Poison hemlock: What Columbus residents need to know about one of Earth's deadliest plants
One of the most poisonous plants known to man has quietly spread throughout the Buckeye State and has recently taken up residence here in central Ohio.
Poison hemlock (Conium maculatum L.) has been on the rise for several years after first spreading throughout southern Ohio. The plant was imported to the United States as an ornamental in the late 1800s from Europe, West Asia and North Africa. Wild plants were relatively uncommon until about 30 years ago. Since that time, poison hemlock has elevated its profile from an uncommon oddity to a common threat throughout Ohio.
This non-native plant is among the most deadly plants on the planet, containing highly toxic piperidine alkaloid compounds that cause respiratory failure and death in all mammals when ingested. It is the plant that was used to kill Socrates as well as the Greek statesmen Theramenes and Phocion.
Although all parts of the plant are poisonous including the leaves, stems, seeds and roots, they must be ingested or enter through the eyes or nasal passages to be toxic.
Contrary to popular belief, contact with poison hemlock does not cause skin blistering or rashes but the potential exists for the toxins in the sap to enter the body if rubbed from the skin into the eyes or mouth. Immediate emergency medical attention is needed if an accidental poisoning from this plant is suspected.
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Where poison hemlock is in Columbus, central Ohio
Poison hemlock survives in nearly any type of soil condition and seems to be most prevalent along woodland edges and particularly along streambanks, ditches and other waterways.
The population of this plant in central Ohio has grown exponentially in just the past two or three years. Each plant produces thousands of seeds, which are spread by wind, water, birds and when mowed. The seeds remain viable in the soil for up to six years.
On a recent drive through several Columbus neighborhoods, patches of this invasive plant were quickly identified on a ravine in Clintonville, along an alley in Franklinton, and outside my window on the Ohio State University campus. Several livestock owners in Franklin County have reported widespread infestations of poison hemlock in pasture fields this summer.
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How to identify the plant
Poison hemlock is in the carrot family (Apiaceae), and is sometimes mistaken for other plants in this same family including Queen Anne’s lace (daucus carota) and wild parsnip (Pastinaca sativa L), which does cause skin rashes and blistering upon contact.
Poison hemlock is a biennial, which means it takes two years to flower and reach reproductive stage. The plant spends its first year as a small, low-growing plant, and bolts during its second season to become up to 10 feet tall with multi-branched stems topped with umbrella-shaped white flowers. The simplest method for identifying poison hemlock is its purple-colored spots on its main stem. Both Queen Anne’s lace and wild parsnip lack these telltale spots on their stems.
Be careful mowing, pulling or cutting poison hemlock
The best time to control poison hemlock is in its vegetative stage during the first year of growth when it is small. The most dangerous time to control this plant is when it is large and flowering in its reproductive stage during its second year of growth. A contact herbicide containing glyphosate (trade name of Roundup) is most effective when the plant is small. The goal should be to control the plant before it flowers and produces seeds.
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Extreme care should be taken when mowing, hand-pulling or cutting these plants and a string trimmer should never be used to cut these plants. When working around these plants, plants proper protective gear should be worn including protective eyewear, gloves, and long-sleeved shirt and pants.
Learn more about identifying and managing poison hemlock here: https://bygl.osu.edu/index.php/search/node?keys=poison+hemlock
Mike Hogan is an associate professor at Ohio State University and an educator at the OSU Extension.
hogan.1@osu.edu
