Poison hemlock (Conium maculatum) and wild parsnip (Pastinaca sativa) are two of our nastiest non-native weeds found in Ohio. Poison hemlock is one of the deadliest plants in North America. Wild parsnip can produce severe, painful blistering. Both are commonly found growing together.
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.
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.
What are the Risks?
Poison hemlock 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 psoralen which presents a completely different mode of action compared to the piperidine alkaloids in poison hemlock sap. Psoralen is a naturally occurring phytochemical grouped in a family of organic compounds known as linear furanocoumarins. Psoralen acts as a photosensitizing compound by inhibiting DNA synthesis in epidermal cells which kills these light-shielding cells responsible for protecting us from long-wave ultraviolet radiation (LWUVR) bombarding us in sunlight.
Severe blistering occurs when affected skin 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. It takes around 24 hours for symptoms to first appear after exposure to LWURV 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.
Another challenge with connecting the dots is that wild parsnip commonly grows in and around other weeds, particularly poison hemlock (Conium maculatum). Gardeners who are exposed to wild parsnip sap while weeding a mixed-patch may mistakenly blame the poison hemlock for their ultimate misery.
Psoralens are also found in a number of 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 the extreme northeast part of the state primarily in and around Ashtabula County. Wild parsnip is found throughout the state and is equally damaging. Of course, giant hogweed has a more threatening sounding common name; wild parsnip just sounds like a vegetable gone wild; which it actually is!
To Mow, or Not to Mow
The potential for poisonings from poison hemlock sap and the extreme skin reaction to the wild parsnip sap means these non-native invasive weeds should not be allowed to grow where they can be easily contacted by people. However, mechanical control through mowing, weed trimming, or hand-pulling is problematic. Certainly, wild parsnip presents a much higher risk with reports of sap spattered by mowers and string trimmers producing phytophotodermatitis on exposed arms and legs of equipment operators.
Still, mowing provides one option for managing poison hemlock and to a lesser degree wild parsnip. However, timing is everything: plants should be mowed in the spring once they've bolted but prior to the appearance of flowers. Waiting until after flowering presents a risk the cut flowers will still mature to seed.
I've watched a gas line right-of-way near my home being slowly converted to a poison hemlock (and teasel) right-of-way over the years because of poorly timed mowing. Each season for the past several years, the right-of-way has been mowed in late August or September.
Of course, this is long after poison hemlock seed had been shed. Mowing at that time of the year failed to cut the low-growing first-season poison hemlock rosettes. What it did accomplish was to expose the rosettes to full sun for the winter and it eliminated plant competition with the poison hemlock flourishing when spring rolled around. It's also providing me with great poison hemlock photo ops!
Chemical Control: Case Study
A strong case can be made for herbicides providing the most effective and safest approach to managing both poison hemlock and wild parsnip. I've watched Kurt Goldick (Conservation and Parks Manager of Glenwood Gardens, Great Parks of Hamilton County (GPHC)) effectively reduce the populations of both of these risky weeds by making properly timed applications of selective post-emergent herbicides. In fact, the results of his efforts have been dramatic with both weeds, particularly wild parsnip, being removed from locations that presented a high risk to the visiting public.
Wild parsnip and poison hemlock are both susceptible to non-selective post-emergent herbicides such as glyphosate (e.g. Roundup). However, "non-selective" means all plants - both good and bad - may be killed and there is a considerable downside to killing the competition as well as the targeted weeds.
Post-emergent herbicides do not affect seeds. Thus, "herbicidal openings" that occur when all plants are killed provide the perfect opportunity for more wild parsnip and/or poison hemlock to spring forth from previously deposited seed. Thus, it's important to have a plan for establishing competitive plants after the wild parsnip dies off such as over-seeding with grasses.
However, Kurt has effectively combined using selective herbicides with the existence of various grasses already growing among the wild parsnip and poison hemlock to remove the risky weeds in favor of the grasses. I've included a number of images taken over the years showing this highly effective approach.
Selective post-emergent herbicides that will preserve competitive plants, particularly grasses, while removing poison hemlock and wild parsnip include 2, 4-D, clorpyralid (e.g. Transline), metsulfuron (e.g. Escort XP), and some 2 and 3-way products such as Triamine (2,4-D + MCPA). However, timing is equally important. Kurt applied after the spring emergence of the targeted weeds but before flowering.