Dying Phlox


Hello, we can’t figure out what’s going on with our lovely phlox which is normally very healthy. We have three different stands of it in the garden, in three different places and all of them are dying. The plants around are fine – any ideas? Thanks very much


Garden phlox are notorious for contracting diseases.   There are a number of leaf spotting diseases known as ‘high humidity’ diseases. These include powdery mildew, leaf blights and stem blights.

Your example in your photo exhibits symptoms of a leaf blight.

Treatment; A sulfur based fungicide will control the disease on new growth but not clear the damaged foliage. Best to remove as much as infected foliage, and remove any fallen leaves from the ground, and  do not place in mulch pile.  Treat plants new shoots in early next spring before the disease rears its ugly head .

To have the most success with your garden phlox, follow these care instruction;

  • Keep soil moist by watering thoroughly on a regular basis
  • Water the soil rather than phlox foliage to aid in disease prevention
  • If you must water overhead, water early in the morning so plants will dry rapidly in the sun
  • Once stems are about six inches tall, eliminate all but five or six stems per plant, then pinch back the growing tips of those remaining stems (you’ll find they become more robust, produce larger clusters of flowers, and are less bothered by powdery mildew)
  • A spring application of manure or compost is beneficial each year
  • If not using manure or compost, you can apply a light application of 10-10-10 fertilizer as new growth emerges each spring, and then again just before plants begin to flower
  • Deadhead flower heads as soon as they complete.