Poison hemlock (Conium maculatum L.) and wild parsnip (Pastinaca sativa L.) are combined in this report because these invasive non-native weeds are increasingly found growing together in Ohio. However, the defense chemicals of these weeds are very different and have vastly different modes of action. This is important to understand relative to management options as well as medical treatments for exposure to these highly dangerous weeds.
Life as a Biennial
Poison hemlock and wild parsnip belong to the so-called carrot family, Apiaceae (= Umbelliferae). They superficially share floral characteristics with other members of the carrot family such as Queen Anne’s lace (Daucus carota); however, this non-native biennial blooms much later in the season.
Poison hemlock has a biennial life cycle. The first year is spent in the “vegetative stage” as a low-growing basal rosette; the stage that is currently very apparent. Plants “bolt” during the second year “reproductive stage” to produce erect multi-branched stems topped with umbrella-like flowers.
Wild parsnip is also reported to have a biennial life cycle. However, it may occasionally behave as a monocarpic perennial spending more than a year in the vegetative stage before flowering once and then dying.
Mature poison hemlock plants can measure 6 – 10 ft. tall. Mature wild parsnip plants are shorter in stature but still impressive at up to 4 – 5 ft. tall. Both are prolific seed producers with seeds remaining viable for 4 – 6 years for poison hemlock and around 4 years for wild parsnip.
Poison hemlock was imported into the U.S. as an ornamental in the late 1800s from Europe, West Asia, and North Africa. Rogue plants remained relatively rare until around 30 years ago. Since that time, poison hemlock has elevated its profile from an uncommon oddity to a common threat.
This non-native is one of the deadliest plants found in North America. It is the plant used to kill Socrates as well as the Greek statemen Theramenes and Phocion. Poison hemlock plants contain highly toxic piperidine alkaloid compounds, including coniine and gamma-coniceine, which cause respiratory failure and death in mammals.
All parts of the plant are poisonous: leaves, stems, seeds, and roots. However, the toxins must be ingested or enter through the eyes or nasal passages to induce poisoning. The toxins 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. Immediate emergency medical attention should be sought if an accidental poisoning from this plant is suspected.
All stages of the poison hemlock plant 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 purplish blotches; Maculatum means 'spotted'. Clusters of tiny white flowers are borne on structures called umbels that look like upside-down umbrellas.
Wild parsnip sap contains psoralen which 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 the 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.
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.
Psoralens are also found in several 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 while wild parsnip sounds like a vegetable gone wild; which it actually is!
Parsnips have been cultivated as a root crop in Europe for centuries, perhaps millennia. The "L." in the scientific name Pastinaca sativa L. means Linnaeus first described the species. Both the cultivated and wild types share the same scientific name; however, it is clear that there are significant differences in toxic biochemical properties between the two types.
It is theorized that the wild parsnip plants in Ohio represent "escapes" from cultivated types brought to North American from Europe and a "reversion" back to a wild type. The wild genes were always there but remained suppressed until revealed through natural selection.
Wild parsnip rosettes have celery-like leaves confined to growing from a short stem near the ground. While in this stage, the plant produces a long, thick taproot.
Flower stalks that eventually arise from rosettes have leaves that are alternate, pinnately compound, branched, and have saw-toothed edges. Each leaf has 5 -15 ovate to oblong leaflets with variable toothed edges and deep lobes. The mature flowering plants have a single, thick, deeply grooved, greenish-yellow stem that sprouts lateral branches topped with hundreds of clusters of the yellow umbellate flowers.
Unfortunately, poison hemlock and wild parsnip are becoming more common throughout Ohio and many other states in the upper Midwest as well as states in the eastern U.S. Worse, owing to the lack of awareness (e.g., identification) or poor management practices, or both, these dangerous non-native weeds are increasingly being found growing in close proximity to people which increases their risks to human health.
Additionally, it is not unusual to find poison hemlock and wild parsnip growing together which can create misinterpretations of exposure symptomology. This may account for some online resources incorrectly attributing skin blistering to contact with poison hemlock.
Mechanical management of poison hemlock can be used if it is certain that no wild parsnip is lurking within the poison hemlock. Still, personal protection equipment is strongly recommended particularly eye protection, gloves, and clothing to cover arms and legs to prevent sap from entering through the eyes or skin wounds. Hand-pulling and tilling are effective options if the area is immediately overseeded with grasses or other competitive plants to help suppress poison hemlock re-establishment from seeds germinating this fall.
Mowing can also be used; however, given that a sizable percentage of the current low-growing rosettes may escape the blade, it’s best to delay mowing to target bolting plants. String trimmers are also effective but present an even greater risk of flinging sap compared to mowing. All mechanical control options should be applied before plants begin to flower! Waiting until after plants flower, or worse after seeds are produced, can increase an infestation by removing canopy competition.
Given the extreme risk of phytophotodermatitis from wild parsnip sap, mechanical control is problematic. Hand-pulling is a high-risk endeavor and not recommended. Likewise, tilling could release a huge amount of harmful sap. There have been reports of sap spattered by mowers and string trimmers producing phytophotodermatitis on exposed arms and legs of equipment operators.
The safest approach to controlling this invasive weed as well as poison hemlock is to use herbicides. Of course, as always, read and follow label directions paying close attention to application sites, recommended rates, warnings against making applications close to desired plants (e.g. trees) or near water, and whether surfactants are recommended to enhance herbicide efficacy.
Both poison hemlock and wild parsnip are susceptible to several selective and non-selective postemergent herbicides. However, keep in mind that non-selective herbicides such as glyphosate (e.g. Roundup) can also illuminate plants that compete with these weeds. Herbicidal openings produced by non-selective herbicides provide perfect opportunities for more wild parsnip and poison hemlock to spring forth from previously deposited seed. Thus, it's important to have a plan for establishing competitive plants such as over-seeding with grasses.
Selective post-emergent herbicides will preserve competitive plants. Herbicides effective against wild parsnip and poison hemlock include clopyralid (e.g. Transline), triclopyr (e.g. Pathfinder II), metsulfuron (e.g. Escort XP), and combination products such as 2,4-D + triclopyr (e.g. Crossbow), or 2,4-D + mecoprop + dichlorprop (e.g. Triamine). Applications made now and before plants start to flower can significantly reduce infestations of both wild parsnip and poison hemlock.