Digging In - Advice on Powdery Mildew, Non-Blooming Hydrangeas

By Scott Aker
Special to The Washington Post
Thursday, January 15, 2009

Q Last summer, my peony developed gray, chalky-looking leaves. Some leaves turned brown and died. I also saw small insects on the leaves. I treated for the insects, but I am not sure how to fix the other problem, should it recur this summer. What do you advise?

A Your peony had powdery mildew, a fungal disease common to peonies, lilacs, zinnias, squash vines and other plants in late summer. This problem has been more common in recent years in our area. Because it appears late in the season, after much of the annual photosynthesis is achieved, it is not a serious threat to the health of your peony. I'm not sure what the insects might have been. Peonies have few insect pests, among them scale insects and thrips. Even those are not common. Ants harvest the sticky, sugary secretions on the bud, though they don't harm the plant.

Peonies thrive in cold climates, even in northern latitudes where the growing season is only four months long. In our climate, they suffer somewhat from our warm night temperatures.

They can survive complete defoliation as long as it does not take place until after early August. However, the plants will be more vigorous if they retain healthy foliage throughout the growing season.

Powdery mildew can be treated easily with an application of horticultural oil or a number of fungicides labeled for the disease. It's important to protect the foliage as soon as we encounter a period of dry weather. Powdery mildew spores are killed by water on the foliage surface, and new infections do not occur during rainy periods. The disease may progress very rapidly when there is a big difference between day and night temperatures, particularly if the plant is in a location with poor air circulation.

I have an old snowball hydrangea that used to bloom beautifully, but in the past two years it has hardly bloomed at all. I don't fertilize beyond an initial dose in the spring, and I don't cut the stalks down. What could be the problem?

Bigleaf hydrangeas like yours fail to bloom for two main reasons: Either the owner has cut back the plant in the fall or spring, or a late frost has killed the buds at a tender stage. Your hydrangea didn't bloom because of cold weather in late winter that killed the flower buds.

Cold injury may take place even when we have a relatively mild winter. If winter temperatures are mild enough, hydrangeas begin to break dormancy. If a cold snap follows, the damage can be severe, even if the cold is not extreme.

Most of the new hydrangeas that have appeared in nurseries in recent years have the ability to bloom on new wood. They will bloom even if the cold kills many of the overwintering flower buds. If you want to be assured that your hydrangea will bloom, you may want to consider replacing it with one of these hybrids.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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