Poison hemlock (Conium maculatum) has received a lot of attention lately with reports of expanding populations statewide and subsequent increased human contact with this toxic plant. However, there is confusion about the risk this plant poses to humans and animals, along with what measures should be taken for protection.

The wild carrot family (Apiaceae) has long been known to include plants with potentially harmful effects to humans. Many plants in this family, including poison hemlock, are dangerous or even deadly when consumed, and many produce other defensive chemical compounds that have serious effects if we simply come into contact with plant parts.

Wild parsnip (Pastinaca sativa), another carrot-family plant, is well known for its photo-reactive sap. Compounds in the sap, called furanocoumarins, cause a painful skin rash if sap contacts skin in sunlight. Every year, many folks are unknowingly exposed to this plant, resulting in a mysterious rash. In my experience, it is the most common wild-carrot family member to cause harm to humans.

The production of these compounds is actually an incredibly interesting plant adaption designed as a defense mechanism against microorganisms, such as fungi, and larger biota like herbivores. Like something out of science fiction, furanocoumarins enter the cells of their victims and are activated by exposure to ultraviolet light to infiltrate DNA and cause rapid death of cells. While this results in painful rashes on humans, it can work to eliminate microorganisms intent on causing harm to the plant.

The production of furanocoumarins has been observed in a large number of plants across multiple families, most notably the citrus family (Rutaceae), where these compounds have been studied for potential health benefits when ingested. In poison hemlock, production of these compounds has been shown to be a stress response, meaning more furanocoumarins are produced when the plant is stressed from things like harsh site conditions or mechanical damage, such as grazing or mowing. This results in varying levels of furanocoumarins present in individual or groups of plants.

So, the risk of rash development from contact with poison hemlock varies based on the health condition of the plant. It’s possible to be exposed to sap and not contract a rash, which has resulted in some confusion about whether or not poison hemlock has the ability to cause skin rashes. However, the risk of death from ingestion of poison hemlock is not questioned.

A different set of compounds produced by this plant, most notably coniine and a few other toxic alkaloids, cause rapid death in very small amounts if they enter the body through ingestion or other means. This attribute has been somewhat sensationalized in recent years, drawing much media attention and creating a general panic surrounding exposure to poison hemlock. That’s not to say precaution shouldn’t be taken around this noxious plant, but it’s highly unlikely that death can result simply from being in proximity of poison hemlock.

While poison hemlock is certainly widespread in rural areas, it’s less of a threat in more developed landscapes. The plant thrives on disturbance and primarily occupies degraded habitats. It’s unlikely to just pop up in your vegetable garden or landscaping, but its highly likely to be present along highways, railroads, field edges and other areas infrequently mowed.

The non-native, invasive plant is commonly observed along fence rows and edges of pastures, which poses a serious threat to grazing animals. If consumed, the plant is equally as lethal to livestock as it is to humans. However, it’s not a preferred forage plant, and grazing animals will typically favor other plants of higher quality over poison hemlock. Nonetheless, quick action needs to be taken if this plant appears in or around grazing areas.

Control of poison hemlock is a bit complicated because the plant, like many others in the carrot family, is biennial, meaning it grows for two years before setting seed and dying back. Control measures center on stopping seed production but vary based on the life stage of the plants. For additional information on appropriate control measures, visit my blog (go.illinois.edu/gardenscoop) for an expanded version of this article.

If you find poison hemlock on or around your property, take precautions to limit human and animal exposure to this dangerous plant. It’s nothing to take lightly, but its also not the end of the world. There are very effective control measures that can easily be accomplished safely by simply being aware of the potential ways this plant causes harm.

Ryan Pankau is a horticulture educator with UI Extension, serving Champaign, Ford, Iroquois and Vermilion counties. This column also appears on his “Garden Scoop” blog at go.illinois.edu/gardenscoopblog.

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