The Pox on Phlox: Powdery Mildew

Janice Weber

DeKalb County Master Gardener

Do you have a love-hate relationship with your garden phlox? The dramatic flowers are a treasure. They attract butterflies and hummingbirds. However, once the high humidity of summer rolls in, white spots of powdery mildew detract from their beauty and threaten the health of your plants.

The best practice is to choose mildew-resistant varieties. The Chicago Botanic Garden evaluated 20 phlox between 1993 and 1996. They rated the plants primarily for their susceptibility to mildew, but also for their appearance, bloom periods, pest resistance, adaptability, and winter hardiness.

The only garden phlox (P. paniculata) to surpass all others was 'Katherine' (lavender with white eye). It had heavy flower production, healthy foliage and good mildew resistance. 'Prime Minister' (white with red eye) was the only phlox with less mildew, but its fallen stems were troublesome.

Some of the best flower producers were 'Bright Eyes' (pink with ruby-red eye), 'David' (white), and 'Franz Schubert' (lilac with dark eye). But they all demonstrated only a fair resistance to mildew. This was a surprise because 'David' is promoted as nearly mildew-proof. 'Eva Cullum' (pink with dark red eye) also had a fair resistance to mildew, but fewer flowers and more winter injury. 'The King' (purple-lavender) had similar problems with mildew and winter damage, but it had lots of flowers.

'Blue Lagoon' (purple) had poor mildew resistance, floppy stems and fewer flowers. 'Fujiyama' (white) bloomed later, when a lot of mildew detracted from its appearance. 'Starfire' (cherry-red) produced an abundance of flowers, but suffered severe mildew infection and plant deaths over the winter. All the garden phlox in the evaluation had at least minor winter injury in one or more years.

Among the summer-blooming phlox (P. carolina) 'Reine du Jour' (white with pink eye) had very good mildew resistance most years and was one of the best for its abundance of flowers over a long bloom period. 'Morris Berd' (light purple) had some mildew resistance and lots of flowers, but it didn't bloom as long.

None of the spring-flowering, low-growing species get mildew. 'Chattahoochee' (lavender-blue with violet eye) was the first to bloom and flowered through June. It was vigorous and healthy until late summer, when it tended to brown out. 'Spring Delight' (rose pink) bloomed from mid-June to early August. It never had high flower coverage, and was damaged each winter. Phlox pilosa (pale purple) was a mass of flowers in mid-May, but sprawled after flowering.

When the Chicago Botanic Garden evaluated phlox for mildew they kept their maintenance practices to a minimum. But there are several things you can do to keep mildew at bay.

Give your plants space and thin out the stems to open them to air movement. Water them at ground level instead of overhead. Be sure to remove the stems and fallen leaves from your phlox when you clean up the garden each autumn. Most phlox like full sun to partial shade. The low-growing species, however, do better in light shade. They all prefer moist, well-drained soils.

A new family of phlox (P. paniculata), the Volcano Phlox series, claims to have little mildew. It's available in 6 colors that grow between 16 and 30 inches tall. They are heavy bloomers with large flower heads from early summer to fall. Cut back the plants by one third after their first bloom. 'Ruby' Volcano, the latest gorgeous color, was one of Garden Gate Magazine's top picks for 2007.

I'll see you in the stampede to the nursery.

If you have questions or comments about this article, home gardening or about the Master Gardener program, call the Master Gardeners, c/o University of Illinois Extension DeKalb County office at (815-758-8194) or email dekalb_mg@extension.uiuc.edu.