Poison hemlock is a member of the Parsnip family, introduced from Europe. It is a biennial that reproduces solely from seed. During the first year of growth a large rosette of leaves is produced; during the second year the plants bolt to produce 4 to 12 foot stems with white flowers that grow in small, erect clusters. The stem is mottled with purple spots. All parts of the plant are poisonous to humans as well as to wild and domestic animals.

Comments: One of the most widespread weeds in Pitkin County. Common in East and West Sopris Creek, Prince Creek, Watson Divide, Snowmass Village, Woody Creek, and Lenado. The entire plant is toxic to humans and animals. The plants contain the intensely poisonous alkaloid coniine and other related alkaloids. Most poisonings occur because the leaves are mistaken for parsley, the roots for parsnips, or the seeds for anise. The plant has such an unpleasant taste and odor that toxic quantities are seldom consumed. If not lethal, recovery occurs within a few days. Symptoms of poisoning are a burning sensation in the mouth and throat, nervousness, trembling, incoordination of body movements, dilation of pupils, muscular weakness, coldness of extremities, weakened and slowed heartbeat, convulsions, and coma. Death occurs through respiratory paralysis. There was a case of hemlock ingestion by local schoolchildren a few years ago. One of their teachers told the children that the plant was a wild carrot and that they could blow on it like a whistle. They did and subsequently took ill. The local hospital called our staff to verify that the plant was hemlock (it was). The hospital prescribed an antidote and the children recovered.

Biological controls: A defoliating moth Agonopterix alstroemeriana, forms conspicuous leaf rolls.

Chemical controls: Often herbicides increase the palatability of plants by affecting the sugar content. Therefore exercise caution when grazing treated areas until plants are dried brown or dead. For additional information contact your local city, county or town natural resource office. (See section VII for phone numbers.)

Cultural controls: Constantly monitor for new infestations. Reseeding of disturbed sites with fast growing native grasses may competitively inhibit hemlock. Maintain range and pasture in good condition.

Mechanical controls: Mowing of pastures before seed heads are formed will effectively reduce hemlock stands.

Locations of Infestations of Poison Hemlock on Roads and Open Space in Pitkin County: