Ask the Master Gardener: Powdery mildew on ornamental plants unsightly but rarely fatal

Unfortunately, once you see powdery mildew, it’s too late to treat it.

Powdery mildew on leaves of a plant.
Powdery mildew on a Veronica ornamental plant.
Contributed / Jennifer Knutson

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

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Answer: It sounds like your Veronica (speedwell) and Phlox 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 and Phlox 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. Unfortunately, once you see it, it’s too late to treat, so don’t start applying fungicides now. If plants are crowded together and air circulation is poor, that can favor an outbreak of powdery mildew. Overhead watering or a lot of rain can also aggravate the problem. 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. When purchasing plants, check the labels for powdery mildew resistance or ask if resistant varieties are available.

Dear Master Gardener: Moss has invaded my lawn. Should I add limestone?

Answer: When moss thrives in a lawn it’s a sign that the grass is weak and has thinned, allowing moss to take over. According to the University of Illinois Extension, some potential causes include excessive shade, compacted soils, poorly drained soils, low soil fertility, high or low soil pH, and/or poor air circulation. Adding limestone is a common “cure” often mentioned for moss control, but is not suggested unless a soil test indicates the need for it. The University of Illinois says, “ferrous ammonium sulfate or ferric sulfate (iron sulfate) can be used to control moss to some extent. The moss will temporarily burn away, but tends to return fairly quickly. Raking out moss is another option, usually followed by reseeding.”

Too much shade for acceptable grass growth is typically the cause for moss invasion. Trees and shrubs can be pruned to add more sunlight and air circulation. Make sure you have the appropriate grass for your site — there are lawn mixes specifically for sunny and shady areas. Getting a soil test is beneficial as it evaluates soil fertility, pH level, and/or problems due to excessive salts or fertilizer materials. The test results will give you the appropriate fertilizer recommendation for good plant growth without adverse effects on the environment.


August Gardening Tips

  • Divide and replant perennials that are overgrown and no longer blooming well. Discard the central part of any clump that has become woody, then replant the outer portions. Wait until spring to divide fall-blooming plants, such as mums.
  • Iris become dormant and slow their growth this month, so now is the time to divide them if needed. Bearded iris flower better when they are regularly divided. Cut the foliage back to 6-8 inches and keep one or two foliage clusters per rhizome.
  • Continue to dead-head perennial flowers to prevent seed set and enable the plant to retain more energy reserves for next year’s bloom.
  • Keep picking zucchini, cucumbers, snap beans, and other vegetables from your garden, even if they are past their prime because the plants remain more productive. Also, it is beneficial to remove rotting produce, as it attracts yellow jackets.
  • Yellow jacket wasps are numerous in August and are attracted to sweet, ripe and rotting food. Be watchful of cans and glasses of beverages, which may harbor yellow jackets you don’t see or expect before your next sip! 
  • Late summer is an excellent time to plant evergreen trees and shrubs; giving them several months to grow new roots. Evergreen trees such as pines, spruces and firs can help save energy when planted on the northwest side of your home, will provide shelter for birds, and are beautiful covered with fresh snow. It is important to water evergreens until the ground freezes any week in which we do not receive an inch or more of rain.
  • Blossom end rot is a lack of calcium caused by uneven watering. It’s characterized by a black, leathery sunken area at the bottom of tomatoes and peppers. Symptoms usually appear in hot, dry weather. To avoid it, water deeply and regularly, mulch well, and don’t fertilize too heavily with nitrogen. Avoid Epsom salts — the magnesium may actually make things worse. Slice off the affected tissue — the rest of the tomato is perfectly good.
  • Raise the height of your lawn mower blades to 3 inches. As heat builds in the summer the taller grass protects the roots and allows deeper root development. Shorter cut levels lead to shallower root systems, making the plants more susceptible to water and heat stress. Mid-August is a good time to plant grass seed. Warm soil and less weed competition greatly improves the chance for successful establishment before winter.
  • Watch for sphinx moths in the early evening. They look very much like hummingbirds.  They are about the same size, also flap their wings rapidly and hover, and they suck nectar from impatiens and other flowers with their long proboscises. 
  • Create a compost pile with grass clippings, leaves, garden debris (plant material), egg shells, and fruit and vegetable scraps. Warm August weather encourages the rapid breakdown of materials by bacteria. Keep it moist and turn it frequently to maintain an aerobic environment to speed up the breakdown process. According to the EPA, home composting can divert 700 pounds of waste per household per year from municipal waste. And, you’ve just created your own “gardener’s gold” to enrich your soil!

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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