How to Grow and Divide Irises

Follow these instructions on how to divide irises and develop new varieties.

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Showy Purple Iris

This enduring perennial, a subject of poetry, painting and countless garden photographs, grows from rhizomes, a fleshy root. The color choices of irises are many. And if the usual spring bloom isn't enough for you, you can choose several different varieties and keep the garden bathed in color from season to season. "You can have irises blooming year-round if you include some of the varieties from the subtropics," says Ray Schreiner, an iris nurseryman in Salem, Ore.

And another great thing about most irises: once established in the garden, they're fairly low-maintenance. The bearded irises are moderately drought-tolerant; the Siberian irises need moist to wet conditions.

One of the most common mistakes in growing irises is planting them too deep. The rhizomes like to sit on top of the soil sunning themselves, so leave the upper part of the rhizome exposed. And go easy on mulch: the only time an iris needs mulch is during the first winter to help the roots set.

Just about the only maintenance that irises need is division every three to four years. The best time to divide is a month or so after bloom. Schreiner recommends one of two methods: digging-and-dividing or thinning old growth while the plant is in the ground.

The variety of irises on the market is enormous, and breeders continue to seek different colors or color variations and select for bigger flowers, more vigor or bigger beards. Hybridizing irises is very easy, requiring only that you brush a stamen of one iris onto the top of the style arm of another.

Irises can be grown in almost any climate, but they need the winter chill to achieve dormancy. This period allows them to store enough energy to rebloom the following year.

Iris growers look for at least seven blooms on the stem, but you can also find varieties displaying a generous 13 or 15. And some irises are rebloomers so you get to enjoy the beauty again, later in the season.

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