need of dividing
Iris form fleshy, elongated rhizomes which sit at soil level. Large roots extend from the lower portion of each rhizome.  When left undivided for many years, they can form thick mats which can grow on top of each other.  At this point, many rhizomes will not produce flowers.

section of undivided iris
To divide irises, use a digging fork to lift a clump of iris out of the soil. Gently pry the rhizomes apart, shaking off soil to see the individual rhizomes and roots. Break apart large sections of rhizomes into sections which have one or two leaf fans, discarding shrunken or older rhizome areas. For easier replanting, trim the leaves to about 6 inches above the rhizome.
Irises with trimmed leaves
Some gardeners treat the open fleshy area with fungicides to prevent disease, but I don’t bother and haven’t had problems with disease or pests on irises in my garden.

Since iris rhizomes like to sit at soil level, I usually dig “W” shaped planting troughs when replanting.  I place the rhizome mid-W, leaving the roots on either side, in the troughs. Firm the soil around the roots and water. It may be best to plant rhizomes in the same direction, so that as they mature and grow, the rhizomes don’t grow into each other.
'W' shaped planting troughs
rhizomes and roots in planting troughs
A Weld County-Colorado Master Gardener once taught me a neat trick to help remember what color, the standards (upright petals) and falls (downward-growing sepals) of each clump in a mixed iris bed. She writes the colors on the iris leaves with a permanent marker when the plant is blooming. When division time comes a month or so later, she has a record of which color of iris is where. 
iris coloration written on leaf

For more information on irises and division, check out Plantalk Colorado # 1076 and #1071


  1. I found out from our local iris expert at MSU extension that irises don't bloom again on a rhizome with a flower stock and to discard that one, but keep the ones attached to it.

    1. Yes, you are correct. I forgot to mention this in the body of the text. Thank you for your comment!

  2. Great blog, Janey! I have been avoiding dividing my iris for years... The blooms are decreasing, so maybe this blog will motivate me? The comment above is really interesting. Thanks, "Anonymous"!