Ask the Master Gardener: Managing powdery mildew on ornamental plants

Powdery mildew is easy to recognize by its characteristic coating of “powder,” which is typically found on the upper sides of leaves. Although it is quite unsightly, it is rarely fatal to the plant.

A Veronica plant with powdery mildew. Photo by Jennifer Knutson

Dear Master Gardener: My Veronica plants have a white powder all over the leaves, but none of my other plants have it. What is wrong with the Veronica and should I do something about it?

Answer: It sounds like your Veronica plants have powdery mildew -- a very common disease found on ornamental plants. Powdery mildews are host specific, so the powdery mildew pathogen found on your Veronicas cannot infect your other plants and vice versa. Powdery mildew is easy to recognize by its characteristic coating of “powder,” which is typically found on the upper sides of leaves. Although it is quite unsightly, it is rarely fatal to the plant. We have had warm, extremely dry weather, which is a condition that favors this disease. If plants are crowded together and air circulation is poor, that too can favor an outbreak of powdery mildew. Now that your plants have the disease it is important not to fertilize them any more this season, avoid overhead watering to help reduce the relative humidity, and in the fall clean up all plant debris around the plants. Do not compost infected plant debris because compost may not heat up enough to kill the fungus. If your plants are too crowded, prune them or remove a few to help increase air circulation, which will reduce relative humidity and infection. Many plants have cultivars that have been bred to be resistant or tolerant to powdery mildew and Veronica is one of them. When purchasing plants, check the labels for powdery mildew resistance or ask if resistant varieties are available.

Dear Master Gardener: How poisonous is water hemlock? Is it found in Crow Wing County?

Answer: Water hemlock, also known as spotted cowbane, is the most violently toxic plant in the United States. It is deadly to both humans and animals. Water hemlock is a native plant in the carrot and parsley family and distinguished by the vein pattern in its leaves, which radiate from the central vein and tend to terminate in the notch of the teeth instead of the tip. Its tuber-like, fleshy roots resemble small sweet potatoes, smell like parsnips, are easy to pull out of the ground, and are the most lethal part of the plant. A piece of root the size of a walnut can kill a cow. Yes, this plant is found in Crow Wing County and most counties in Minnesota. Water hemlock is usually found in wet areas such as marshes, wet streams, and along lakeshores. The white, umbrella-like flowers bloom from June to August and look similar to Queen Anne’s Lace.

Dear Master Gardener: Is now a good time to divide my perennials?


Answer: It depends. Spring and summer blooming perennials can be divided now. Don’t divide perennials when they are blooming and try to divide them on a cloudy, overcast day so they experience less stress. It would be ideal if you divided your plants when there are a couple days of rain in the forecast to soak them well, but since there was a 99% chance of rain one evening last week and it didn’t rain, you probably shouldn’t count on that. Perennials with fleshy roots such as peonies, Oriental poppy, and Siberian iris are best divided in the fall, four to six weeks before the ground freezes. Other perennials that can be divided now and into September are: Ajuga, Allium, Astilbe, Brunnera, Columbine, Coreopsis, Creeping Phlox, Heuchera, Hosta, Lady’s Mantle, Lilies, Pulmonaria, Salvia (divide when center of plant dies), Tiarella, and Veronica. Hemerocallis (daylilies) can be divided right after they are finished blooming or in spring.

Dear Master Gardener: I would like to have a small water feature without digging a pond. What kind of container can I use? What plants do you recommend? Is it legal, or even possible, to get water lilies from the lake and transplant them into a water garden?

Answer: Creating a water garden – even one as simple as a large container of water – opens up a whole new world of plant possibilities. A container water garden can be created from a whiskey barrel lined with plastic, plastic trash can, large ceramic pot, or a stock tank. If it’s attractive you can leave it in sight, otherwise sink it in the ground leaving a few inches above ground to keep soil from washing into the container. The container should be large enough for the plants you are growing and placed where it gets at least six hours of sun per day. Miniature water lilies, lotus, and many other water plants do well in as little as 20-30 gallons of water. You can even add a little pizzazz by adding fish, a small fountain, or a submersible light on a timer.

The white and yellow water lilies you find in local lakes are not protected species. However, they are important to keep in lakes because they provide habitat for largemouth bass and sunfish, and the seeds are eaten by waterfowl, so it may be best to leave them in their natural habitat and purchase non-native lilies for your water garden. Keep in mind that non-native water lilies are regulated invasive species in Minnesota. That means they are legal to possess, sell, buy, and transport, but may not be released into public waters. Water gardens often include floating, submerged, and edge plants. Floating plants include water hyacinth, giant velvet leaf, water lettuce, and water lilies. Although not a floating plant, lotuses are a lovely choice and hold their leaves and flowers one to eight feet above the water (depending on the variety). Submerged plants grow under water and include: hornwort, hairgrass, and sagittaria. The Parrot Feather plant grows mostly underwater, but often has above water stems that can be very attractive in a container. Edge plants to consider are: sweet flag, canna, marsh marigold, water plantain, and sedges. Local garden centers and on-line sources usually only sell plants that are allowed in our state and will recommend plants that do well in container water gardens.

You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A Master Gardener will return your call. Or, emailing me at and I will answer you in the column if space allows.
University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.
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