Thursday, July 22, 2010

Iris pallida, also known as variegated sweet iris, has a particularly striking leaf, with elegant vertical stripes of cream and white, and the added benefit of being less prone to rot.

Rot's the reason to keep the rhizome relatively dry and close to the surface. It's one of the two plagues specific to bearded irises. The second is the iris borer. The brown moth lays her eggs on garden debris in late summer and fall. The tiny borer larvae hatch in the spring, climb up the iris leaves, chew into them, and eat their way down inside the leaves, reaching the rhizomes by midsummer. In the rhizome they grow into 1-1/2-inch-long caterpillars with a worm look—reddish brown head and pink body. After chewing tunnels through the iris rhizomes, they spin themselves a shiny chestnut-colored chrysalis, hatch, and breed a new generation.  YUCK  or YUCK !!   The holes the caterpillars chew make the rhizome susceptible to bacterial rot, and it turns slimy and foul-smelling.

Sure, irises come in nice colors and opulent, velvety textures, you're thinking, but why go to the trouble if borers will swoop in and ruin them? There is a safe and fairly effective organic mode of borer control. In fall, clean up and destroy the old stems, leaves, and leaf litter where the mother moth lays her eggs. This, I think, is why iris growers traditionally cut their irises back to a tidy 6-inch fan, which isn't really necessary. A chemical solution is to apply insecticide dust to the iris plants once a week from first growth until flowering. Here, you run the risk of killing off some beneficial insects.

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