I have seen this plant in cultivation twice but the owners were not aware of its poisonous character. Local along roadsides and alluvial banks of streams and locally abundant along the old canal in Huntington, Wabash, and Miami Counties and found, no doubt, farther down the canal. Noted along the Ohio River in Dearborn County and as a weed in fields between Madison and Hanover in Jefferson County.
Duration: Biennial Nativity: Non-Native Lifeform: Forb/Herb General: Tall, hairless biennial herb; 0.5-3 m tall; stems streaked or spotted with purple blotches; large taproot; pungent odor. Introduced from Eurasia. Leaves: Leaves pinnately compound, finely divided, and sometimes toothed. Flowers: Loose inflorescence of umbels; flowers white. Fruits: Seeds ribbed (crenulate), about 2 mm long. Ecology: Widely distributed in moist disturbed areas, streams, and canyons from 5,000-7,500 ft (1525-2285 m) and lower; flowers June-September. Notes: The purple streaked/spotted stems and habitat of moist areas leads to poison hemlock. Also look for the finely dissected leaflets and loose axillary and terminal umbels. Host for Black Swallowtail butterfly. Poison hemlock can be easily controlled with the herbicide 2,4-D. No effective biological control techniques are known, but mechanical removal (hand pulling, grubbing, or mowing) is effective if done prior to flowering. Ethnobotany: All parts of this plant are highly toxic. Notorious from ancient times as the poison that Socrates drank. Differentiated from many other members of Apiaceae by the purple-spotted stems. Etymology: Conium is from -koneion,- ancient Greek name for this species, while maculatum refers to spotted, referring to purple splotches on the stems of leaves or on petals. Synonyms: None Editor: SBuckley, 2010
Stem freely branched, to 3 m, purple-spotted; lvs 2-4 dm, broadly triangular-ovate, 3-4 times pinnately compound, the ultimate segments ovate-oblong, 4-10 mm, toothed or incised; umbels 4-6 cm wide, the terminal one blooming first but soon overtopped by others; fr broadly ovoid, 3 mm, the pale brown ribs very prominent when dry; 2n=22. Native of Eurasia, now widely intr. as a weed in waste places from Que. to Fla., w. to the Pacific. This is the hemlock of classical antiquity. All parts of the plant are very poisonous.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.