Farmers and non-farmers alike enjoy the view of cattle or sheep grazing in lush green pastures. Abundant rainfall in 2010 has provided more than enough moisture to boost grass and legume growth in our grazed acreage.

Unfortunately, in some of those pastures a huge white flowered source of poison often stands out.

Poison hemlock is a tall, erect biennial that produces a vegetative rosette the first season of growth and an upright reproductive stem in the second season. Stems are mostly hairless and light gray-green with distinct reddish-purple spots. They are hollow, except at the nodes, and may grow up to 10-feet tall. Leaves are basal in the rosette stage and alternate on the stem in the reproductive stage. The basal and stem leaves are finely dissected with a lace-like appearance, resembling the nonpoisonous wild carrot. The flower heads at the top of the plant are large, one to three inches in diameter, white, and umbrella-shaped. Flowers develop into a green, deeply ridged fruit with several seeds. The plant has an unmistakable and disagreeable mouse-like odor.

Poison hemlock was introduced from Eurasia as an ornamental plant, and is currently found in about half of the counties in Ohio. It grows in waste areas, especially partially shaded, poorly-drained sites, stream banks, or edges of cultivated fields. Seeds germinate in autumn, and seedlings grow late into fall. It is one of few green plants in pastures in late winter and is among the first plants to green up in the early spring. This plant is sometimes confused with wild carrot or wild parsnip.

All parts of this plant are poisonous, though roots are more toxic than leaves or stems. Leaves are especially poisonous in the spring, whereas the root becomes more toxic over time. Eight alkaloids are found in this plant, with coniine and conicine being the most common. Livestock can be severely injured by eating the leaves or fruits of poison hemlock. The strong odor usually prevents consumption of the green plant, but animals can be poisoned by eating contaminated fresh hay or silage. Signs of poisoning include dilated pupils, weakness, staggering gait, and respiratory paralysis two to three hours after ingestion.

Consumption of the weed can also cause birth defects in animals.
Thankfully, the taste of leaves and seeds to livestock is unpleasant, so toxic quantities are seldom consumed when ample desirable feed is available for the animals. However, several deaths of livestock and humans are attributed each year to poison hemlock. Whistles made from hollow stems of poison hemlock have caused death in children.

Herbicides are the best way to control poison hemlock. Crossbow and Banvel are fairly effective on small poison hemlock, but taller plants may need to be controlled with glyphosate. Mowing after the plants have bolted and before setting seed will prevent seed production.

For more information on poison hemlock, consult Ohio State University Extension Bulletin 762-00, “Poisonous Plants,” at http://ohioline.osu.edu/b762/b762_24.html and Bulletin 866-98, “Identifying Noxious Weeds of Ohio,” at http://ohioline.osu.edu/b866/b866_6.html Evidently, weather conditions this year are darn near perfect for the proliferation of this nasty weed. I have also seen poison hemlock growing above standing wheat and late first cutting hay stands; primary around the perimeter of those fields.

Readers with questions or comments for Roger Bender may write to him in care of this publication.