My irises are starting to come up (Hooray, Spring is HERE!) but I noticed that every bunch started coming up in a circle. I wondered if this meant it was time to divide them.
I found a great article at the UMN Extension Center site on Irises: http://www.extension.umn.edu/distribution/horticulture/M1267.html
There are two opportunities for dividing Siberian Iris. To insure flowering it is best to divide in early spring as new growth just appears. Waiting until the new growth has started may stress out the plants and prevent them from flowering that season. Dig up the clump and cut through the thick root system, keeping at least six growing points with adequate roots in each clump. Keep these sections moist, plant immediately, and then water in thoroughly. In late summer or early fall it is possible to cut back the foliage to about six inches, then dig and divide the plant as before. Replant, water in, and mulch well for winter survival. Siberian iris don’t require division as often as bearded iris to perform at their best; their tough crown often requires a strong arm to cut them apart.
So, based on this (I have great faith in extension center websites), my own understanding of when plants need dividing and this little blurb on the ask.com site:
If your irises have grown into a circle with a dead zone in the middle, it’s time to thin.
I think I will tackle splitting these asap before they get too big. There are about three clumps of irises coming up in that circle pattern. I may do one or two now and save the last for the Fall. Just to see if it makes a difference in the health of the transplants, or the ease of dividing to do them in the early Spring or Fall. I have a feeling my sister is going to want to take all my excess rhizomes, but I have a few ideas of where I’d like to fill in with new patches of iris. These have been very healthy over the years and I’m thrilled that I have three bunches ready to be split.