Phlox is a highly versatile perennial with powerful blooms that can extend through much of the season. From spring-flowering, low-growing creeping phlox (Phlox subulata), to its taller, summer relative, garden phlox (Phlox paniculata), there are many cultivars with an endless variety of flower and leaf color, height and bloom time.
Garden phlox's powerful presence and rainbow of colors, from pinks, purples and blues, to reds, oranges and white, can add incredible visual impact to the late summer garden. That is, if they make it that long.
Unfortunately, phlox is susceptible to being eaten by animal visitors such as deer and being covered with an unattractive, white substance known as powdery mildew.
Powdery mildew is a group of fungi that attaches to the leaves and stems of plants, including phlox, bee balm and peonies. It makes the surfaces of the plant look almost dusted with white and brown specs.
Its spores are spread by wind and once attached, the fungi absorbs nutrients from the plant in order to continue spreading. While this sounds extreme and doesn't look very nice, if the affected plant is still growing and flowering as it should, you don't need to do anything to treat the mildew.
If you notice your phlox is suffering from stunted growth, dying foliage or decreased flowering, or if you want to test out possible improvements, there are some non-chemical, pollinator-friendly methods you can use to manage fungi.
When planting a new garden, try to select phlox varieties that are powdery-mildew resistant. This is a good step toward preventing future problems.
Phlox paniculata 'David' is a 24-48-inch tall white-flowering garden phlox with resistance to powdery mildew. The 26-30-inch Purple 'Goliath' phlox (Phlox paniculata 'Goliath'), 36-inch salmon-toned 'Orange Perfection' (Phlox paniculata 'Orange Perfection') and 12-20-inch light blue 'Blue Flame' phlox (Phlox paniculata 'Blue Flame') also are good choices for powdery mildew resistance.
For existing gardens, try to relocate your phlox to a new place in your garden. Try an area that gets more sun and has more airflow to see if this makes a difference.
Some varieties of powdery mildew prefer moist environments, while others are happy to live in dry conditions. If you have the fungi and your environment is very shady, moist and tightly spaced, relocating your phlox could make a difference.
Before transplanting, adjust your soil nutrients to see if anything improves. Get a soil test from University of Vermont Extension (pss.uvm.edu/ag_testing) to make sure you match the best amendments to the plant requirements.
Still having trouble? Talk to the experts at your local greenhouse or nursery about their experience with different phlox varieties and how they manage powdery mildew.
Or plan to attend the 19th annual Phlox Fest at Summersweet Gardens at Perennial Pleasures Nursery and Tea Garden in East Hardwick, which has guest speakers, garden tours and an opportunity to view more than 160 phlox varieties in bloom. Among the featured speakers is retired UVM Extension horticulturist Dr. Leonard Perry. The festival runs from July 31-Aug. 15. Go to summersweetgardens.com/phlox-fest for details.
Bonnie Kirn Donahue is a UVM Extension master gardener and landscape designer from central Vermont.