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Getting to the root of phlox problems

Jessica Walliser
| Thursday, Oct. 12, 2017, 8:55 p.m.
The bees and butterflies adore phlox, but it is not a plant without problems.
Jessica Walliser
The bees and butterflies adore phlox, but it is not a plant without problems.

Question: I have a purple garden phlox that I planted in my front yard landscape in 2013. It bloomed beautifully this year, but I have a concern. As it was beginning to bloom, a few different stems turned brown and died. I don't recall that happening in the past years. Could that be a sign there is a problem or it may be diseased? It is slowly dying out and my concern is for next year. Also, I wanted to split it and replant part of it in a different area. I am unable to do that now.

Answer: Garden phlox (Phlox paniculata) is a gorgeous summer bloomer that has no doubt found a home in many Pennsylvania landscapes. In my own garden, I grow about six different varieties of garden phlox, ranging in color from white and pink to purple and lavender. The bees and butterflies adore it almost as much as I do, but it is not a plant without problems, as you've come to learn. But, thankfully, most of the problems faced by phlox are easy to manage.

Among the most common ailments seen on phlox is a fungal disease known as powdery mildew, though I do not think this is what's happening with your plant. Powdery mildew appears as a white, talcum powder-like dusting on the foliage, often starting with the lowest leaves first. It's largely an aesthetic issue that makes the plant look not-so-good, but it seldom causes significant injury to the plant. There are many powdery mildew-resistant phlox varieties, such as “Fancy Feelings,” “Frosted Elegance,” “Shortwood” and “Peppermint Twist” and I suggest planting them whenever possible.

Spider mites can also be problematic on phlox, especially during summers that are dry and hot. Their damage first appears as stippling and discoloration on the upper leaf surfaces. Eventually the leaves turn brown and crispy and fall from the plant. When infested leaves are flipped over, you'll see a fine webbing and tiny, barely visible specks moving on the leaf undersides. Spider mites can cause the branch dieback you describe when the infestation is severe. Symptoms are often noticed on the lower leaves first and the problem progresses up the stems. Spider mites can easily be controlled with horticultural oil or insecticidal soaps applied to both upper and lower leaf surfaces according to label instructions.

Phlox are also sensitive to irregular irrigation, especially if the plants are overwatered, as sometimes happens during years of excessive rainfall like the one we just had. Signs of overwatering are a lot like the signs of under-watering, including plant wilting, reduced flowering, and stem death. Be sure to keep phlox plants well watered during times of drought, but do not allow the bed where they're planted to become water logged at any point during the growing season.

There's one final possible cause for your phlox woes and it's a doozy: aster yellows. Aster yellows is caused by a phytoplasma, and it makes the leaves and stems of susceptible plants turn yellow and stunted. The flowers may also be affected by this pathogen, and if they are, they'll be distorted and have petals that don't fully color-up. Sometimes a broom-like cluster of small stems (called a witch's broom) can emerge from the main stem of the plant. Aster yellows can affect hundreds of different plants, including many perennials, annuals, and even vegetables, and is most often spread by aster leafhoppers feeding on the plants. There is no cure for this pathogen, and once an infection is confirmed, the plant should be removed from your garden immediately to keep it from spreading. If the flowers on your phlox were large and healthy, rather than small, discolored and malformed, aster yellows is probably not to blame, but it's worth watching for in the future.

All this being said, I should note that many of the phlox plants growing in my own garden show a similar kind of dieback during almost every growing season, mostly due to spider mites. But every spring, they return happy and healthy, and I don't really do anything to manage the spider mites. The plants bounce back and bloom beautifully with little ill effects. I've had them for 10 years now, and they keep on trucking, despite their regular branch dieback.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com.

Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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