Phlox paniculata Lyon ex Pursh `Eva Cullum' plants were grown under seven photoperiods following 0 or 15 weeks of 5 °C to determine the effects of photoperiod and cold treatment on flowering. Photoperiods were a 9-hour day extended with incandescent lamps to 10, 12, 13, 14, 16, or 24 hours; an additional treatment was a 9-hour day with a 4-hour night interruption (NI). Noncooled plants remained vegetative under photoperiods ≤13 hours; as the photoperiod increased from 14 to 24 hours, flowering percentage increased from 20 to 89. Flowering of noncooled plants took 73 to 93 days. Flowering percentage was 19, 50, or 100 when cooled plants were held under photoperiods of 10, 12, or ≥13 hours or NI, respectively. Time to flower in cooled plants progressively decreased from 114 to 64 days as the photoperiod increased from 10 to 24 hours. Reproductive cooled plants had at least three times more flowers, were at least 50% taller, were more vigorous, and developed seven or eight more nodes than did noncooled plants. Photoperiod had no effect on height of flowering plants.