Water well until soil is completely moist. Cut old stems to the ground in late fall or early spring using bypass pruners. Apply a light application of organic fertilizer to the top of the soil in early spring. Mulch around, but not on top of, plants with 3 inches of organic mulch. Summer phlox grows in Zones 4 to 8 of the U.S.D.A. Plant Hardiness Map.
Summer phlox is susceptible to powdery mildew.
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