Pittsburgh is in the native range of Phlox paniculata, but the flower is so popular in gardens, and persists and spreads for so many years after planting, that we always have to suspect any individual plant of being a garden escape. So this one may have been, but if so it made its escape long ago. It was growing well back in a thicket in a wooded area in Scott, where it was blooming in early July.
Phlox is so much beloved in Pittsburgh that any plant with a vaguely similar inflorescence is likely to be called “Phlox,” Dame’s Rocket (Hesperis matronalis) and Soapwort (Saponaria officinalis) being two notable examples. This, however, is the real thing.
The flowers come in a range of colors from white through pink to purple. In the Pittsburgh area at least, the species shows a remarkable affinity for the edges of old cemeteries, where perhaps it was once a popular planting.
Gray describes the genus and the species:
Calyx somewhat prismatic, or plaited and angled. Corolla with a long tube. Stamens very unequally inserted in the tube of the corolla, included. Capsule ovoid, with sometimes 2 ovules but ripening only a single seed in each cell. — Perennials (except a few southern species), with opposite and sessile perfectly entire leaves, the floral often alternate. Flowers cymose, mostly bracted ; the open clusters terminal or crowded in the upper axils. (Phlox, flame, an ancient name of Lychnis, transferred to this North American genus.) Most of our species are cultivated in gardens.
§ 1. Herbaceous, with flat (broad or narrow) leaves.
* Stem strictly erect; panicle pyramidal or ellipsoid, many-flowered; peduncles and pedicels very short; corolla-lobes entire.
P paniculàta L. Stem stout, 0.5-1.5 m. high, smooth, or puberulent or villous above; leaves oblong-lanceolate and ovate-lanceolate, pointed, large, tapering or rounded, the upper often heart-shaped at the base; panicle ample, pyramidal-corymbed, calyx smooth or glandular-hispid, the teeth awn-pointed; corolla pink-purple varying to white. (Including P. acuminata Pursh, P. glandulosa Shuttlw., and P. amplifolia Britton.) — Open woods, Pa. to Ill., Kan., and southw.; escaped from cultivation northw. July-Sept. — Highly variable in outline of leaf, pubescence of leaves, stems, calyx, and corolla, hut without concomitant characters.