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Connecticut Garden Journal: Tips for preventing powdery mildew on phlox

Mehltau am Phlox with mildew spreading on its leaves.
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Powdery mildew is a fungus that can attack the plant's leaves turning them yellow and white and making a mess of your phlox plants.

One of the stars of the perennial flower garden is phlox. Phlox paniculata and Phlox maculata are the two most common tall, garden phlox species. They feature plants that grow 2- to 5-feet tall, grow best in full sun and like well-drained, moist soil. You can grow them in part shade, too, but they won't flower as well and are more susceptible to diseases. Tall garden phlox are clump forming perennials and you can divide them in spring.

One of the biggest concerns with growing phlox is powdery mildew. This fungus can attack the plant's leaves turning them yellow and white and making a mess of your phlox plants. The easiest solution is to grow powdery mildew resistant varieties. 'David' is the classic white. 'Shortwood' is a rosy-pink colored variety that grows 3 feet tall. 'Goldmine' is a bright magenta colored flowers and 'Candy Store' comes in a range of bright colors.

If you do have a favorite garden phlox variety that gets mildew each year, you can reduce the damage. Space plants further apart so they don't touch each other. This allows for better airflow, drier leaves and less disease. Reduce overhead watering for the same reason.

There are also some preventive sprays. Horticultural oil and a baking soda solution create an environment on the leaf surface the fungal spores don't like so won't grow. These sprays only work on healthy leaves and won't stop an infection that's already started. For the baking soda, mix 1 tablespoon of baking soda with 1/2 teaspoon liquid dish soap in 1 gallon of water. Spray early and often, especially after a rain.

Charlie Nardozzi is a regional Emmy® Award winning garden writer, speaker, radio, and television personality. He has worked for more than 30 years bringing expert information to home gardeners.