Family Euphorbiaceae (Spurge Family).
A native of the Midwest, but so often cultivated in the East that it has become an established weed, especially in old city lots and along railroad tracks. The showy part of this plant, as with its relative the poinsettia, is the leaf. Only a close view reveals the comparatively insignificant flowers. This colony grew along a residential street in Beechview. It makes an attractive cut flower, but the stems have a milky sap that may be irritating to the skin.
From Gray’s Manual:
EUPHÓRBIA L . SPURGE Flowers monoecious, included in a cup-shaped 4-5-lobed involucre (flower of older authors) resembling a calyx or corolla, and usually bearing large thick glands (with or without petal-like margins) at its sinuses. Sterile flowers numerous and lining the base of the involucre, each from the axil of a little bract, and consisting merely of a single stamen jointed on a pedicel like the filament; anther-cells globular, separate. Fertile flower solitary in the middle of the involucre, soon protruded on a long pedicel, consisting of a 3-lobed and 3-celled ovary with no calyx (or a mere vestige). Styles 3, each 2-cleft; the stigmas therefore 6. Pod separating into three 1-seeded carpels, which split elastically into 2 valves. Seed often caruncled (ours only in §§ 5 and 6). — Plants (ours essentially herbaceous) with a milky acrid juice. Peduncles terminal, often umbellate-clustered; in the first section mostly appearing lateral, but not really axillary. (Named for Euphorbus, physician to King Juba.)
§3 . PETALÒMA Boiss. Uppermost leaves with conspicuous white petal-like margins, whorled or opposite, the others scattered; erect annuals, with leaves equal at base and entire, and with lanceolate deciduous stipules; involucres 5-lobed, in an umbel-like inflorescence.
E. marginàta Pursh. (SNOW-ON-THE-MOUNTAIN.) Stem stout, 3-9 dm. high, erect, hairy; leaves sessile, ovate or oblong, acute; umbel with three dichotomous rays; glands of the involucre with broad white appendages. —Minn. to Mo., Col., Tex., and S. C.; spreading eastw. to O., and frequently escaping from flower-gardens.
