Poison hemlock (Conium maculatum L.) and wild parsnip (Pastinaca sativa L.) are currently in a growth stage that makes them susceptible to early-season management. Targeting these dangerous plants with herbicides applied now will prevent flowering and seed production later this season.
These non-native invasive weeds are combined in this report because they are increasingly found growing together in Ohio. Both belong to the carrot family, Apiaceae, and produce umbrella-like flowers referenced in the old name for the family, Umbelliferae. They also have biennial life cycles requiring at least two years to grow from seed to mature flowering plants.
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 (Variable) Biennial
Plants with a biennial life cycle spend the first season in the vegetive stage. The low-growing “rosettes” use carbohydrates acquired through photosynthesis to produce a robust root system.
Plants “bolt” during the second-year reproductive stage to produce erect multi-branched stems topped with umbrella-like flowers. The mature plants die after producing seeds.
It’s important to 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 the growth stages within a group of poison hemlock and wild parsnip plants are seldom synchronized. It’s common for first-season vegetive plants to be mixed with second-season reproductive plants.
Seed viability as well as the timing of seed germination also affects what we see. Poison hemlock and wild parsnip are prolific seed producers with hemlock seeds remaining viable for 4-6 years and parsnip seeds remaining viable for around 4 years. New and old seeds produced by both of these plants may germinate in late summer, early fall, to early spring. As a result, first-year rosettes commonly range in size from small plants if seeds germinated in the spring to larger plants if seeds germinated in the fall.
Also, some plants take longer than two years to complete their development. Wild parsnip may occasionally behave as a monocarpic perennial spending more than one year in the vegetative stage before flowering once and then dying. It’s suspected poison hemlock may also be capable of behaving as a monocarpic perennial although research has not confirmed this speculation.
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 our body through our eyes, nasal passages, or cuts in our skin 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 accidental poisoning from this plant is suspected.
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.
Flowering plants have hollow, 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. Mature poison hemlock plants can measure 6 – 10 ft. tall.
It's commonly reported that Wild Carrot (Queen Anne’s Lace, Daucus carota) may be mistaken for poison hemlock or vice versa. However, white flowers and parsley-like leaves are the only things this non-native has in common with poison hemlock. The flat-topped flower umbels look nothing like poison hemlock, the stems are hairy, and the bristly leaves are single pinnate. More importantly, wild carrot blooms in mid-summer long after poison hemlock has bloomed, and plants are collapsing after producing seeds.
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.
Skin blistering 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 not become widespread in Ohio with confirmations confined to the northeast part of the state.
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 America 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 hollow stem that sprouts lateral branches topped with hundreds of clusters of the yellow umbellate flowers. Mature wild parsnip plants are normally shorter in stature compared to poison hemlock. While some plants may top 6 ft, most mature plants are 4 – 5 ft. tall.
Unfortunately, poison hemlock and wild parsnip are becoming more common throughout Ohio. Worse, these dangerous non-native weeds are increasingly being found growing near people which increases 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.
Timing is everything! The graphic below shows the best and worst times to implement management tactics.
The safest approach to controlling these dangerous invasive weeds is to use herbicides. This minimizes risks associated with direct contact. As always, read and follow label directions paying close attention to recommended rates and whether surfactants are recommended to enhance herbicide efficacy.
The application window for controlling poison hemlock and wild parsnip is now opening in southern Ohio. Surrounding vegetation has largely not yet started to grow which makes first and second-year plants very evident.
Keep in mind that both of these weeds bolt in the spring; flowers and seeds are produced in early summer. It’s critical to make herbicide applications before the plants produce flowers.
Non-Selective Post-Emergent Herbicides:
These herbicides kill a wide range of plants after they have sprouted from seeds. However, keep in mind that non-selective herbicides such as glyphosate (e.g., Roundup) can also illuminate plants that compete with poison hemlock and wild parsnip.
Herbicidal openings produced by non-selective herbicides provide perfect opportunities for more parsnip and hemlock to spring forth from previously deposited seeds. Thus, it's important to have a plan for establishing competitive plants such as over-seeding with grasses.
Selective Post-Emergent Herbicides:
These herbicides kill a select range of plants after they have sprouted from seeds. Selective herbicides can be chosen that will preserve competitive plants. For example, grasses are strong competitors against these opportunistic weeds. Herbicides that spare grasses but kill “broadleaf weeds” like poison hemlock and wild parsnip will preserve and enhance this competitive edge.
Selective herbicides effective against wild parsnip and poison hemlock include, but are not limited to, clopyralid (e.g. Transline), metsulfuron (e.g. Escort XP), and combination products such as those that contain 2,4-D, mecoprop, and dichlorprop (e.g. Triamine). Again, applications must be made before plants start to flower to effectively reduce weed infestations.
These herbicides interfere with the successful establishment of targeted weeds from seed. Unfortunately, I’m not aware of any published data on the efficacy of preemergent herbicides against poison hemlock or wild parsnip.
To Mow, or Not to Mow
Poison hemlock can be controlled by mowing bolting plants before they produce flowers. However, low-growing rosettes may escape the blade, and seeds are unaffected. Thus, an infestation will not be eliminated in one mowing season.
Improper timing can actually enhance poison hemlock infestations. The images below show a high-risk location where poison hemlock has been consistently mowed from late summer to early fall after plants had already released their seed. Worse, the late-season mowing removed shading from taller competitive plants exposing the hemlock rosettes to full sun. As the result of years of ill-timed mowing, poison hemlock has ascended from a rarity in this location to a dominant plant.
CAUTION: sap from poison hemlock presents a serious hazard. There is a potential for the sap to become aerosolized with the mechanical removal of actively growing plants. String trimmers and open flail mowers should not be used.
Care should also be taken with shrouded mowers. Personal protection equipment (PPE) including eye protection, gloves, and clothing to cover arms and legs is strongly recommended. Clothing and gloves should be removed and washed as soon as possible after mowing.
Poison hemlock plants may be hand-pulled before flowering and disposed of in a safe manner. The same PPE recommendations for mowing should be applied.
In my opinion, wild parsnip should not be removed mechanically or by hand-pulling. There is simply too much of a risk presented by errant sap. Thus, if wild parsnip is growing among poison hemlock, herbicide applications are the safest option to remove both of these dangerous weeds.