[header] How to divide a phlox plant [title] Determine when to divide phlox. [substeps] Divide phlox while it still looks healthy. The most common mistake home gardeners make when growing phlox is waiting until the plant shows signs of failing or over-crowding before they divide it. 
As a rule of thumb, allow the diameter of the plant cluster to grow to the same width as the plant's height before dividing it. Divide the phlox sooner if the plant becomes over-crowded, or if the center of the plant fails to bloom and produces less foliage.