My phlox leafs are going yellow what could be the problem?


The perennial Phlox grows in many shapes and sizes: some low creepers, others tall, bushy plants. The common denominator is their fragrance, and wide range of colours. The phlox family  (polemoniaceae) has over 300 species worldwide, from herbs to shrubs. Without knowing your variety we can only give you general information.

The yellowing of phlox leaves is generally due to a pest, or fungal problem. The P. paniculata  is now available in powdery-mildew resistant cultivars: mastering the powdery mildew problem has long been an issue with many plants, from grapes to herbs. In this case, a judicious pruning of the affected leaves, to free up light and air in and around the plant, could be an easy start to a remedy. An annual application of a sulfur fungicide (see your local nursery) will help stymie the disease, and prevent its appearance. Above all, follow basic good practices:

  • Space plants adequately to allow good air movement throughout the foliage.
  • Water plants early in the day so leaves dry quickly.
  • Do not over fertilize (especially with nitrogen).
  • Remove diseased plant debris at the end of the season to minimize survival of the fungus over the winter.

Then, there could be spider mite culprits, easily identified by webbing spread across the plant. Insecticidal oils and soaps work best, but only if they are applied onto the mites directly. Again, a good cleaning up of the phlox stand, with your secateurs, could even promote another flush of seasonal blossoms.

Attached here is a related Toronto Master Gardener article, written to another gardener, with similar concerns about their Phlox mildew

Hope this helps with housekeeping advice for your phlox, and may your blossoms always be sweet!