UT Extension News: The Problem with Poison Hemlock

June 10, 2022

I’ve seen several posts and discussions on social media recently about poison hemlock. For a lot of people, it seems this is the first year they have noticed this weed, but it has been around Smith County for several years. I know this because I wrote about it back in 2015. I realize some of you weren’t here in 2015, and I forgive the rest of you for not remembering that article. Anyway, let’s talk about it.

I’m not sure when it first arrived in Tennessee, but it caused a bad day for Socrates over 2,400 years ago. In 399 B.C. the Athenian philosopher was famously condemned to die by drinking a cup of liquid distilled from the poison hemlock plant.

“Poison hemlock is one of the most toxic plants in North America and all parts of the plant are toxic,” says Neil Rhodes, retired University of Tennessee Professor and Extension Weed Management Specialist. “Both animals and humans have died due to accidental ingestion. Deaths are often associated with mistaken identity. Children have been fatally poisoned by making whistles or pea shooters from the hollow stems. Other human deaths have occurred where the plant is mistaken for wild parsnips or parsley.”

The plant is also a danger to grazing livestock. Less than a pound of consumed hemlock can be enough to poison cattle. Thankfully, human and livestock deaths from hemlock poisoning are rare, due to the plant’s unpalatability. The exception to this is right after herbicide application. At this point the palatability increases and animals are more likely to graze the plant. Livestock should be excluded from infested areas prior to applying herbicide. They should continue to be excluded until the plants have died and are brown and dry.

The best way to prevent exposure to the plant is by being able to recognize and destroy it. The weed is capable of reaching heights of three to four feet and it produces long, triangular compound leaves that range from eight to 16 inches long. Poison hemlock’s most distinctive feature is the many handsome, small, white flowers when it blooms. “Knowing how to recognize poison hemlock by sight allows for physical removal and disposal of initial introductions of this weed,” says Rhodes. “Fortunately, most pasture infestations of this weed are very localized rather than being scattered across the entire pasture.”

Sometimes infestations are too large to remove by hand. In those cases, herbicides may be needed. Two times of the year, either in November or March to April, are best for treatment. Plants that are still in the rosette stage (before the stem elongates and the plant grows vertically) are more easily controlled with herbicides than after they mature.

With thorough coverage that is achievable with spot sprays, products such as 2,4-D, Brash, GrazonNext HL, and Duracor are fairly effective. Before spraying, be sure to thoroughly read the herbicide label and follow all directions and precautions. Again, realize that plants that are growing upright and nearing the seed head stage will be very hard to kill with herbicides. Bush-hogging to prevent seed formation would be a better option at that point.

For more information about poison hemlock or other weeds, go online to smith.tennessee.edu and check out our weed control page under the agriculture tab. There you can find fact sheets on Poison Hemlock and many other common weeds, as well as UT recommendations for control.