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Plant Talk

Inside The New York Botanical Garden

Tip of the Week — 8/18/08

Posted in Gardening Tips on August 18 2008, by Sonia Uyterhoeven

Pondering Powdery Mildew

Sonia Uyterhoeven is Gardener for Public Education at The New York Botanical Garden.

Gardening is about observation. I have noticed over the past several years that my phlox and peonies that are out in full sun never suffer from powdery mildew while those that are in partial shade suffer miserably. What then are the lessons for the gardener?

Powdery mildew tends to create a problem for gardeners in middle to late summer. Unlike many fungal problems that need good moisture to take hold, powdery mildew thrives with warmth (temperatures around 70s-80s) and high humidity.

What can we do to alleviate the problem? Not surprisingly, good maintenance practices and smart gardening is the key to healthy plants.

  • Do not overcrowd your plants; this will lead to poor air circulation and will create problems with the humidity level around susceptible plants.
  • During the winter the fungus survives on plant debris, so it is important to clean up around infected plants.
  • Using high nitrogen fertilizers (the first number on fertilizers) promotes weak, leafy growth; try a balanced fertilizer with low numbers and apply only once in the spring to give perennials a boost for the season.
  • Choose disease-resistant cultivars whenever possible.
  • Follow the maximum “the right plant for the right place.” A healthy plant is always more resistant to disease problems.

If your favorite perennials do get attacked with powdery mildew, you have several options. When powdery mildew attacks my peonies late in the season, I just cut them back once the leaves get unsightly. If your bee balm has succumbed and is beyond repair, then cut the foliage back; you will get a new flush of foliage but no more flowers for the rest of the season. If it is a late-season bloomer that you’re set on keeping, then remove the worst stems and apply an environmentally friendly product to the rest of the plant as soon as you notice the powdery mildew. Some options include products that contain neem oil, sulfur, or potassium bicarbonate.