This was reprinted with permission from Joe Boggs, OSU

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 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 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.

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.

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.

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.

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. Apply after the spring emergence of the targeted weeds but before flowering.

For more information, contact the Pulaski County Extension Service at 606-679-6361. Learn about timely events or things to do in your home gardens by becoming a fan of Pulaski County Horticulture on Facebook, or following @hortagentbeth on Twitter, kyplants on Instagram, and Pulaski County Horticulture YouTube channel.

The Pulaski Co Extension office is open to the public by appointment only through the month of June. Extension employees are still on the job and can be reached via office phone. Read the entire directive on the Pulaski County Cooperative Extension website at pulaski.ca.uky.edu.

The Lake Cumberland Master Gardeners are temporarily out of pine straw. Another load will be coming soon.

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