Poison hemlock (Conium maculatum) is an annual or biennial herb which is native to Europe and western Asia. It has been introduced to America and Oceania as an ornamental plant. C. maculatum can grow up to 9 feet in height and resembles a carrot plant. It has large lacy leaves and a usually hollow stem which has purple spots. It produces small white or greenish-white flowers which present as flat clusters like an umbrella which blossom in the spring. Small fruits appear in the summer. When the leaves, stem or flowers are crushed, they produce an offensive "mousy" odor.

Toxic components
C. maculatum contains eight piperidinic alkaloids, however y-coniceine and coniine are generally the most abundant and the predominant cause of acute and chronic toxicity. Coniine is a pyridine derivative similar in structure and function to nicotine. The toxicity of the plant varies depending on the stage of growth, environmental conditions (rain, temperature, cloud cover), and soil. Although all parts of the plant are toxic, the most toxic parts are full formed fruit that is still green, ripe fruits, and stems.

Signs of poisoning develop 1 to 2 hours after ingestion. The toxic dose in horses is 15.5 mg/kg. Horses don't typically graze hemlock unless food is scarce or it is contaminated in hay or haystuffs.