Weed of the Month: Poison Hemlock

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Originally published on TheHorse.com

Common name: Poison Hemlock
Scientific name: Conium maculatum L.

Life Cycle: Biennial
Origin: Eurasia
Poisonous: Extremely

Poison hemlock

Poison hemlock is distributed across the United States and grows most frequently along fence borders in shady and moist areas. Seeds germinate in the fall or early spring, and flowering occurs May through July, depending on location. This robust plant can grow up to 10 feet tall. The leaves are alternate or basal (grow from the lowest part of the stem) and are three to four pinnately (featherlike) compound. The weed is sometimes confused with wild carrot (Queen Anne's lace). Stems are erect, smooth, and hollow and have purple mottling. This purple mottling is one characteristic that enables you to distinguish poison hemlock from wild carrot.

This plant is extremely poisonous to horses and humans. All plant parts contain the poisonous alkaloids; however, the fruits contain the greatest concentration of the alkaloids. Poison hemlock gives off a bad odor when crushed, and horses rarely eat this plant because of its low palatability. Poison hemlock plants harvested with hay maintain the toxic properties; care should be taken to avoid feeding hay containing this plant.

Poison hemlock control is relatively easy with herbicides. Mowing and hand-weeding should occur well before flower production to prevent seed production. Consult your local cooperative extension service personnel for herbicidal control in your area.

William W. Witt, PhD, a researcher in Plant and Soil Science at the University of Kentucky, provided this information.

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Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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