urgent garden question: preventing mildew on phlox

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.

  1. margaret says:

    Yes, and a beauty–thanks, Hilary, and welcome to A Way to Garden. For those who have not seen it, ‘David’ is white-flowered, and maybe 3 to 3 and a half feet tall. Great suggestion.

  2. margaret says:

    Marina, welcome to A Way to Garden. Wow–30 plants of a supposedly resistant variety all mildewed! Not so happy-sounding. If the tops are good and healthy and green, there’s reason for hope, however. Like so many things in gardening, one has to just wait and see. Keep them stress free from here on out if you can, watering when dry and so on.

  3. marina says:

    I have PHLOX DAVID. about 30 plants also they are all infected with powdery mildew. i did not see them until all of the plants go mildew and now i spray it with Defender i don’t see any improvement but all the top part of the plant is nice and green i also have cut up all of the leafs that has mildew. i wonder if they are going to bloom this year.
    any ideas

  4. Jennifer Kearney says:

    I found a suggestion online to deal with powdery mildew on phlox – milk!

    You use 1 part milk to 9 parts water, put it in a squirt bottle and spray it on all the parts that are affected.
    It’s almost miraculous in the way it makes the mildew vanish – it’s not all permanently gone yet from the plant, but enough disappears with each treatment to keep the plant going through bloom season.

    It seems to act as a foliar feeding as well for the phlox.

    1. Margaret says:

      Welcome, Jennifer. What a hilarious twist on home-remedies, isn’t it? “Got Milk?” taken to a new meaning. :) Hope to see you soon again.

    1. Margaret says:

      Welcome, Lois. I assume you mean tall garden phlox, Phlox paniculata, which in the Northeast is a high-summer thing (like around the last few weeks). The answer is no in my experience, which makes sense if you look at how/where the blooms are produced (and how there are no buds the rest of the way down the stems to call into subsequent action).

      1. Nancy says:

        I cut back some of my phlox late in the summer and blooms reappeared so if it was done earlier I’m assuming more blooms would appear but usually;y mine go all summer long so I don’t cut flowers off. Just try and experiment.

    1. Margaret says:

      Welcome, Lisa. Here you can just say hello, ask or chat away! On the forums, if you register (give username and email address) you can ask/answer freely (for free!). Enjoy!

  5. Mary says:

    Just thought I’d let Lois know that I have cut the dead heads back after blooming on almost all the varieties of Phlox I have and mine have bloomed a second time. I have been able to keep them going until nearly frost.

    1. Margaret says:

      Welcome, Mary and that’s fantastic. Mine bloom and bloom…but always from up top, or close to there (so I don’t cut hard into the stems the way I would cut back some other perennials way down low). Thanks for the updates, and do come again soon.

  6. Krys says:

    I have beautifully colored tall phlox. They are purple in the morning and fuschia pink in the afternoon. They actually do change colors, I thought my eyes were deceiving me and took pictures to prove it was happening. I would love to buy more of them for other areas of my yard (the ones I have seem to be slow growing) and haven’t a clue what they are called or where to get them. Any ideas?

  7. Stella says:

    My tall phlox bloomed in mid-June, but then my deer herd found them and chewed off all the blossoms. I thought it was all over, but the plants sent up new buds and have been blooming through August & September. While we expect a the frost any night now, the plants are thriving, so Bambi and the rest of the herd actually helped this time.

  8. Joan King says:

    Hi! Just found your site and have lots of questions! I planted tons of impatiens last year only to have them all die off. Took the chemical spray route and animal spray route and even fox urine to no avail. Found out through an article int the paper from Cornell that they all had a disease and could not replant for two years as it was in the soil. Can you help with this?

  9. Joan King says:

    After my couple of hundred dollar loss last year on the diseased impatiens I would like to know what I can plant in that shaded area instead. Will the disease effect those that I plant? It is large area around a very large pine tree. My poor hubby spent weeks making the garden. I would love to add color. Would begonias work or hydranges?

  10. Ann Marie says:

    My Mom and I have struggled with powdery mildew on Phlox for years. I have added more Phlox to my garden. I love the long blooms. I am concerned about next summer. I saw a little powdery mildew this year. I am interested in the spray with baking soda and botanical oil, and will give that a try next year. My favorite White Phlox David withered away to nothing early this summer. I should have tried to cut it back like I see someone did here. I know that Phlox recover quickly because I sometimes accidentally break them off. To my dismay!

  11. LORRAINE K Hughes says:

    I sprayed my Phlox with the baking soda mix, three days in a row as advised on a website..now it has been several days & I haven’t seen any improvement, What should my next step be..spray again, cut them back? I would hate to lose them as some were from my Mother’s garden. Thanks for any help!

    1. margaret says:

      Hi, Lorraine. I assume your phlox already had powdery mildew showing? The baking soda mix is meant to be preventive, not curative — as in, it is to be used before mildew breaks out. Powdery mildew will not kill your plant, just make it look a mess the rest of this summer. Here is more information about growing phlox.

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