Garden Phlox

Garden phlox blooms when the garden needs it most — July into September, when spring's show is over and fall's asters are still weeks away. The flower heads are dense and fragrant, and they draw swallowtail butterflies and sphinx moths in the evening. But the plant comes with a condition that borders on certainty: by late summer, the lower leaves will turn white with powdery mildew. The question is not whether mildew will appear, but how much of the plant it consumes before frost.

The most consequential decision you make with phlox happens before you plant it — which cultivar you buy. Some varieties, bred for mildew resistance, can make it through August with mostly green foliage. Others, especially older cultivars, are white from the ground up by mid-July. 'David', a tall white-flowered selection, is the standard for resistance. 'Robert Poore' and 'Jeana' are nearly as reliable. If you plant a variety without checking its mildew rating, you are planting the wrong variety.

Spacing and air circulation help, but they do not prevent the disease — they only delay it. Plant phlox with at least two feet between clumps, and thin each clump in spring to five to seven stems, removing the weakest and most crowded growth. Water at the base, not overhead, and mulch to keep soil moisture even; drought-stressed plants tend to succumb faster. When mildew does appear, remove the worst-affected lower leaves and drop them in the trash, not the compost.

Phlox is a clumping perennial, and the clumps exhaust themselves over time. The center of an old clump tends to die out, leaving a ring of stems around a bare patch. Every three to four years, dig the clump in early spring, discard the spent center, and replant only the vigorous outer divisions. This renews the plant and reduces disease pressure in the process — old, crowded growth is where mildew takes hold first.

Deadheading prolongs bloom somewhat, but the real reason to remove spent flowers is to prevent self-sowing. Garden phlox seedlings revert to a dull magenta and spread aggressively; they are not an improvement. Cut the flower stalks back to a side bud as the blooms fade, and you may get a second flush in September.

In fall, after a hard frost, cut the stems to the ground and remove all debris. The mildew spores overwinter on dead foliage, and leaving the stems standing is an invitation for the disease to start earlier the following year.