It contains highly poisonous alkaloids toxic to all classes of livestock and humans . It has poisoned many who have mistaken it for parsley. Poison hemlock is often found on poorly drained soils, particularly near streams, ditches, and other surface water. Stems are stout, hollow, ridged, and mottled with purple spots.
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 . It grows throughout the U. S. except from eastern Montana to northeastern Minnesota and south to Nebraska. It has an extensive presence in Idaho.