Question. 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, PA
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 keep the white stuff at bay.
Question. I have a "patch " of lilacs, (i.e., one big bush with multiple canes). 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
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; these things breed disease like mad. As with roses, remove any old mulch in the spring and replace it with an inch or two of fresh, 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.
Question. 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
- ---Dan in Reston, VA
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 some strange kind of native bee, 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 cabbage worms (a notorious caterpillar pest) by more than half! So try and deter rather than kill them. Spray the bark at night with an organic repellent, 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.
Don't seal or otherwise try and repair any damage; let the plant take care of that naturally. And finally, don't worry about this becoming a seasonal problem. No one knows why they pick certain 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. My lilac has been out there for quite a few years now and this summer was the first time I had bark-stripping wasps stop by. I decided to let Nature take its course and the lilac looks none the worse for wear.
I wish I could say the same of myself.