Poison hemlock is a weedy biennial species with one to several upright stems which are freely branched rising from 50 to 300 cm high. The stems are covered with distinct longitudinal ridges and purples spots. The leaves are pinnately divided three to four times into many small leaflets. The larger leaves are commonly 15-30 cm long. The lower leaves clasp the stem while the upper leaves are on long stalks. The foliage has a strong musty odor.
The inflorescence is a compound umbel. The rays of the umbel are subequal, reaching a length of 1-4 cm at maturity. The tiny flowers are white and lack sepals. The paired seeds are several mm long, ribbed, concave in cross-section, and a light brown color.
Poison hemlock is commonly found in moist disturbed places such as the borders of pastures and cropland and along streams and ditches. It tolerates poorly-drained soils.
Poison hemlock is a native of Europe. It is now established over much of North America. It is fairly common in disturbed places west of the Cascade Mts.