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.
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.
Identification
The old name for the carrot family was Umbelliferae which refers to the umbel flowers. The flowers are a key family feature with short flower stalks rising from a common point like the ribs on an umbrella. Wild parsnip’s flat-topped yellow flowers have a characteristic Umbelliferae arrangement while poison hemlock’s white flowers are arranged in distinct clusters.
All stages of the poison hemlock plant have dark-green to bluish-green leaves that are 3-4 times pinnately compound. The deeply cut parsley or carrot-like leaflets have sharp points.
All stages of the wild parsnip plants have light-green pinnately compound leaves that strongly resemble celery leaves. Each leaf has 5 -15 ovate to oblong leaflets with variable toothed edges and deep lobes. The leaf structure is even evident on newly germinated seedlings.
Poison hemlock plants have hairless stems powdered with epicuticular wax that gives the stems a blueish-green color. The stems are also covered in obvious purplish blotches; Maculatum means 'spotted'. Mature poison hemlock plants can measure 6 – 10 ft. tall.
Wild parsnip’s coarse green stems are deeply grooved. Mature wild parsnip plants can rise as high as 8 ft.; however, most mature plants only grow to 5-6 ft.
Management
1. Know the Life Cycle:
Poison hemlock and wild parsnip have biennial life cycles. A generalized biennial life cycle is illustrated in the graphic below.
The first year is spent in the “vegetative stage” as a low-growing basal rosette. Plants “bolt” during the second year “reproductive stage” to produce erect multi-branched stems topped with umbrella-like flowers. Plants die once they produce mature seeds.
Keep in mind that the graphic above provides a generalized view of a biennial life cycle. In reality, there can be considerable variability in the timing of events meaning that poison hemlock and wild parsnip plants are seldom synchronized within an infestation. It’s common for first-season vegetive plants to be mixed with second-season reproductive plants.
2. Exploit the Life Cycle
Successful management of poison hemlock and wild parsnip should center on exploiting vulnerable points in their biennial life cycles. The most vulnerable points in the biennial life cycles of poison hemlock and wild parsnip are presented in the graphic illustrations below.
A. The Role of Mowing (Only for Poison Hemlock!)
Mowing is an option for managing poison hemlock, not wild parsnip. Even then, equipment operators should use personal protection equipment (PPE) and mower blades should be shrouded. Remember that there is evidence that the deadly sap may become mechanically aerosolized to present an inhalation risk. Thus, string or blade trimmers should never be used.
Mowing is not a safe option with wild parsnip owing to the extreme risk posed by the sap. There’s even a risk of exposure to the blister-inducing sap while cleaning mowers. Of course, using string or blade trimmers would be a potentially disastrous decision.
Note that the most effective window for suppressing poison hemlock by mowing is when plants are bolting but before plants bloom. However, most mowers will pass over the low-growing rosettes and even if they are cut, the plants will produce new stems.
Improper timing with mowing poison hemlock can expand rather than eliminate an infestation. This is illustrated in the images below taken on a gas line right-of-way near my home. The right-of-way borders a walking trail that I frequent and there was a time when poison hemlock was rare. However, years of mowing in late summer to early fall has allowed poison hemlock to expand from an uncommon weed to a dominant weed.
Poison hemlock is a poor competitor with other plants. Mowing the dead poison hemlock plants in late summer to early fall does nothing to suppress the deadly weed. In fact, cutting down the weed canopy exposes poison hemlock seed to the sun which enhances seed germination and new-plant establishment. Worse, there’s no competition from other plants which gives poison hemlock free rein to run rampant, which it has!
B. The Role of Herbicides
Herbicides provide the safest route to eliminate poison hemlock and wild parsnip. Indeed, it’s the least risky route for suppressing wild parsnip.
Pre-emergent herbicides prevent new plants from developing from seed by halting successful seed germination and seedling establishment. Unfortunately, I cannot cite research data on the efficacy of any pre-emergent herbicides on poison hemlock or wild parsnip.
Post-emergent herbicides kill plants as they are growing. Non-selective post-emergent herbicides, such as those with the active ingredients glyphosate or pelargonic acid (e.g., Scythe), are effective but as indiscriminate plant killers, they also eliminate plant competition.
Herbicidal openings produced by non-selective herbicides provide perfect opportunities for wild parsnip and poison hemlock to spring forth from previously deposited seed. The images below show how a glyphosate application produced an expansion of a poison hemlock infestation. Thus, it's important to have a plan for establishing competitive plants such as over-seeding with grasses (family Poaceae).
Selective herbicides kill targeted plants, but spare non-target plants. The most effective time to apply post-emergent selective herbicides is when poison hemlock and wild parsnip plants are bolting but long before plants bloom. The application will kill first-year plants and second-year plants before they produce seed.
Late summer or fall herbicide applications are not effective because poison hemlock and wild parsnip seeds may germinate in late summer, fall, or spring. Late summer and fall applications would miss plants arising from spring-germinating seeds.
Wild parsnip and poison hemlock are susceptible to a wide range of selective post-emergent herbicides. Herbicides that target broadleaf weeds but spare grasses should be considered because grasses are effective competitors against poison hemlock and wild parsnip. These include clopyralid (e.g., Transline), metsulfuron (e.g., Escort XP), triclopyr (e.g., Triclopyr 4), and products that contain a combination of 2,4-D, dicamba, mecoprop, and dichlorprop.
Of course, as with using any pesticide, it’s important to closely read and follow label directions. Some post-emergent herbicides can seriously damage trees if applied over the root zone.
3. Enhance the Competition
The most effective long-term poison hemlock and wild parsnip management plans focus on enhancing plant competition. As noted above, grasses are strong competitors against these high-risk weeds.
The images below illustrate how heavy infestations of poison hemlock and wild parsnip were eliminated using post-emergent selective herbicides to suppress the weeds allowing grasses to dominate. I’m not aware if over-seeding was involved; however, that’s a viable option if the pre-existing grass density is too low to provide effective competition with the weeds.
