Poison Hemlock (Conium maculatum) and Wild Parsnip (Pastinaca sativa) belong to the carrot family Apiaceae (= Umbelliferae). These non-natives can be found throughout Ohio and are commonly found growing together. Both present serious human health risks, but in very different ways.
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.
Poison hemlock and wild parsnip typically bloom and begin to produce seed in late May to early June in southwest Ohio. As shown in the image below, poison hemlock is already producing seed when Queen Anne’s Lace (Daucus carota, family Apiaceae) is in full bloom.
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.
Why Worry?
Poison hemlock is one of the deadliest plants now found growing in many areas of North America. The iNaturalist map below shows its general distribution in Ohio and the surrounding states.
Poison hemlock is best known for its use by the Greeks to kill Socrates as well as the Greek statesmen Theramenes and Phocion. 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.
These toxic compounds can have deleterious effects on major systems in mammals, including the gastrointestinal tract (nausea and vomiting), respiratory tract (bronchoconstriction and bronchorrhea), nervous system (neuromuscular respiratory paralysis), and cardiovascular system (rapid heartbeat – then slow heartbeat).
However, Poison hemlock toxins must be ingested to induce poisoning or enter the body through other means such as through the eyes or nasal passages. Poison hemlock sap does not cause skin blistering, although individuals who are hypersensitive to the plants may experience some slight skin irritation.
A cautionary tale is provided by a story titled, “Hiding in Plain Sight” published in the “Life + Health” section of Good Housekeeping (April 2022, pgs. 21-25). The article describes a life-threatening encounter in southwest Ohio with a homeowner spending 109 days in a hospital. The homeowner used an electric chainsaw to cut through the dense, stiff stems. He inadvertently aerosolized the sap, which he inhaled. This means that mowers should be shrouded and plants should not be cut using weed saws or weed trimmers.
Wild parsnip is also commonly found throughout North America. The iNaturalist map below shows its general distribution in Ohio and the surrounding states.
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.
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 life cycle graphic 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 vegetative plants to be mixed with second-season reproductive plants.
Exploit the Life Cycle: Timing is Everything
Successful management of poison hemlock and wild parsnip centers 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.
Poison Hemlock and Wild Parsnip are beginning to “bolt” in southwest Ohio. This heralds the beginning of the most effective target for the application of selective or non-selective postemergent herbicides to manage these dangerous non-natives. The herbicides will kill the “reproductive” plants before they produce flowers and seeds, and may also remove seedlings and plants in the vegetative, first-year “rosette” stage.
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.
Selective herbicides kill targeted plants, but spare non-target plants. 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 read and follow label directions. Some post-emergent herbicides can seriously damage trees if applied over the root zone.
A Word About Mowing
Mowing is another effective management strategy, but is NOT recommended for wild parsnip. Mowing should only be considered for the control of poison hemlock. 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.
Timing is critical. There’s clear evidence that late-season mowing can play an important role in the spread of these dangerous non-natives. For example, Poison hemlock and wild parsnip are not very competitive with other plants and mowing a utility right-of-way in late summer can reduce canopy competition, allowing rosettes to flourish.
Mowing after plants have died but are still holding onto seed can spread the seed of these non-natives. Seed falling onto the mower decks can be distributed to other locations. Thus, mowing can expand rather than eliminate an infestation. Debris should be removed from mowers, particularly mower decks, before equipment is moved to new locations.
Support 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 image below shows an effort to establish grasses in a right-of-way that was dominated by poison hemlock.
The images below illustrate how post-emergent selective herbicides can be used to suppress poison hemlock and wild parsnip, allowing pre-existing grasses to dominate. The dangerous non-natives were removed in a single herbicide application.
