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Beware of Poison Hemlock

Poison hemlock is an invasive plant that is highly toxic as all parts of the plant are poisonous during all stages of growth. First: the poison hemlock begins as a rosette of leaves that resembles a fern and stays close to the ground. Second: the plant grows upwards and can reach 10 feet. Third: the plant develops a grouping of small white flowers. Fourth: the stem is smooth and has purple splotches, like bruising. Photos from the Minnesota Department of Agriculture Website.

Mary Beth Sallee

Managing Editor

Hart Co. News Herald


Poison hemlock is one of the most deadly plants in North America, and it’s spreading across Kentucky at an alarming rate.

Known as an invasive species, poison hemlock is native to Europe, western Asia, and northern Africa. In the 1800s, it was introduced to North America as an ornamental garden plant.

Poison hemlock very much represents its name as all parts of the plant are poisonous, including the leaves, stems, roots, flowers, and seeds. In fact, even the dead canes remain toxic for up to three years. Historically, poison hemlock was used by the ancient Greeks to poison condemned prisoners. It was even what killed Socrates after he consumed a potent hemlock infusion.

Brice T. Leech Jr., a Natural Resources Specialist at Mammoth Cave National Park, stated that the plant contains piperidine alkaloids, which are highly toxic to both humans and animals.

Poison hemlock is a highly toxic plant that can cause severe medical episodes and even death in humans and animals. Photo: Minnesota Department of Agriculture Website.

“It is toxic when ingested or when entering the body through mucus membrane, like the eyes or nasal passages,” Leech explained. “Symptoms can come as early as 15 minutes after ingestion (or exposure). Initial symptoms can include seizures, nausea, vomiting, abdominal pains, confusion, weakness, dizziness, rapid or slow heart rate, dilated pupils, and alternating low and high blood pressure. Complications of ongoing seizure activity can include an increase in body temperature, swelling of the brain, blood coagulation disorders, muscle breakdown, and kidney failure. Death is usually caused by respiratory failure or ventricular fibrillation. Seizures have been known to last as long as 96 hours, leading to coma.”

There is no known antidote for poison hemlock. For an individual who has ingested or been exposed to poison hemlock, treatment normally consists of supportive care once a diagnosis has been determined. According to Leech, there have been cases in the United States involving medical emergencies caused by poison hemlock.

“In 2021 in southwest Ohio, a landowner was using an electric chainsaw to eliminate some weeds on his property, not knowing the types of plants he was cutting,” Leech said. “The landowner began to feel sick while dragging the plants away from the site, then began to have trouble breathing. He was taken to the hospital and was found negative for COVID (he was vaccinated). His symptoms worsened. When his daughter showed pictures of poison hemlock, he identified this was what he was cutting. He spent 109 days in the hospital and ultimately required heart surgery. His doctors believed the mode of entry was through inhalation of the aerosolized sap from the poison hemlock, primarily due to the damage to his lungs.”

“Another tragic occurrence from a couple of years ago, small children were playing in the backyard,” Leech continued. “They began eating this plant in the yard. All children but a 3-year-old spit the plant out of their mouths. This 3-year-old was found dead 1.5 hours later. This may not be definitive evidence of toxicity due to poison hemlock, but it does show the caution that should be addressed while handling this plant. This is obviously a very dangerous plant to handle. Extreme caution should be used if this plant is found in your yard if you have children or animals.”

Farmers should also be on the lookout for poison hemlock as it is also very toxic and deadly to livestock and other animals. The plant is often found along fence rows but also in open pasture fields. Forage producers should also watch for poison hemlock in hay fields as the plant is toxic in both its vegetative state and when dry.

Poison hemlock is a member of the carrot family. It is a biennial plant, meaning it grows for two years and then dies. It reproduces by seed dispersion. In its first year, it grows as a rosette of leaves that resembles a fern and stays close to the ground. In the spring of its second year, the plant grows upwards and can grow up to 10 feet.

In the late spring and summer, poison hemlock has a flower head called an umbel that is similar to Queen Anne’s Lace. Left: The poison hemlock umbel consists of a grouping of small white flowers. Right: the flower head on Queen Anne’s Lace. Photos: Minnesota Department of Agriculture Website.

Water hemlock is in the same family as poison hemlock and is also very toxic. The leaves of water hemlock are more lanceolate and serrated and are not as fern-like as poison hemlock’s leaves. The flower heads/umbels of poison hemlock and water hemlock look similar.

The white blooms that appear on poison hemlock as it matures is similar to Queen Anne’s Lace.

“In the late spring/summer, poison hemlock has a flower head called an umbel that is similar to Queen Anne’s Lace (QAL),” Leech said. “The umbel consists of an upside-down umbrella-shaped grouping of small white flowers. QAL grows to about three feet tall, whereas poison hemlock can grow up to 10 feet (though beware when distinguishing the two). The stem of poison hemlock is smooth and has purple splotches, like bruising. The stem is hollow and can be about one inch in diameter. The leaves are fern-like. It grows quickly in disturbed sites. It can infest roadsides, field margins, ditches, marshes, meadows, and low-lying areas. It prefers shaded areas with moist soils.”

Additionally, poison hemlock also differs from Queen Anne’s Lace in that the umbel on QAL has lance-shaped bracts underneath the flower head, whereas the poison hemlock does not. The flower groupings on poison hemlock also appear more starburst-like. The flowers of QAL bloom much later in the season than poison hemlock. QAL also has hairs along its stem.

Left: The stem of the poison hemlock is smooth and has purple splotches, similar to bruising. It is also hollow and can get about one inch in diameter. Right: Queen Anne’s Lace has a hairy stem and does not have purple blotching. Photos: Minnesota Department of Agriculture Website.

Giant hogweed also has similar flower umbels to poison hemlock, as well as similar green stems with purple-ish splotches. However, it differs from poison hemlock as giant hogweed has prominent course hairs and large leaves.

If poison hemlock has been identified on a person’s property, the plant should be removed immediately but with extreme caution.

“PPE (personal protective equipment) should be used. Do not use trimmers, as they spread sap and small quantities of the plant in all directions,” Leech said. “When plants are young and not very dense, pulling works well in its second year in the spring (before flowering). Mowing a dense thicket will work during the same time in spring, before flowering. During these attempts, the wearing of gloves, long sleeves, long pants, and goggles (eye protection) should be used. If these physical methods are not desired, herbicides can be used. Glyphosate, 2-4-D, triclopyr herbicides, or prescribed combinations of these have shown good efficacy when sprayed in the fall when in the rosette stage or spring before bolting up.”

Unfortunately, poison hemlock is not a one-and-done management project. Wherever the plant has become established, there is more than likely already a build-up of a seed bank. Poison hemlock seeds from one growing season can remain viable for 4 to 6 years. Therefore, a person must check for new poison hemlock plants for several years in order to ensure that the removal of the poisonous plant is complete. Areas where poison hemlock was removed, should be reseeded with a plant that is competitive for space. If this is not done, the open space where the poison hemlock was removed leaves an invitation for the plant to become reestablished.

For additional information on poison hemlock or for questions and concerns, contact your local Extension Office.

A grouping of poison hemlock. Photo: Minnesota Department of Agriculture Website


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