May 24, 2021 - 11:25am --

Poison Hemlock and Wild Parsnip are Bolting and Blooming

Author Joe Boggs

Poison hemlock (Conium maculatum) is one of the deadliest plants in North America.  Wild parsnip (Pastinaca sativa) sap can produce severe, painful skin blistering.  Both are commonly found growing together in Ohio and both are beginning to “bolt” and bloom meaning the clock is quickly winding down for controlling these non-native nasties.

These non-native weeds are members of the carrot family, Apiaceae.  The old name for the 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.  Queen Anne’s lace (a.k.a. wild carrot) (Daucus carota) is often used as the poster child for carrot family flowers.  This non-native blooms much later in the season.

Poison hemlock produces white flowers on stalks that create a more rounded look.  Wild parsnip has intense yellow flowers with the stalks producing a more flat-topped appearance.

Poison hemlock has a biennial life cycle. 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 are bolting with some already producing flowers in southern Ohio.

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.  Like poison hemlock, wild parsnip plants are also bolting and beginning to flower in southern Ohio.

Mature poison hemlock plants can tower as much as 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.

Why Worry?

Poison hemlock plants contain highly toxic piperidine alkaloid compounds, including coniine and gamma-coniceine, 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 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, killing 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.

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.

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; wild parsnip just sounds like a vegetable gone wild; which it is!

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 as long as proper precautions are followed including wearing personal protective gear and equipment cleanup with soap and water.  However, timing is everything:  plants should be mowed once plants have bolted but before heavy flowering.  In other words, RIGHT NOW in southern Ohio!

I’ve watched a gas line right-of-way near my home being slowly converted to a poison hemlock (and teasel) right-of-way over the years because of poorly timed mowing.  Each season for the past several years, the right-of-way has been mowed in late August or September.

Of course, this is long after poison hemlock seed had been shed.  Mowing at that time of the year failed to cut the low-growing first-season poison hemlock rosettes.  What it did accomplish was to expose the rosettes to full sun for the winter and it eliminated plant competition with the poison hemlock flourishing when spring rolled around.  It’s also providing me with great poison hemlock photo ops!

Chemical Control Case Studies

Given the problematic nature of controlling poison hemlock and wild parsnip by physical removal, herbicides may be the best option particularly in areas where the weeds present a clear and present danger to the public.  I’ve observed both poison hemlock and wild parsnip being effectively managed in two parks in southern Ohio with properly timed herbicide applications.

Glenwood Gardens which is part of the Great parks of Hamilton County system, began targeting these non-native weeds last year with selective post-emergent herbicides and have had excellent results.  Voice of America (VOA) MetroPark which is part of the Butler County MetroParks system has declared war on poison hemlock and wild parsnip this season with dramatic results.

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), metsulfuron (e.g. Escort XP), triclopyr (e.g. Triclopyr 4), and combination products such as those that contain 2,4-D, mecoprop, and dichlorprop (e.g. Triamine).  Applications made now can significantly reduce infestations of both wild parsnip and poison hemlock.  However, with plants beginning to flower, the control clock is winding down.