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Over the Garden Fence

Where gardeners come to find out what's happening out in the yard.

Time to Divide Your Iris

Posted by Richard Hentschel -

Down the Garden Path

Richard Hentschel, Extension Educator

Iris can be a great addition to any perennial garden. Bearded Iris is a more traditional plant, but the Siberian Iris can also add additional visual elements to the bed. Siberian Iris typically has thinner, but taller, foliage in a solid green color. Bearded Iris has a shorter wider blade and is blue green in color. There are countless versions of colors and flower types with Bearded Iris. Like other perennials, there have been years of breeding to create the flowers that gardeners enjoy so much.

Iris really need to be dug and divided every few years, depending on how vigorous they are in the location where they are planted. Iris tubers are planted two or three inches below the soil surface. Over time the tubers grow together and over one another until the tubers are clearly visible at the soil surface. This exposes them to the elements while they should be below ground. Overcrowded tubers lead to less flowering and more foliage. That should be reason enough to dig, divide and replant.

As the Iris become overcrowded and tubers appear near the surface, something else is happening below ground that really impacts the health of the Iris planting. When you have an older overgrown planting, then the well-known Iris Borer moves in to feed and destroy the tubers. The planting begins to look weak and thin. These are not cute little boring insects that do just a little damage, but very large borers that can tunnel through and hollow out entire tubers. This leaves little food for the Iris to grow and contributes to the poor conditions we see. The borer also brings along bacterial rot which causes a smelly soft rot in the tuber as and after the borer feeds.

The basic life cycle of the Iris borer starts out with eggs hatching in the spring, after overwintering on old iris foliage and plant debris. Those eggs produce a tiny smooth-skinned caterpillar which climbs up the new foliage and then tunnels back down the leaf into the tubers. The leaf will show a streak where the caterpillar is tunneling downwards. The borer feeds until full grown and will leave the tuber to pupate in the soil. Both male and female moths emerge to mate and lay eggs on iris again in late August and September so the process can start all over again next spring.

Iris Borers are just about full grown but still in the tubers right now. This is the time to dig and divide. While the plants are out of the soil, you can remove the old damaged tubers, borers and all. The borers will be about two inches long and quite active. Iris need very little of the storage tuber to reestablish in the bed. Another benefit of this garden activity is that you can clean away those pesky perennial weeds that are always hard to remove otherwise. What you should end up with are just the leaves with fibrous roots and a bit of tuber. It is easier to handle the iris if you cut off half or more of the foliage as you work with them while you are replanting. Water well and keep the soil moist till cold weather sets in.

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