• By Adam Probst, Extension Agent

Agriculture & Natural Resources

Look for poison hemlock now!

Source: Dr. JD Green, UK Weed Scientist; Dr. Michelle Arnold, UK Extension Veterinarian Poison hemlock is toxic to a wide variety of animals, including man, birds, wildlife, cattle, sheep, goats, pigs and horses. People are usually poisoned when they mistakenly eat hemlock for plants such as parsley, wild carrot or wild anise.

Cattle seldom eat hemlock but they will if no other forage is available or it is incorporated in hay or silage. A question commonly asked is how much do cattle need to eat to kill them. Unfortunately, the answer is not clear cut. There is considerable variation in the toxic alkaloid content of the plant depending on stage of growth, season, moisture, temperature, time of day, and geographical region (southern plants are more toxic than northern plants). The conium alkaloids have two major effects: 1) rapid, sometimes fatal effects on the nervous system and 2) they are teratogenic agents (causing birth defects in calves and pigs). Cattle have died by eating as little as 0.2-0.5 percent of their body weight in green hemlock. Poison hemlock is teratogenic, if it is eaten by a cow in the first trimester of pregnancy.

The principle strategy for poison hemlock control is to prevent seed production, which can be a challenge, since a fully mature plant is capable of producing 35,000 to 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 and begin to look for emergence of new plants in the fall.

The best time of year for herbicide treatment is during the late fall (November) or early spring (March). In grass pastures and hayfields, herbicide products containing 2,4-D can be effective when applied to young, actively growing plants that are 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.

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