Poison hemlock (Conium maculatum) and wild parsnip (Pastinaca sativa) are two of our nastiest non-native weeds found in Ohio. Poison hemlock can kill you while wild parsnip may make you wish you were dead. Both are commonly found growing together and continuously wet conditions caused both to flourish this growing season. The size of some infestations has been remarkable.
Poison hemlock and wild parsnip are members of the carrot family, Apiaceae. The old name for the family was Umbelliferae which refers to the umbel flowers. They are a key family feature with short flower stalks rising from a common point like the ribs on an umbrella. In fact, the origins of both umbel and umbrella can be traced to the Latin word umbra which means shadow or shadow.
Poison hemlock produces white flowers on stalks that create a more rounded look; perhaps a bit more like an umbrella. Wild parsnip has intense yellow flowers with the stalks producing a more flat-topped appearance.
Both are biennial weeds meaning that it takes two years for plants to produce seed. The seeds currently being produced will give rise to plants that spend their first year as low-growing basal rosettes. The plants produce a long, thick taproot while in this stage.
During their second year, plants "bolt" by producing erect, towering stalks and multi-branched stems topped with umbel flowers. Mature wild parsnip plants may top 6' tall while poison hemlock plants can tower to as much as 8 - 10' tall. Both are prolific seed producers
Wild parsnip plants have leaves that look vaguely like celery, another member of the carrot family. Mature plants have a single, thick, deeply grooved, greenish-yellow stem that sprouts lateral branches topped with flowers.
All stages of poison hemlock plants have bluish-green leaves that are 3-4 times pinnately compound. The deeply cut parsley-like leaflets have sharp points. Flowering plants have hairless, light-green to bluish-green stems that are covered with obvious reddish-purple blotches. However, the blotches may occasionally coalesce to cause stems to appear an almost solid color.
Why Should You Care?
Poison hemlock is one of the deadliest plants in North America. Plants contain highly toxic piperidine alkaloid compounds, including coniine and gamma-coniceine, which cause respiratory failure and death in mammals. The roots are more toxic than the leaves and stems; however, all parts of the plant including the seeds should be considered dangerous.
The toxins must be ingested or enter through the eyes or nasal passages to induce poisoning; they do not cause skin rashes or blistering. Regardless, this plant should not be handled because sap on the skin can be rubbed into the eyes or accidentally ingested while handling food.
Wild parsnip sap contains psoralens which are naturally occurring phytochemicals grouped in a family of organic compounds known as linear furanocoumarins. Psoralens kill epithelial skin cells by inserting themselves into the DNA in the cell's nucleus. These are the cells responsible for protecting us from long-wave ultraviolet radiation (LWUVR) that bombards us in sunlight.
Severe blistering occurs when skin affected by the psoralens is exposed to LWUVR. The synergistic effect is called phytophotodermatitis (a.k.a. Berloque dermatitis) and the burn-like symptoms, as well as skin discoloration, may last for several months. However, connecting skin blistering to exposure to wild parsnip sap can be a challenge. The cause and effect are muddled by time because symptoms do not appear for around 24 hours after exposure to LWUVR and severe blistering doesn't peak for another 48 to 72 hours.
Another challenge is that wild parsnip commonly grows in and around poison hemlock. Gardeners exposed to wild parsnip growing among poison hemlock may mistakenly blame the poison hemlock for their ultimate misery.
It is becoming too late to effectively manage either of these weeds in southern Ohio, but there may still be time to reduce infestations in the central or north parts of the states. However, it's important to remember that once flowers mature, seeds will still be produced on plants that have been cut down.
While it may be too late for control, it's not too late to suffer from the toxicity of both of these plants. They will remain a risk until collapsing later this season.
Don’t be Fooled
Apiaceae is a large family that includes many innocuous plants. The roots of wild carrot, or Queen Anne's lace (Daucus carota), are sometimes eaten raw or cooked. Unfortunately, they bear a striking resemblance to poison hemlock roots and misidentifications have been responsible for a number of accidental poisonings.
During this week's BYGL Zoom Inservice, Erik Draper showed pictures of garden angelica (Angelica archangelica) which is sometimes cultivated for its edible roots and stems as well as its perceived medicinal properties. The stems are a deep purple. As noted above, poison hemlock stems are commonly covered in reddish-purple blotches, but those blotches may occasionally merge to produce an almost solid color.
I never considered fennel (Foeniculum vulgare) to be a possible look-a-like plant for either poison hemlock or wild parsnip. However, last season, I was told by an avid gardener that while she and a friend were walking along a trail in Ohio, her friend grabbed some poison hemlock seeds thinking they were fennel seeds. Thankfully, the gardener stopped her friend from eating them.