Growing Concerns 59: Beating iris borer and clematis wilt

Iris borer

I have a large perennial garden, with my favorite bearded iris having a prominent role. However, so do the iris borer who each summer decimate my iris. I dig them up, cut out the worms and replant. Bingo, by August the following summer, the iris get attacked again. Is there no preventative on the market to kill the borers before they kill the iris? - S.C. -

No preventives, only controls. If disposing of borer grubs you find in the roots is the extent of your effort, you need to do more.

Here's the borer's life cycle and how to interfere.

Winter: eggs wait on old iris debris. So, destroy all old foliage each fall.

April - early May, when new iris foliage is 5-6" tall: eggs hatch, tiny larvae chew on and tunnel into leaves. They're vulnerable before they enter the leaf. Dust or spray iris with an insecticide every two weeks during this time. Use diatomaceous earth, horticultural oil, paraffin-based Sunspray, or a contact insecticide such as Malathion.

July: larvae move into roots as they become large pinkish-white grubs. Dispose of those you find in the roots. Others have moved out into the soil, formed a pupa, and are about to become adults. Either sift the soil around the plant to remove these, or rotate the iris to new locations. Carolyn Study at Valley Nursery says they've never had enough borers to worry about, though they've grown iris for many years to sell along with other perennials, annuals and produce. The reason for this is probably constant movement: when the iris are divided, they're frequently moved, sometimes almost a quarter mile away.

August - September: pupae in the soil mature. Grey, 1-1/4" adult moths emerge, lay eggs in clusters on iris foliage and iris debris. This is why crop rotation pays off. Though adults can fly, they may not find the new location, and their chances of being intercepted and eaten by predators is much greater than if they only had to fly a few inches.

Double-up possibility while iris take a break

Bottom line: clean the iris bed well in late fall, and move your iris around, even take them out of that bed for a time. Letting go of favorites is hard, but change can be rewarding. Try unusual bulbs such as quamash (Camassia cusickii, blue; Camassia leichtlinii, blue or white) close planted with late-emerging, late-blooming balloon flower (Platycodon grandiflorus) or low and leafy sea kale. Quamash will fill the iris time slot, late season balloon flower is a bonus.

More about iris borer... with photos!

Clematis wilt

I need your help as soon as possible with my clematis vines. I have them on either side of the garage, each one on a trellis. Last year they bloomed beautifully with an abundance of blooms and buds and suddenly as the weather warmed up they withered and died. This year I moved the trellises a foot away from the garage thinking the heat on the garage is killing them. Same thing happened - loads of flowers and buds, but then hot weather and they all withered. Do clematis hate the full sun? The heat? E.S., Somerset

You're dealing with clematis wilt, a widespread problem that mystifies many. May is the time to deal with this fungus disease, so tickle this column to your spring calendar!

Clematis wilt fungi enter the plant through nicks or breaks in the vine, reproduce, and clog up the stem's circulation. Branch tips wilt as a result and then within a week the whole vine above the point of entry blackens and dies.

Look closely at a stem that died. If it was killed by wilt, you'll probably find discolored (rotted) spots where the fungus invaded, just below the lowest pair of wilted leaves. The injury through which the fungus entered may have been caused by wind rocking the plant, a slug chewing the stem, or a gardener tying up the vine. Wilt fungi are particularly active when it's very humid, as the weather often is in July when the clematis is just coming into full bloom.

If the clematis is well established before the fungus strikes, it usually has enough energy left to produce new stems from below the point of attack. New shoots may escape attack and so keep the plant alive into the next year.

The fungus spores can lay in wait a long time, on fallen leaves from past victims and even on unrelated plant species. So replacing the vine is not a solution but a set-up in which the new plant is the patsy.

To cure this problem you'll have to accept the fact that the fungus does exist within your yard and your soil, and that you can't eradicate it. Concentrate on making your vines inhospitable to fungi, through the use of a systemic fungicide containing Benomyl.

Drench the soil at the base of the plant with a Benomyl mixture, and the roots will take up the fungicide and move it all through the plant. Mix it per the label directions and apply it once in May, once in June. Use one half gallon to two gallons of mixture per plant - less for a youngster, more for a big, established vine.

Benomyl has its drawbacks: it kills earthworms; it also kills beneficial fungi that normally keep certain insects in check or break down dead organic matter. So use this and all fungicides only where needed - on the afflicted plant.

The late, great Christopher Lloyd wrote the book "Clematis" (1989, Capability's Books, P.O. Box 114, Deer Park, Wisconsin, 54007). It's a worthwhile investment for anyone seriously interested in clematis. Mr. Lloyd spent a lifetime growing clematis of every type and we're grateful that he wrote to share what he learned.


Originally published in two parts, 9/3/94 and 9/10/94