Poison Hemlock: Have you seen this weed?

Stan Smith, PA, Fairfield County OSU Extension

Each passing year it seems I get a few more calls regarding “this weed that looks like a real tall wild carrot.” Indeed, the population of poison hemlock along field edges, in fence rows, around barn lots, and now even growing throughout hay fields seems to have reached new proportions this year.

Poison hemlock is a biennial member of the carrot family – Conium maculatum – which can cause respiratory failure and even death when ingested by livestock or humans. It may, at times, be confused with giant hogweed – Heracleum mantegazzianum – a plant with many similarities and also quickly spreading in Ohio.

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In fertile soils poison hemlock may easily grow up to 10 or 12 feet, producing small white flowers that are typical of the carrot family. The plant began flowering around Ohio a few weeks ago. The herb has a smooth, purple-spotted stem; dark, glossy-green and fern-like triangular leaves. It has a fleshy white taproot. Both the leaves and roots have a disagreeable parsnip-like odor.

All parts of the plant are poisonous including the leaves, stems, seeds and roots. Simply handling the plant can cause toxic reactions in humans. Perhaps poison hemlock’s most famous claim to fame was when it was used to execute Socrates in 329 B.C.

The taste of leaves and seeds to livestock of poison hemlock is unpleasant, so toxic quantities are seldom consumed when ample desirable feed is available for the animals. Cattle can usually survive poison hemlock if consumed in amounts less than 0.4% of their body weight (4 to 5 pounds for mature cows) although abortions are possible at lower rates. The toxicity of the plant changes little if fermented with silage or dried in hay.

Crossbow, dicamba and 2,4-D are fairly effective on small poison hemlock, especially when treated in the fall. Taller plants in the spring may need to be controlled with glyphosate. Mowing after the plants have bolted and before seed set will prevent seed production.