This is the plant “by which criminals and philosophers were put to death at Athens,” as Gray observes with a humor as dry as an herbarium specimen. Most notoriously, the juice killed Socrates. It’s a European import that’s very common in Pittsburgh along roadsides and at the edge of the woods. The plants resemble their relative Queen Anne’s Lace (Daucus carota), but are much taller, with looser umbels borne profusely up and down the strong stems. Often the main stem has a distinctive whitish bloom.
Needless to say, Poison Hemlock is very poisonous, so mixing it up with edible members of the same family can be a bad mistake. This plant grew on a hillside in Beechview, where it was blooming in the middle of June.
Gray describes the genus and the species:
CONIUM L. POISON HEMLOCK
Fruit somewhat flattened at the sides, glabrous, with prominent wavy ribs; oil-tubes none, but a layer of secreting cells next the seed, the face of which is deeply and narrowly concave. Poisonous biennial, with spotted stems, large decompound leaves with lanceolate pinnatifid leaflets, involucre and involucels of narrow bracts, and white flowers. (Koneion, the Greek name of the Hemlock, by which criminals and philosophers were put to death at Athens.)
C. maculatum L. A large branching herb, in waste places, Que. to Del., Pa., and westw. (Nat. from Eu.)