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Poison Hemlock Background Poison hemlock is native to Europe. It contains highly poisonous alkaloids toxic to all classes of livestock and humans. An extract of this species was used to execute Socrates in ancient Greece; it has also accidentally poisoned many who have mistaken it for parsley. Poison hemlock is often found on poorly drained soils, particularly near streams, ditches, and other water surfaces. Description Poison hemlock is a biennial that grows up to 10 feet tall. Stems are stout, hollow, ridged, and mottled with purple spots. Leaves are shiny green, 3 to 4 times pinnately compound, and clasp the stem at the obvious nodes. Crushed foliage has a disagreeable, mousy odor. Flowers are small, white, and borne in umbrella-shaped clusters about 3 inches across in early summer. Seeds are ridged and flattened, with 2 seeds borne together. The plant has a thick, white taproot, which, when cut longitudinally near the top, reveals thin, horizontal walls through a hollow chamber. Control A biological control agent (a defoliating moth) provides good to excellent but inconsistent control. Herbicides are also available. Source: Weeds of the West, The Western Society of Weed Science, 1992