URBANA — 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.
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 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.
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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.
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.
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 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.