We have discovered, through trial & error, that PHLOX paniculata or Border Phlox as they are more commonly known, resent being cut back hard at the end of the growing season - this maybe a trait that is more apparent when they are grown in pots and left outdoors to overwinter. We would recommend cutting the old flowering stems down to approx. 6 inches in autumn and applying a layer of mulch - either well-rotted manure or garden compost, to protect the crown of the plant.
Phlox paniculata have a bit of a reputation for being prone to powdery mildew, don't let this put you off, they will provide colour and glorious scent over a long period of time and be a great asset to the garden.
Powdery mildew strikes when plants are under stress, it can be controlled to an extent by providing the right location and growing conditions. These Phlox ideally need a sunny to lightly shaded spot with good airflow and a hearty, well-drained but moisture-retentive soil. Avoid planting them where Phlox have been planted previously.
Reducing the size of clumps by removing some of the flower stems in early summer will aid air movement and mulching will increase the humus content of the soil, which in turn will prevent the soil drying out and increase its fertility. Lastly, remember to water plants during prolonged dry spells.