The leaves are pinnately dissected three to four times with rather small ultimate segments. The flowers are small and white and are arranged in umbrella-shaped clusters. Fruits are broadly ovate and are flattened laterally. Flowering occurs between June and August.
Poison hemlock is a member of the carrot family.
The plant was introduced from Eurasia and is now a widespread weed along waterways and disturbed sites throughout North America. The toxic properties of poison hemlock have been known since antiquity. The poison was used by Romans and Greeks for suicides or to dispatch enemies or criminals. The plant contains toxic alkaloids of which one, coniine, has been identified. Unlike water hemlock, the toxic compounds occur primarily in the stems, leaves, and fruits rather than in the roots.