Miss Floribunda: Protect your roses from insects

You can add Elmer's Glue-All while pruning to prevent insects from boring into rose stems. Photo courtesy of Lowes.

Dear Miss Floribunda,

Thank you for introducing Dr. Huey last month. I’d never heard of this rose before, though I certainly knew the problem it causes. You mentioned hard winters and accidental injury being possible causes of Dr. Huey’s takeover. I have noticed something else that causes the top rose to die back and leave only Dr. Huey rootstock. After I prune my roses the stems often darken beneath the cut. When I cut the dark part off I find a circular mark in the center of the wood. I have been told it’s the mark of a “borer” but don’t know exactly what’s doing the boring or what to do about it.The more I cut back the worse the problem gets. Systemic poisons have been recommended by gardeners who have success with roses but I don’t want to go that route.  Some suggested putting nail polish, liquid wood and even school glue on the cuts. I was desperate enough to try these messy experiments and had no success. What do you suggest before I give up on roses in complete exasperation?

Coming Unglued on Gallatin St.

Dear Coming Unglued,

You are quite right that Dr. Huey can turn up when a rose bush is damaged by insects and/or inexpert pruning. I have turned for advice to my rose mentor Citizen Cane, who knows all about pruning, insects and horticultural remedies—including glues. He suspects you have used Elmer’s School Glue to seal branch cuts instead of Elmer’s Glue-All, which was probably what the “ding-a-lings” meant to recommend. The school glue is water-soluble so it washes off in the rain whereas the other is as gentle but  forms a kind of vinyl shower cap over the stem or branch. This keeps out  the humidity that invites invading insects to lay their eggs in the soft pith. No sharp ovipositor (egg-laying apparatus) can penetrate the tough cap. Cane warns that nail polish and liquid wood contain toxic chemicals that further harm the rose bush. While I have conscientiously avoided naming any commercial brand in the past, my mentor insists that nothing compares to that one glue and he asserts that it is better than any expensive horticultural product you would find at a nursery. Here’s the process he uses: after clipping he squeezes the glue over the raw wood, then spreads and removes excess with a wooden popsicle stick and a moist paper towel.  He uses sharp clean high-quality bypass (not anvil) pruning shears, and he carries them around in a can of bleach and water to keep them disinfected. Continue to be watchful, and if you see the tell-tale blackening of the wood, cut back to below where you last see a circular mark. The mark shows where the larvae of the boring insect has penetrated.

What insect is actually doing the damage? In our area there are several possible suspects, but the most likely is the stem-boring sawfly (Hartigia trimaculate). The parents are black and yellow wasps that become evident in May when your roses first bloom and you are cutting bouquets. They lay their eggs in the pith of the cut rose. When the eggs hatch, the larvae create chambers where they develop into adults and then gnaw their way out later in summer. The second generation lays more eggs, the larvae of which tunnel downward to spend the winter near the base of the plant where Dr. Huey waits.

Now I’m going to both make a confession and present a very different suggestion for the unambitious home gardener. This is “anecdotal” and based on nothing more than my own experience. When I first began planting roses I knew so little about pruning I decided not to try it. I had no problem with borers, though admittedly my rose bushes were leggy and overgrown. One year on a hot summer day I decided to try to shape them. Few of the hybrid tea roses survived but floribundas, shrub roses and the antiques aka OGR (Old Garden Roses) seemed more forgiving. This was due to their innate toughness but also because I did not prune the fluffy bushes more than very lightly. I was aware that opulence rather than simple elegance was their appeal and dead-heading was all that was needed. Success was half and half with climbers. As it happened, one of my other concerns at the time was to find advice about how to meet the challenge of very hot summers. When I read that in stressfully hot and humid southern gardens climbing roses should be left alone, except for a little deadheading of spent flowers, I decided to limit pruning in general. When I re-acquired more hybrid teas, I decided to prune only twice a year. I would remove dead wood and make a few wake-up cuts in very early spring. Then in December I’d cut back the tallest bushes to waist height to prevent the wind-whipping in January that can seriously disturb root systems, and if needed, remove criss-cross branches on hybrid teas. While I cannot claim to produce roses I’d dare enter in competitions at least  Dr. Huey has not returned to my garden.

The Hyattsville Horticultural Society will not have a monthly meeting till September but instead will visit the Kenilworth Park and Aquatic Gardens on the morning of Saturday, August 20 to admire the fabulous lotuses. Please join us.