Poison hemlock (Conium maculatum) is a non-native, invasive, herbaceous, biennial plant. Poison hemlock prefers moist soils but can easily adapt to a variety of conditions, and has the capacity to invade large portions of pastures and disturbed sites. Due to the toxicity threat to livestock, it is imperative that this plant is properly identified and controlled in pastures and hay fields.
Poison hemlock. Image by John Cardina, The Ohio State University, via Bugwood.org
Toxicity
Poison hemlock poses a significant threat to livestock. It contains various alkaloids causing poisoning. These toxins impact both the nervous and reproductive systems. General symptoms of poisoning from poison hemlock in livestock vary from nervousness, trembling, incoordination, dilated pupils, weak heartbeat, cold extremities, coma, to death by respiratory failure. Additionally, alkaloids in poison hemlock impact the development of embryos. Livestock impacted include swine, cattle, horses, sheep, goats, elk, and turkey. Of these, the most sensitive to poison hemlock toxins are swine and cattle.
Livestock poisoning most often occurs in the early spring, as poison hemlock is one of the first plants to emerge. Poison hemlock germinates in the fall, and as one of the last available green forages can also lead to late fall poisoning events.
Livestock can be exposed to poison hemlock through fresh forage, harvested silage, and contaminated hay. Silage will not reduce the plant’s toxicity. Additionally, seeds of poison hemlock contain toxins and can contaminate harvested grains.
Please visit Cornell University’s Department of Animal Sciences page on plants poisonous to livestock for more information on the impact of poison hemlock on livestock
Humans may also be poisoned by poison hemlock, although it is far less common than livestock poisoning. Typically, poisonings occur with children playing with stems or adults mistaking the plant for a similar-looking edible species. Human symptoms from exposure to alkaloids of poison hemlock are similar to those of livestock.
Identification
Seedlings: When poison hemlocks seedling first emerge, cotyledons are longer than wide and tapered to a rounded tip, with predominant veins on the underside. The seedling itself also typically has a purple base. The first true leaf is highly divided and compound, pale green, and smooth; later leaves are ferny and triangular with deeply lobed leaflets.
Poison hemlock seedling. Image by Ohio State Weed Lab, The Ohio State University, via Bugwood.org.
Young poison hemlock plant. Image by Ohio State Weed Lab, The Ohio State University, via Bugwood.org.
Leaves: The leaves of poison hemlock are fern-like and delicate: compound, toothed, and finely divided. They are a glossy green color with paler underside. Leaves are arranged alternately on the stem and range in length from 1 to 16 inches. When crushed, poison hemlock gives off a pungent order that has been compared to the smell of mouse urine.
Mature plant: In its first growing season, poison hemlock produces a rosette, or a cluster of leaves with no vertical stalk, and strong roots. Rosette leaves are glossy green and can be over 2 feet long. The next growing season, the plant produces branching, erect stems 2-10 feet tall with alternately arranged leaves. Stems of poison hemlock are slightly grooved, hairless, and hollow except at the nodes. Stems also have distinctive purple blotches throughout, and give off a pungent odor when crushed. After seeding the plant will die, leaving stems behind that persist into the winter.
Poison hemlock leaf. Image by Robert Vidéki, Doronicum Kft., via Bugwood.org
Poison hemlock stem. Image by Eric Coombs, Oregon Department of Agriculture, via Bugwood.org
Flower/seeds: Poison hemlock flowers from June-August, producing umbrella-shaped clusters of tiny, white flowers. Poison hemlock relies solely on seeds for reproduction. Poison hemlock seeds small, tan-and-brown striped, and a flattened oval shape. Seeds can germinate immediately or stay dormant and germinate in the next two to three years.
Poison hemlock flower. Photo by Jeff Stachler of Ohio State University, via Bugwood.org.
Poison hemlock seed head. Image by Jan Samanek, Phytosanitary Administration, via Bugwood.org.
Species Similar to Poison Hemlock
Table 1: Shows poison hemlock (Conium maculatum) similarities and differences in identification compared to its similar species .| Species | Similarities | Differences |
|---|---|---|
| Queen Anne's lace (Daucus carota) | white, flat-topped flowers in clusters; fern-like leaves | hairy stems with no purple splotches |
| Giant hogweed (Heracleum mantegazzianum) | white, flat-topped flowers in clusters | white hairs at stem nodes (where leaves sprout from stem) |
Management
Chemical control
Chemical control is more effective seedlings or rosettes rather than mature plants, and is an effective option for large infestations. Chemical control will need to be to be repeated until the soil seedbank is depleted.
Cornell University’s Crop and Pest Guidelines and Cornell’s Turfgrass and Landscape Weed ID app offer suggestions for conventional and alternative chemical control options.
In addition, University of California’s Statewide Integrated Pest Management Program’s page on poison hemlock provides guidance thorough guidance on chemical control of poison hemlock. Make sure to check your state’s regulations before selecting a chemical management strategy, as they vary from state to state.
Non-chemical control
Early detection of this species is important, as larger infestations are harder to control. Smaller infestations can be dug up by hand, removing the plant and taproot. The plant should be thrown away plastic bags, as dead and dried plant material still contain potent levels of toxins. Mowing may not be an effective means of control unless carried out multiple times a season. Plants can regrow and flower again after being mowed. Plowing or repeated cultivation of newly germinated plants can prevent establishment. After control measures are taken, impacted areas should be monitored until the soil seedbank is depleted of poison hemlock seeds.
References
Cornell University, College of Agriculture and Life Sciences, Department of Animal Sciences- Plants Poisonous to Livestock: Piperidine Alkaloids. https://poisonousplants.ansci.cornell.edu/toxicagents/piperidi.html
Cornell University’s Turfgrass and Landscape Weed ID app. Identification and control options for weeds common to turf, agriculture, and gardens in New York; uses a very simple decision tree to identify your weed. http://turfweeds.cals.cornell.edu/plant/control/147
Cornell University’s Turfgrass and Landscape Weed ID- Weeds and Human Health: The Severe Seven. http://turfweeds.cals.cornell.edu/weeds-and-human-health
Cornell University’s Turfgrass and Landscape Weed ID app. Identification and control options for weeds common to turf, agriculture, and gardens in New York; uses a very simple decision tree to identify your weed. http://turfweeds.cals.cornell.edu/plant/identify/150
Cornell University’s Weed Ecology and Management website. Contains a wealth of information on ecological management of agricultural and garden weeds. Look for a revamp of this site in 2021.
The Ohio State University- Ohio Perennial and Biennial Weed Guide: Poison-Hemlock. https://www.oardc.ohio-state.edu/weedguide/single_weed.php?id=114
University of California Agriculture & Natural Resources -Statewide Integrated Pest Management Program, Pests in Garden and Landscape: Poison Hemlock. http://ipm.ucanr.edu/PMG/PESTNOTES/pn74162.html
Washington State Noxious Weed Control Board: Poison Hemlock. https://www.nwcb.wa.gov/weeds/poison-hemlock