COLUMN: Check fields for poison hemlock

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Pervasive weed lives up to its name

By David Harrison

Poison hemlock is abundant again this year (though not in bloom yet).  Although often seen along roadways, fence rows etc, it has expanded in recent years into grazed pastures and hay fields.

The concern not only comes from its invasive nature, but also because it is one of the most toxic plants in the world.

Poison hemlock is a biennial that reproduces only by seed. It is capable, however, of completing its life cycle as a winter annual if it germinates early during the fall. Flowers and new seed are typically 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 which 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 which are smooth with purple spots randomly seen along the lower stem. 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.

Cattle, horses, and goats are considered to be the most susceptible farm animals to the poison. Symptoms of poisoning can occur rapidly anywhere within 30 minutes to two hours.

Late fall or early spring 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.