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Question of the Week © 2015 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

Lilac Troubles: White Mold & Bark Eating 'Bees'

Q. I've been noticing white stuff on the trunk of my lilac bush. Is it harmful? If so, what can I do? The lilac gets a lot of sun—from around 1:30 pm until sunset.

---Linda in West Chester

A. Sorry Linda, but that is NOT a lot of sun, and it's the way wrong kind of sun for a lilac. Like roses, tomatoes and other disease-prone plants, lilacs need morning sun to dry overnight dew and dampness off their leaves. If other plants are blocking that early sun, see if you can prune or remove them. Otherwise, do everything you can to increase airflow to the area.

This includes pruning the lilac immediately after flowering every Spring to open up the center and increase the internal airflow. Remove some entire older branches down at ground level first, then take out some of the younger branches that are crossing or otherwise restricting airflow. Remove all the spent flower heads promptly as well. Do these chores immediately after the flowers have faded and it will improve the following year's flowering as well as help keep the white stuff at bay.

Q. I have a "patch" of lilacs, (i.e., one big bush with multiple trunks). Every year a white flaky substance appears on the trunks and branches, as well as a white coating on many of the leaves. A lot of the branches covered with the dreaded white stuff are dead the next spring. What can I do?

----Win in Haverford PA

A. I just can't exaggerate what notorious drama queens these plants are about airflow. My lilac—a very hardy disease-resistant variety—is growing near a bunch of ancient, inherited ferns, and anytime the ferns try to grow anywhere near the outskirts of the lilac, the wussy plant gets a white moldy coating on the trunk. When this happens (once every couple of years), I rip the offending ferns out right away, apologize to Her Majesty, and—believe it or not—just wipe the white stuff off with a damp cloth. Occasionally, I've come home from a summer vacation and 'Christmas in July' has spread to a few leaves as well, so they get pulled off and trashed.

It can't be stressed enough: Lilacs need airflow! Don't plant them in a crowded area; and make sure their area stays nice and clear all around. If the white stuff still shows up, remove and destroy any affected leaves immediately and wipe the white stuff off the trunk and branches. If you want to be fancy, mix some baking soda into the water you use for your washrag. Or even better, use compost tea for the wiping down; that'll remove the gunk and leave a disease-fighting residue behind.

In addition, never feed a lilac chemical fertilizer or use a wood, bark or root mulch underneath it; both breed disease like mad. As with roses, remove any old mulch in the spring and replace it with an inch or two of fresh, high-quality yard-waste compost. That will feed the plant for the season and prevent disease spores from incubating down there. And if the season is wet, cloudy and damp, remove that mulch and replace it with fresh compost mid-season.

Q. I thought I had an infestation of bees feasting on my lilac's branches, but now I'm not sure; they're aggressive, golden-yellow, about an inch-and-a-quarter long and have wasp-like wings. What are they and how do I get rid of them?

---Ed in Clarksville, Maryland

We have wasp-like bees eating the bark on one of our lilacs. Nothing I've tried will keep them away. Help!

---Dan in Reston, VA

These two Washington, DC area gardeners emailed me within a few days of each other in mid-September; a surprising time of year to have this kind of problem, as wasps and hornets are pretty much done for the season at that point.

That's right—the culprit is wasps, not bees. Carpenter bees do famously hollow out breeding galleries in soft woods like cedar, but they don't strip the bark off of trees. Paper wasps and hornets (themselves a type of wasp), however, DO perform this kind of mischief so that they can use the bark as building material for their papery nests. Some sources feel they also feed on the sap that flows as a result of their peeling.

Because they ARE wasps and NOT bees, they are aggressive and WILL sting if confronted, so be careful. And although these insects are a nuisance when they strip bark, they are also very useful predators that eat lots of caterpillars and other garden pests; having a hornet's nest near a garden has been shown to reduce the number of cabbageworms by more than half! So try and deter rather than kill them. Spray the bark at night with an organic repellant, like neem or one of the garlic oil products sold for outdoor mosquito control. Or wrap it with strips of row cover, sheer curtain, or other gauzy material.

Heck, wrap the whole plant in row cover if you can do it. That'll keep the hornets away while still letting light and air reach the plant.

Don't seal or otherwise try and repair any of their damage to the bark; let the plant take care of that naturally. And finally, don't worry about this becoming a regular problem. No one knows why these insects pick specific targets in certain years, but each colony only lives for one year, and successive generations don't tend to return to the scene of the crime.

Gardens Alive exclusive update!: The Question of the Week above originally appeared way back in 2010 on a show that we have chosen for a repeat this week (its Pledge Drive time in Philly). In the five years since, I've learned more about these creatures.

They are still wasps, but at least THIS September (true to the timing of the original questions) they were European Hornets on my lilac. These are big menacing-looking insects that, unlike other hornets, do NOT sting. Yes, they DO look fierce, but they are not a stinging threat. (I proved that through a variety of maneuvers that would have had regular hornets stinging me all over.)

And yes, they were stripping the bark from my lilac this year. It's probably the third time this has happened in 15 or so years of the lilac being out there (so it's not an annual problem). They hung around for a about a week and a half, seemed to devour a lot of bark and then were gone. The lilac—which bloomed its absolute best ever this past Spring—never seemed to suffer any ill effect and still looks great a month later.

I'm guessing that although it looks troubling, their September snacking doesn't harm the plant. In fact, it may stimulate it to grow better!