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Time to divide your Iris

Posted by Richard Hentschel -

Iris can be a great addition to any perennial garden. Bearded Iris is more the traditional plants, yet 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, blue green in color. There are countless versions of colors and flower types with bearded iris since, like other perennials, there has been years of breeding to create what gardeners enjoy so much.

Iris really need to be dug and divided every few years, depending on how vigorous they are in the location they are planted in. The iris tubers are planted two or three inches below the soil surface and over time those tubers grow together and over one another until the tubers are clearly visible at the soil surface. This exposes them to the elements. 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 near the surface become visible, something else is happening below ground that impacts the health of the iris planting. In older, overgrown plantings, the well-known iris borer moves in and begins to feed and destroy the tubers. The plants begin to look weak and thin. These are not cute little boring insects that do just a little damage, they are very large borers that can tunnel and hollow out entire tubers. This leaves little food for the iris to grow with which contributes to the overall poor condition of the plant. The borer also carries 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 an egg hatching in spring, after having overwintered on old iris foliage and plant debris. Those eggs produce a tiny smoothed skinned caterpillar which actually 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 and both male and female moths emerge to mate and lay eggs on iris again in late August and September to start the process all over again next spring.

Iris borers are just about full grown but still in the tubers right now, so 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 always hard to remove otherwise. What you should end up with are just the leaves with fibrous roots and bit of tuber. It is easier to handle the iris if you cut off half or more of the foliage while you are replanting. Water well and keep the soil moist till cold weather sets in.


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