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Changing how you water peonies can prevent this common problem

Gardening columnist Don Kinzler answers questions about powdery mildew, as well as a trick to mulching around trees and how to know when squash is ripe and ready to be picked.

Peonies with Powdery Mildew, August 20, 2022.jpg
Powdery mildew is a disease caused by a fungus that creates a gray-white coating on leaves and stems.
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Q: Attached are photos of the peonies I care for at our church. To me, it looks like they have powdery mildew, which has happened the past three years or so. They’re on the east side, get plenty of sun and air circulation, and are watered by an underground sprinkler system. What, if anything, can be done about this and when? — Deb C.

A: You are right, the peonies are showing symptoms of powdery mildew, which is a disease caused by a fungus that creates a gray-white coating on the leaves and stems.

Although powdery mildew can appear in many situations, overhead sprinkling with an underground irrigation system usually sets the stage for severe mildew on peonies. These sprinkler systems are often set to water lawns several times a week, and the frequency of wet foliage easily spreads powdery mildew.

The preferred way is to water only the soil around peonies, keeping the leaves and stems dry. That’s the first line of defense, although natural rainfall and humidity can create mildew conditions also.

More gardening columns from Don Kinzler
In this week's Fielding Questions, Don Kinzler offers advice for caring for a weeping fig, tips for thinning apples, and tells readers it's not too late to wrap trees to prevent sunscald damage.

To further prevent powdery mildew, in May while the foliage is still green and healthy, apply a flower and vegetable fungicide containing the active ingredient chlorothalonil, which is readily available at most garden centers. The fungicide is a preventative, nipping the fungus in the bud before the disease can progress.

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Once powdery mildew forms, there's no way to erase it — the mildew continues for that growing season. Follow the label for frequency of application. Fungicides won't be as effective if the sprinkler system regularly gets the peonies wet. Maybe there's a way to adjust the underground sprinklers to avoid hitting the peonies.

After the first killing frost of fall, cut the peony stems down to near ground level and dispose of all stems and foliage, which can help reduce disease inoculum.

Q: I'm going to try your method of killing grass around my trees . How do I separate the grass edge from the mulch, especially to keep mowers from getting tangled up with the mulch and mulch getting out into the lawn? — Donna C.

A: In most municipal tree plantings on boulevards, and with my own trees, no separation barrier or edging is installed at the perimeter of the mulch. If an underlayment of either cardboard or landscape fabric is installed under the mulch, the grass won't grow into the mulch, and the mulch will stay separate from the grass.

Shredded bark mulch, instead of wood chips, nests together quite tightly, and doesn’t migrate out into the grass. For mowing, the wheel of the mower can ride on the edge of the mulch, and usually won't pick up the shredded bark, if lawns are being mowed at the recommended 3-inch height.

Some homeowners do install edging in a circle around the tree, and then fill the circle with mulch, but I've found the mulch also works fine without edging.

Q: It’s my first year growing buttercup squash. How do I know when they’re ripe and ready to be picked? — Jenny M.

A: There are three ways to determine ripe squash. First, the skin loses its glossy appearance and become dull. Second, the “ground spot,” the non-green circular marking, usually turns from yellow to orange. Lastly, test with your thumbnail. A ripe squash will not be easily dented, while underripe squash can be easily dented.

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Many gardeners prefer to delay squash harvest until after the first fall frost has collapsed the foliage.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.
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