Poison Hemlock: Toxic to Horses and Other Animals

Although poison hemlock is often seen along roadways and in other nonagricultural sites, in recent years it has expanded into grazed pasture lands and hay fields. Here’s how to keep your horses and other livestock safe.
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Poison Hemlock
FIGURE 1: Mature poison hemlock plants growing in hayfield. | Photo: University of Kentucky

Poison hemlock (Conium maculatum) is toxic to a wide variety of animals including man, horses, birds, wildlife, cattle, sheep, goats, and pigs. People are usually poisoned when they mistake hemlock for edible plants such as parsley, wild carrot, or wild anise and consume it inadvertently. The first notable example of human poisoning was when Socrates ingested a tea made from poison hemlock (which contained the toxic piperidine alkaloids coniine and gamma-coniceine) and died in 399 B.C.

As the plant begins to send up flower stalks, the leaves are alternately arranged on the main stem. Each individual leaf is pinnately compound with several pairs of leaflets that appear along opposite sides of the main petiole. As the plant matures, poison hemlock can grow upwards to about 6 to 8 feet tall (Figure 1, above left). At maturity the plant is erect, often with multi-branched stems, and forms a deep taproot. Poison hemlock has hollow stems which are smooth with purple spots randomly seen along the lower stem that help distinguish it from other plants similar in appearance. The flowers, when mature, are white and form a series of compound umbels (an umbrella-shaped cluster of small flowers) at the end of each terminal stalk. Although poison hemlock is often associated with areas with moist soil conditions, it can also survive in dry sites.

Although poison hemlock is often seen along roadways, abandoned lots, fencerows, and other noncropland sites, in more recent years, it has expanded into grazed pasture lands and hay fields. Poison hemlock is classified as a biennial that reproduces only by seed. It is capable, however, of completing its lifecycle as a winter annual in Kentucky if it germinates during the fall months. Flowers and new seed are typically produced in late May and June. Plants emerge as a cluster of leaves that form a rosette. Poison hemlock is most noticeable at this stage of growth in late fall through early spring with its parsley-like leaves which are highly dissected or fern-like (Figure 2). The individual leaves are shiny green and triangular in appearance

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