1. Conium maculatum L. (poison hemlock)

Pl. 205 c, d; Map 850

Plants biennial, glabrous, often glaucous. Stems 50–300 cm long, erect or ascending, purple-spotted or mottled. Leaves alternate and usually also basal (1 to several basal leaves usually present at flowering), the basal and lower stem leaves long-petiolate, the median and upper leaves short-petiolate to nearly sessile, at least the lowermost sheathing bases somewhat inflated. Leaf blades 3–40 cm long, broadly ovate to ovate-triangular in outline, those of the basal and lowermost stem leaves 3 or 4 times pinnately compound, the ultimate leaflets 5–35 mm long, oblong-lanceolate to ovate, narrowed at the base, pinnately lobed, the lobes narrowed or tapered to a blunt or sharp point at the tip; those of the median and upper leaves progressively reduced, 2 or 3 times pinnately compound, the leaflets similar to those of the lower leaves. Inflorescences mostly terminal, compound umbels or more commonly loose clusters or panicles of compound umbels, mostly long-stalked. Involucre of 4–8 bracts, these 2–6 mm long, much shorter than the rays, spreading to reflexed at flowering, narrowly lanceolate to narrowly ovate, with broad, thin, papery margins and sharply pointed tips, some adjacent bracts sometimes fused toward the base. Rays numerous, 1.5–3.0 cm long. Involucel of 4–9 bractlets, these shorter than the flower stalks, similar to the bracts but smaller. Flowers mostly numerous in each umbellet, the stalks 3–6 mm long. Sepals absent. Petals obovate, rounded or notched at the tip, white. Ovaries glabrous. Fruits 2.5–3.5 mm long, ovate to broadly elliptic-ovate in outline, flattened laterally, glabrous, dark brown with pale ribs, the mericarps often slightly narrowed along the commissures, with 5 ribs, these blunt and somewhat corky. 2n=22. May–August.

Introduced, scattered to common nearly throughout the state (native of Europe, Asia, Africa; widely introduced in North America). Banks of streams, rivers, and spring branches; also roadsides, railroads, ditches, pastures, fencerows, and open, disturbed areas.

Conium maculatum contains toxic alkaloids, including coniin and conicein, and is extremely poisonous to humans and other animals when ingested. This species has been implicated as the hemlock with which the Greek philosopher Socrates was poisoned. Although plants are not considered particularly palatable to livestock, landowners should take care to remove plants from pastures and fencerows, where they have a tendency to accumulate when other plants are suppressed or eliminated by grazing.