Water hemlock earns its reputation. Considered among the most violently toxic plants in North America, it grows in wet meadows, swamp margins, pond banks, and roadside ditches throughout its range, often in the company of more benign carrot-family relatives. Its hollow stems are purple-striped or mottled and show distinctive cross-partitions at the nodes — features that, once learned, make identification more reliable. The leaves are 2 to 3 times pinnately divided, with a critical identifying detail: the veins run to the notches between the teeth, not to the tips, distinguishing water hemlock from related species.

It grows as a low rosette in its first year, then in the second season sends up a stout stalk to 8 feet, topped with the small, white, umbrella-like flower clusters typical of the Apiaceae family. Blooming from spring through fall, the flowers draw a range of bees, wasps, and butterflies, and the plant serves as a host for the Black Swallowtail butterfly. Water birds consume the fruits without apparent harm. The roots are the most toxic part, but all parts of the plant are dangerous, and no part should be handled or ingested. Its presence in a planting should always be noted clearly.