We don’t often think of July as the time to be digging up our perennials and dividing them. Usually this task is relegated to either the early spring before much new growth has developed, or in the fall after the plants have gone into dormancy. Bearded iris are special, however, in that they should be divided soon after they have finished blooming. Although dividing them in spring or fall will not lead to the plants’ demise, it will lead to a loss of a season of bloom. Dividing iris shortly after they have finished blooming allows the plants to establish themselves and set buds for next year, which means that you will not be without bloom for a year.
Iris need to be divided every three years or so; the clump tends to die out in the middle, and the centre of the clump then is comprised of a whole bunch of big old rhizomes which put forth no green growth – the new growth will always be around the perimeter of the clump on newly formed rhizomes. It is these new pieces of rhizome around the outside of the clump that should be saved and replanted when the clump is dug up.
The old rhizomes from the centre can go to the compost heap while the younger rhizomes are carefully broken off where they are attached to the old rhizomes. Each new rhizome will have a few roots on its underside and the planting holes for the new rhizomes should be deep enough to allow these roots to be spread out and covered over with a good covering of soil. The tops of the rhizomes, however, are barely covered with soil; iris resent being planted too deeply and the tops of the rhizomes are often visible above the soil.
It is an opportune time to amend the soil while the division process is underway; iris like a rich, but well-drained soil, although they are not fussy plants and will thrive in most common garden soil. The addition of a bit of sand to the soil will ensure that the rhizomes do not get too wet which could lead to rot during an extended wet spell. The leaves can be left alone, although some people cut the foliage back by a third. Cut at an angle; the cut-back foliage will not be too unattractive during the rest of the growing season. If the leaves are not cut back the roots of the newly planted rhizomes may not be able to supply the leaves with all the nutrients they require and the leaves may turn yellow, then brown, at the ends. Cutting the leaves back often prevents this from happening.
Midsummer division of iris varieties other than the bearded iris (e. g. grassy iris, Siberian iris) is not as imperative as they do not seem to die out in the centre of the clumps. They are more of a clumping plant and their rhizomes are smaller so early-spring or autumn division will not result in a total loss of a season of bloom. Division of these plants is done similar to many other perennials, whereby the clumps are dug up and simply reduced in size before replanting, by slicing the clump into several smaller pieces.
If you have recently divided your bearded iris and they do not require division this year, be sure to remove the flower stalks after the blooms have faded. This improves the appearance of the plants and allows the nutrients to be used to develop next year’s buds rather than seeds.
– Albert Parsons writes from Minnedosa, Manitoba