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 some 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 in contact with plant parts. 

Risk from skin contact 

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 the presence of 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 unknowing 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 in the citrus family (Rutaceae), where these compounds have been studied for potential health benefits when ingested. In poison hemlock, the production of these compounds has been shown to be a stress response, meaning that 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 plants or groups of plants.  

So, the risk of rash development from contact with poison hemlock varies based on the health condition of the plants in question. 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.

Risk from ingestion of poison hemlock

The risk of death from ingestion of poison hemlock is not questioned, however. 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. Not to say that 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 plants.

Risks to livestock

Poison hemlock is a non-native, invasive plant that 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.

Where does poison hemlock occur in Illinois?

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 pop up in your vegetable garden or landscaping.
• It's highly likely along highways, railroads, field edges, and other areas that are infrequently mowed. 

How to control poison hemlock

Control of poison hemlock is a bit complicated because the plant, like many others in the carrot family, its 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. Since this plant is biennial, be aware that 2nd-year plants are tall and in full flower right now, but 1st-year plants may also be present that are much smaller and look almost like a ground cover right now. The foliage has a similar, fern-like appearance on both life stages, which can help with the identification of smaller, 1st-year plants.    

When it is flowering: At this point in the season, chemical control is not a great option since the plant has initiated flowering and may be getting close to seed set. However, it is an opportune time to hand pull or remove plants with a sharp shovel since plant resources are low due to the high investment required to flower and set seed. Use a sharp shovel to sever the tap root below the soil line. Or plants can be hand-pulled if soil moisture permits. Pulled or cut plants must be bagged and set to the landfill as seed production can still occur. 

Mowing: Mowing is a great option to stop or limit seed production if other measures cannot be taken. Repeated mowing can actually eliminate the plant if consistently applied over several growing seasons. However, be sure to mow before seed production begins or your mower may spread seed to new locations during future mowing.

Chemical control: For guidance on effective chemical control, Wisconsin Extension has a fact sheet available by clicking here.

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, which can easily be accomplished safely by simply being aware of the potential pathways this plant causes harm.

More information

For more information on poison hemlock and a few lookalike members of the wild carrot family, please read this past Garden Scoop Blog.

Learn more about Illinois invasive species. 

References

• Berenbaum, M. 1981. Patters of furanocoumarin production and insect herbivory in a population of wild parsnip. Oecologa (Berl.). 49:236-244.
• Cadierno, V. 2015. Meal-catalyzed routes for the synthesis of furanocoumarins and coumestans. Green Synthetic Approaches for Biologically Relevant Heterocycles. 77-100.
• Del Rio et al. 2014. Furanocoumarins: biomolecules of therapeutic interest. Studies in Natural Products Chemistry. 43: 145-195.
• Fatma et al. 2004. Antifungal compound from induced Conium maculatum L. plants. Biochemical Systematics and Ecology. 32: 1097-1108.
• Hotti, H. and Rischer, H. 2017.  The killer of Socrates: Coniine and Related Alkaloids in the Plant Kingdom. Molecules. 22(11):1.