Poison Hemlock (Conium maculatum) and Wild Parsnip (Pastinaca sativa) have reached developmental stages that place these weeds outside of effective management options in southwest Ohio. Wild parsnip is producing seeds while poison hemlock is in full bloom meaning seeds can still develop on plants that are cut or sprayed with an herbicide.
These non-natives can be found throughout Ohio and are commonly found growing together. Both belong to the carrot family Apiaceae (= Umbelliferae)) and both present serious human health risks, but in very different ways.
Poison hemlock usually receives the most attention because of its use by the Greeks to kill Socrates as well as the Greek statesmen Theramenes and Phocion. Indeed, I’ve heard that poison hemlock is responsible for Socrates’ last words, “I drank what?”
The genus name Conium is Greek meaning to spin or whirl, and refers to the symptoms of hemlock poisoning. All parts of the plant contain highly toxic piperidine alkaloid compounds, including coniine and gamma-coniceine, which cause respiratory failure and death in mammals.
Poison hemlock toxins must be ingested to induce poisoning or enter the body through other means such as through the eyes or nasal passages. Inhalation of aerosolized sap is also a concern. A story titled, “Hiding in Plain Sight” published in the “Life + Health” section of Good Housekeeping (April 2022, pgs. 21-25) described such an encounter in southwest Ohio with a homeowner spending 109 days in a hospital. Poison hemlock sap does not cause skin blistering.
Photo: Joe Boggs
Wild parsnip sap causes severe skin blistering. The sap contains the chemical psoralen that acts as a photosensitizing compound by inhibiting DNA synthesis in epidermal cells. This eventually kills these light-shielding cells responsible for protecting us from ultraviolet radiation (UV) bombarding us in sunlight.
Severe blistering occurs when the affected skin is exposed to UV radiation. The synergistic effect is called phytophotodermatitis (a.k.a. Berloque dermatitis) and the burn-like symptoms and skin discoloration may last for several months.
However, connecting skin blistering to exposure to wild parsnip sap can be a challenge. It takes around 24 hours for symptoms to first appear after exposure to UV radiation and severe blistering typically doesn't peak until 48 -72 hours. The time required for symptoms to appear after exposure to the sap means the effect may be disconnected from the cause. This can result in misplaced blame with poison hemlock or other weeds being incorrectly blamed for the skin blistering.
Psoralens are also found in other members of the Apiaceae family including the notorious giant hogweed (Heracleum mantegazzianum) which has captured national attention in the past. However, giant hogweed has only been confirmed in Ohio growing in relatively small infestations in the northeast part of the state. Wild parsnip is found throughout the state and is equally damaging.
Of course, giant hogweed has a common name that sounds more threatening. The name “wild parsnip” sounds like a vegetable gone wild, which it is! Wild parsnip is the escaped form of the vegetable we grow in our gardens.