I thought for a while and suggested they might be overcrowded or stressed from last year’s dividing activities. She said she had not divided any of her Iris, ever, and asked when to do that. I said not until after the blooming was complete and not to cut back leaves until the divisions were made. She seemed satisfied with the answers and went on to get a nice big juicy breakfast burrito from the booth next to ours. Imagine smelling that wonderful aroma all morning and not having enough time to buy one for yourself, much less eating one. AND they were all gone when we broke down the booth!
Well, I have been thinking of that question now for a couple of days and decided to look up Iris on the http://www.ext.colostate.edu/ website. Remarkably and somewhat surprisingly I was mostly correct!
According to “Plantalk Colorado” 1018-Perennials: How to divide
“Plants that begin to produce more foliage at the expense of flowers may be overgrown. To determine if plants are overgrown, count the number of stems that emerge from the base. If five or more strong stems are present the plant may need to be divided.” (I found one clump with ten in my collection)
“When dividing plants, pick a cool spell in the spring or fall, and gently dig up a clump with as many roots as possible. Expose the central rhizome or stem by shaking or washing away the soil. Then, with a sharp knife, cleanly slice the plant into several new pieces. Leave a minimum or two to three growing points, or it may take many years for the plants to recover. Place in newly prepared soil and keep them moist until they re-establish. Share surplus divisions with friends.” (Most likely when I do this in August I will be looking for parked cars with the windows rolled down!)
“It’s a good idea to start with a single clump of one kind of plant until you master division, rather than risking an entire planting.” (It is wise to have a first aid kit handy, sometimes sharp knives slip!)
“Iris should be divided every three to four years. Divide the rhizomes, which are underground stems, after the plant have finished flowering, but no later than August.” (I was close!) “Discard any segments that are mushy or riddled with holes. Separate the healthy rhizomes into segments with one fan of leaves and several feeding roots. Cut the leaves back to six inches. When setting the new plant spread the roots out in the soil and position the top third of the rhizome above the soil surface. Arrange the foliage to face outward away from the center of a group.
Other helpful information can be obtained from Colorado State University Extension fact sheets:
Perennial gardening
Flowers for Mountain Communities
Herbaceous Perennials