Why are the leaves curling?
Certified GKH Gardening Expert
If the edges (or any other part) of the leaves aren't turning brown, it's probably nothing to worry about, but if the leaves are turning brown or have a white powdery substance on them, they may be suffering from not enough water or powdery mildew. Phlox also needs plenty of air circulation in order to thrive, so if the plant is overcrowded, that could be stressing it out and causing the leaves to react. This article provides additional information about growing Phlox: https://www.gardeningknowhow.com/ornamental/flowers/phlox/growing-garden-phlox.htm