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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Growing Garden Phlox

This article was published originally on 7/14/2010

There are many perennials that bloom reliably every summer. One that is particularly impressive in my garden this summer is the garden phlox (Phlox paniculata). There are over 60 phlox species native to North America. While many of these species are suitable for Midwestern gardens, garden phlox is probably the most popular. Plants range in height from 1 ½ feet tall to over 5 feet tall. Blooms first appear in mid-June in Iowa and may last for a month or more. Flower colors include lavender, white, orange/peach, pink, red, and many with light or darker centers or "eyes". This prairie native prefers sunny sites with moist, well-drained soils and good air circulation. 
The "Achilles' heel" of many cultivars is susceptibility to powdery mildew. Powdery mildew is a fungus that creates the appearance of a fine white dust or powder on the plant foliage. Powdery mildew rarely kills affected plants but it can reduce vigor and be unsightly in the landscape. Twenty or thirty years ago, most of the cultivars of garden phlox were susceptible to powdery mildew.  The practical management option was to pick the best color for your garden and hope that powdery mildew wouldn't be horrible that year. If you were a perfectionist, you sprayed your plants every couple of weeks with a fungicide to prevent powdery mildew. Today there are many newer introductions that are more resistant to powdery mildew and therefore don't require fungicide treatment. Even the best of the best, however, can occasionally get powdery mildew in hot, humid years. So, give them plenty of room and do not overcrowd them in the garden. This will give plants plenty of air circulation. Frequent division may also be necessary to further promote air circulation and keep plants vigorous.
The cultivar 'David' is the standard for powdery mildew resistant phlox and has been what the newer cultivars are measured against. The Chicago Botanic Garden conducted a trial of phlox in an attempt to determine which cultivars performed the best in the Midwest. Performance was based on many factors including (but not limited to) flower production and incidence of powdery mildew.   Below are some of the top rated cultivars from the Chicago Botanic Garden trial of garden phlox. For more information on the trial, see the Chicago Botanic Garden web site

Cultivar Flower Color Height Comments
Bright Eyes Pink with dark eye 2-3 feet Fair resistance to powdery mildew
David White 3-4 feet Fair resistance to powdery mildew
Eva Cullum Pink with dark eye 2-3 ½ feet Fair resistance to powdery mildew
Franz Schubert Lilac with dark eye 2-3 feet Fair resistance to powdery mildew
Katherine Lavender with white eye 3-4 feet Good resistance to powdery mildew
Prime Minister White with red eye 2-3 feet Good resistance to powdery mildew
Robert Poore* Lavender   Newer cv.; considered good resistance
*not evaluated at Chicago Botanic Garden

This doesn't mean you shouldn't try other garden phlox – like the one pictured below ('Peppermint Twist'). Some of the newest cultivars haven't been in production long enough for sufficient testing and powdery mildew may arise (or maybe not!).   
Phlox 'Peppermint Twist' in bloom.  Photo by Cindy Haynes.
Phlox 'Peppermint Twist' in bloom. Photo by Cindy Haynes.