Asked June 25, 2018, 7:12 PM EDT
Arapahoe County Colorado
Iris rhizomes can be completely dry for quite some time and still be viable. There are several tests you can use to see if they are still good. First, is the rhizome shriveled? If so, it may not grow or bloom. Second, does the rhizome feel dense and have some weight to it. The rhizome that is light and feels weightless is weak or even dead. Finally, if you are still not sure that the rhizome is alive, a small scraping or slice into the rhizome can tell you if it is still alive. If the inside is tan to yellowish white, it is probably still alive and can grow with proper soil and water. Make that shallow cut though so you don't introduce a weak area for infection.
Now is a great time to plant your rhizomes, even when it is so hot. The plants tend to be dormant in summer heat. You do want the roots of newly planted irises be well established before the end of the growing season. Plant your iris at least four to six weeks before your first hard freeze or killing frost. in Colorado, that can be as early as mid September and almost always by Halloween. Iris bloom best with six to eight hours of full sun each day so location is important.
Plant your rhizomes at or just barely below the surface of the ground. Irises should be planted so the tops of the rhizomes are visible and the roots are spread out facing downwards in the soil. However, in extremely hot climates or with very light soils, cover rhizomes with up to one inch of soil. Tamp the soil firmly to anchor the rhizomes until new roots begin to grow, and water well. It is a common mistake to plant Irises too deeply. If your rhizome "pushes itself out of the ground", loosen the soil slightly underneath the rhizome and gently push it back down to the correct depth. You can also add a small amount of soil on top of the rhizome and lightly pat the soil down.
Newly planted rhizomes need moisture so their root systems develop. After planting, water well and continue watering deep enough to penetrate the shallow root system. Less frequent deep watering is better than frequent shallow watering.
I hope that this helps and your iris are beautiful next spring.
Colorado Master Gardener