READER DAN asked a great question that reveals a tip you can use right now to prevent powdery mildew this summer on your garden phlox:
“The biggest problem I encountered was with the Phlox I planted,” Dan wrote. “The leaves developed a terrible fungus and it slowed its growth terribly. When I first spotted it, I did some research and decided to use an organic fungicide. That worked alright, but the fungus came back. Then, I tried an organic remedy I found on the internet: spraying with a milk solution. That worked less well. The poor plants were so overcome with the black fungus that they eventually withered without flowering late in the summer. I finally cut them down. Now they are starting back with strong green growth and I’m pleased, but I wondered if you had any advice for treating Phlox fungus.”
One of the best non-chemical ways to deal with powdery mildew, I replied, which Phlox paniculata is so prone to in our humid summers, is to prevent it by improving air circulation in and around the plant. Do this as shoots emerge by “thinning” the clumps—just use your finger (or a pruner if need be) to knock out like a third of the shoots, to space the remaining ones less closely together. The plant will breathe better, so to speak, and that will help prevent the fungus from overtaking it.
Another good preventive step is to spray starting before the outbreak occurs with a homemade anti-fungal like a solution of water, baking soda and horticultural oil. Rosarians do this with their roses to prevent black spot, another fungal pest; recipes vary but it’s approximately a teaspoon of baking soda and a teaspoon of horticultural oil (from the garden center) in a quart of water, or a tablespoon or so of each in a gallon. Other gardeners swear by horticultural oil alone.
Another truly preventive method: Grow mildew-resistant varieties, which give you the best shot of success. The Chicago Botanic Garden’s Plant Evaluation program is an excellent source for PDFs on the best cultivars of many kinds of plants, including Phlox (click on and download Issue #13 PDF at http://www.chicagobotanic.org/research/plant_evaluation/). This newer story on phlox with Mt. Cuba Center will also provide lots of advice.