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Poison hemlock increasing in grazed pasturelands

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Mature plants grow 6 to 8 feet tall

By David Harrison

Poison hemlock has been increasing locally during the past several years.  Although this plant often was seen along roadways and fencerows, it has expanded into grazed pasturelands and hay fields.

The concern not only comes from its invasive nature, but the fact that it is one of the most toxic plants in the world. Throughout history, the toxicity of poison hemlock is well known for accidental deaths of humans and animals. It was the toxin used to poison the philosopher Socrates.

Poison hemlock is classified as a biennial that reproduces only by seed. It is capable, however, of completing its lifecycle as a winter annual if it germinates early during the fall. Flowers and new seed typically are produced in late May and June.

Although plants emerge as a cluster of leaves that form a large rosette, it is most noticeable in early spring with its parsley-like leaves that are highly dissected or fern-like. The individual leaves are shiny green and triangular in appearance. As the plant begins to send up flower stalks, the leaves are alternately arranged on the main stem.

Mature poison hemlock can be 6 to 8 feet tall. At maturity the plant is erect, often with multi-branched stems and a deep taproot. Poison hemlock has hollow stems that are smooth with purple spots randomly seen along the lower stem. This helps distinguish it from other plants similar in appearance. The flowers, when mature, are white and form a series of compound umbrella-shaped cluster of small flowers at the end of each terminal stalk.

All classes of livestock are known to be affected. Cattle, horses and goats are considered the most susceptible farm animals. Symptoms of poisoning can occur rapidly anywhere within 30 minutes to two hours depending on the animal, quantity consumed and other factors.

Initial symptoms can include nervousness, trembling, muscular weakness and loss of coordination, dilation of pupils, coma and eventually death from respiratory paralysis. Lethal doses for cattle are considered to be in the range of 0.2 to 0.5 percent of the animal’s body weight. Poison hemlock is also known to cause fetal deformation when pregnant animals consume the plant.             

Fortunately, most animals tend to avoid grazing poison hemlock if other forage is readily available. However, animals may be more prone to consume green plants during the late winter and early spring when other forage species are more limited. All parts of the plant, including the seeds, are considered toxic. Toxicity may be somewhat reduced in dried plants, but the potential for toxicity still exists, particularly when a sufficient quantity is consumed in dried hay. Therefore, extreme caution should be considered before feeding animals hay known to contain poison hemlock.

The principle control strategy for poison hemlock is to prevent seed production that can be a challenge since a fully mature plant is capable of producing 35,000-40,000 new seeds. It is too late to utilize herbicide control methods after plants have produced flowers. Therefore, mechanical control efforts (if feasible) such as mowing or cutting down individual plants should be initiated just before peak flower production to avoid or reduce the amount of new seed being produced. 

Make note of areas heavily infested with poison hemlock this spring and begin to look for emergence of new plants in the fall. During the late fall (November) or early spring (March) is the best time of year for herbicide treatment. In grass pastures and hayfields herbicide products containing 2, 4-D can be effective when applied to young, actively growing plants in the rosette stage of growth.  Spot treatments with products containing 2, 4-D, triclopyr or glyphosate can also be used depending on the location.