The lone voice of horticultural reason

Iris Borer Alert!

8-14-1997 — Here’s a great question to test your gardening knowledge: If a garden writer is typing his weekly column late on a Tuesday night and still smells of chlorine beach, what’s the column going to be about?

Iris borer alert! Iris borer alert! I’ve heard and read about the problem for years but never encountered it in my garden, diligent as I am to cut back all iris foliage in the fall and remove it from my property (more on that later). But an infestation has been building in the western suburbs the past five years and last week I discovered it’s claimed me. If you have irises on your property, pay attention, please, so that this week you can become part of the solution, not the problem.

  1. Here’s what to look for and here’s what to do:
    You probably have borers if: your irises didn’t bloom this spring or bloomed sporadically; buds appeared but turned brown and watery; leaves acquired saw-toothed edges in early summer; leaves turned or are now turning yellow and/or brown, often in streaks and splotches. Bearded irises are most at risk but you will find borers in old clumps of Siberians, too. If your irises all bloomed this spring and today appear healthy, with tall, perfect leaves, you’re probably OK, but should inspect a clump or two anyway, just to be safe.
  2. Dig up an entire clump with your perennial fork. Shake and rub off all the dirt. Divide irises by snapping them apart with your hands, like opening a fortune cookie. Healthy rhizomes should snap as loudly as twigs. Sometimes you’ll find they twist apart, almost unscrew from each other. Be ruthless, go fast and don’t worry about hurting the plants-you’re going to be scrubbing every inch of them with Clorox bleach and a stiff brush in a minute, they can handle a little snapping. It’s best to break them down to single rhizomes, that is, the white fleshy part with three to ten roots dangling from it and a fan (sometimes two) of three to five leaves off one end, like an ostrich’s tail. Also break off the darker portions of long rhizomes that don’t show any roots-that’s last year’s growth.
  3. You are looking for holes in the rhizomes, little tunnels and caves, and mushy brown, rotted parts. Many of you are going to find all the above. If you do, keep checking and you will undoubtedly come face-to-face with the little rascal himself, a one-inch and longer grub-like worm, white turning reddish-brown at the head, wriggling, chomping and mad as hell. Extricate them from within rhizomes using a thin piece of wire bent into a hook. Iris borers are the larvae of a small gray moth, Macronoctua onusta; if you fish, save the “worms” in a bucket with a little sandy soil, for they are unsurpassed as bass and walleye bait.
  4. Toss any collapsed, mushy or mostly devoured rhizomes into the garbage can. Make a pile of those you’re going to save. Grab one and cut the fan to 4″, flip it around in your hand, grab all the roots like a barber grabs a hank of hair and in one cut trim a third off the roots. Do this over a bucket so you can collect all the fan and root snips and remove them from your property. Next scrub each plant with undiluted bleach, immediately splashing each in a bucket of clean water. Spread them on the lawn and rinse them all again thoroughly with a hose. This takes care of any disease organisms that might attack weakened rhizomes. An alternative to individual scrubbing is to soak rhizomes in a solution of one part bleach to nine parts water for an hour. Remove and rinse thoroughly.
  5. Leave them on the grass to dry or place them on screens raised on sawhorses; dry for two days. Direct sun is fine. Then place them in a brown grocery bag, sprinkle liberally with bulb dust, close the bag and shake thoroughly. Leave them in the bag for two to three days. You must use an insecticide brand containing methoxychlor-Bonide is one. Sevin, the common insecticide dust used by vegetable gardeners, won’t cut it.
  6. Remove all roots, rhizome mush and larvae from the soil and sprinkle it with bulb dust. Add some 10-10-10 or 10-20-10 granular fertilizer and mix in with a spade. Good time to add coarse sand if the soil isn’t sandy and well drained.
  7. Replant your irises leaving the entire fan and top of the rhizome exposed. Spacing irises 6″ apart in a circle with two in the middle makes for a nice show next spring. Water them in moderately and water once a week in periods of no rain, through September. After the first hard frost cut the fans down to 1″. In the spring, spray irises and the ground around the rhizomes with Cygon when the new leaves are 4″ tall. Spray again mid-summer. This spraying should be done every year. Plus always cut back the foliage in October to 1″ and remove it from your property-the foliage is where the moths lay their eggs, which started the mess in the first place. Remember, irises must be divided every three to four years, healthy or not.

The main cause of this infestation are homeowners who don’t garden but bought a yard containing irises. They’re alongside the house, or in the side yard, or growing up through the weeds in a strip in back where the leaves are dumped. They’ve stopped blooming, the homeowners don’t know why, don’t care, don’t understand that their yard has become an unchecked, literal breeding ground for iris borers. Ask your neighbors, tell your friends, check on the property of the nice old lady down the street. We do not get beauty without work. This is gardening.

Don Engebretson
The Renegade Gardener