Troubleshooting
Annual Phlox problems — and how to fix them
Annual Phlox (Phlox drummondii) is generally forgiving once you match its basics, but a few issues come up again and again. Here is what each one looks like, why it happens, and the fix.
Powdery mildew
Common in warm, humid conditions with poor airflow. Space plants adequately, water at the base, and apply a preventive sulphur fungicide if mildew appears early in the season.
Summer heat collapse
Flowering declines sharply in temperatures above 30°C. Treat as a cool-season annual; start seeds early and allow plants to finish before midsummer heat arrives.
Damping off at sowing
Seedlings are prone to damping off if sown too densely or kept too wet. Sow thinly in sterile compost, ensure good drainage, and do not overwater germinating trays.
Prevent annual phlox problems before they start
Most annual phlox issues are care-mismatch, not bad luck. Dial in the basics:
Looking for annual phlox in general rather than its problems? Read the full annual phlox care guide — light, water, soil and feeding.
Annual Phlox problems — FAQ
Why is my annual phlox powdery mildew?
Common in warm, humid conditions with poor airflow. Space plants adequately, water at the base, and apply a preventive sulphur fungicide if mildew appears early in the season.
Why is my annual phlox summer heat collapse?
Flowering declines sharply in temperatures above 30°C. Treat as a cool-season annual; start seeds early and allow plants to finish before midsummer heat arrives.
Why is my annual phlox damping off at sowing?
Seedlings are prone to damping off if sown too densely or kept too wet. Sow thinly in sterile compost, ensure good drainage, and do not overwater germinating trays.