Poisonous Hemlock (Conium maculatum)

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Poison hemlock (Conium maculatum) in full bloom. Photo credit: Pedro Tenorio-Lezama, Bugwood.org

Poisonous Hemlock (Conium maculatum) is an extremely poisonous plant. There is great concern over this plant due to the personal safety of humans, foraging domestic animals and wild life. Most of the cases of poisoning to humans has been the result of foragers misidentifying the plant, believing the root to be that of the edible cow parsnip, or the leaves that of wild celery. Although the species grows mainly in non-crop areas, it’s highly invasive nature has caused it to increasingly invade crop edges. This causes additional health concerns as well as posing a serious economical threat; in addition to the horticultural and environmental threats it has long posed.

Distribution of Poison Hemlock

Poison hemlock is native across northern Europe, western and central Asia and North Africa . It has been introduced widely outside its native range and can be found in Canada, many parts of the USA, southern Africa, China, New Zealand, Australia, Brazil and Norway. It has also been found in South America, and Micronesia (USDA-ARS 2016.gov. of Can). In Canada it occurs in AB, BC, NB, NS, ON, QC, SK  (Government of Canada rev. Nov. 2017 citing Brouillet et al. 2016).

To view one of the best distributional maps I found out there visit Conium maculatum (poison hemlock) (cabi.org).

Habitat: Poison hemlock prefers moist soils in sunny locations and is commonly found along streams and creek-beds, ditches, road sides and low-lying areas. The species however is also tolerant of shade and drier conditions and can be found growing along the edges of cultivated fields, unmanaged yards and disturbed sites and waste areas (CRD rev. Jan. 2019). The species is typically hardy to USDA hardiness zones 4-8.

Identification

Over-all Growth Pattern: Poison hemlock grows up to 2-3 meters tall (6-10′) and 1-2 meters wide (3-6′). It is extensively branched, with distinctive purple blotched, thick, hollow, hairless stems and fern like foliage. Flowers are white umbels.

Leaves:  The pinnately compound leaves are hairless, bright green, fern-like, finely divided, toothed on edges and have a strong musty odor when crushed. The petioles are light green and purple blotched when mature.

Poison hemlock (Conium maculatum) new seedling. Photo credit: Ohio State Weed Lab , The Ohio State University, Bugwood.org
Poison Hemlock (Conium maculatum) seedling. Photo credit: Ohio State Weed Lab , The Ohio State University, Bugwood.org
Poison Hemlock (Conium maculatum) Photo credit: Robert Vidéki, Doronicum Kft., Bugwood.org
Poison Hemlock in flower. Photo credit: Barry Rice, sarracenia.com, Bugwood.org
Poison hemlock (Conium maculatum) patch. Photo credit: Barry Rice, sarracenia.com, Bugwood.org

Stems: Poison hemlock plants are most identifiable by the reddish or purple spots and streaks that appear on these light green, thick, smooth, hairless, hollow, well branched stems. These markings are particularly noticeable toward their base. Giant hogweed also has these reddish or purple markings but their stems have stiff bristles. Cow parsnip may also have a few purple spots but their stems are deeply grooved and covered with fuzzy hairs and are usually unbranched.

Poison Hemlock (Conium maculatum) stem. Photo credit: Ohio State Weed Lab , The Ohio State University, Bugwood.org

Flowers: The flowers are white and consist of five notched petals that are incurved and roughly all the same size. They are arranged in compound umbels that measure approximately 5-8 cm (2 to 3 inches) across. The plant typically flowers in April-July here in Ontario.

Seed:  Poisonous hemlock are prolific seed producers adding to their invasive quality. They produce between 1,500 to 39,000 seeds per plant, about 80% of which are viable (Woodard CA, 2008). The seeds can persist in the soil for up to six years (Csontos, 2008). Plants disperse about 90 percent of their seed in September through December, with the remainder dispersed by late February (Baskin and Baskin 1990). This lengthy dispersal period allows poison hemlock to produce new seedlings continuously for several months (Baskin and Baskin 1990).

Roots: The long, whitish tap roots are forked and measure about 1-2 cm in diameter. They may easily be mistaken for wild parsnips.

Life Cycle of Poisonous Hemlock

Poisonous hemlock is a bi-annual. The seeds germinate in the spring (although they may also germinate in fall). The plant typically forms a rosette of leaves the first year then in it’s second year, sends up a thick, hollow stem that is well branched. White umbel flower heads appear at the ends of branches usually April-July. The plant then sets seed into the winter and dies. Reproduction and spread is mainly by air and water bourn seed. Relocating soil that contains it’s seed is also a means of spreading the plant.

An infestation of Poison Hemlock (Conium maculatum). Photo credit: Steve Dewey, Utah State University, Bugwood.org

Symptoms of Ingesting the Plant

All parts of this plant contain toxic alkaloids that can be fatal even in small amounts. The alkaloids can affect nerve impulse transmission to your muscles, eventually killing you through respiratory failure. The toxic compounds are coniine, g-coniceine, and related piperidine alkaloids (USDA ARS  rev. June 2018). For a list of symptoms for live stock who have ingested the poison visit How It Affects Livestock. For a list of symptoms for people visit Hemlock poisoning symptoms.

Management

An ounce of prevention is worth a pound of cure. To prevent poison hemlock from establishing in your area prevent the spread of it’s seed by staying away from areas infected with the plant. Seeds can be spread by shoes, vehicles, infected hay, animal fur, water, etc. Since the plant reproduces by seed alone once they have germinated in an new area preventing them from setting seed is the main objective in their control. Where protective eye wear, gloves, long sleeved shirts and pants when controlling this plant.

Physical Control:

Ideally dig up the young plants in spring before they get to large and while the digging is easier. Place all plant debris in sealed black garbage bags. If you are unable to remove the entire root, chop or cut the root at least 3cm below the ground. If they are already in seed place a bag over them and tie it at the bottom, before you begin, to help contain the spread of the seed. Check on the site during germination time for new seedling and remove.

Repeated mowing will help to reduce poison hemlock seed production and stress its carbohydrate root reserves. Repeat tilling also helps to control poison hemlock but it is quick to re-germinate in disturbed soil.

Biological Control:

The defoliating Hemlock moth (Agonopterix alstroemeriana) feeds solely upon poison hemlock.

Chemical Control

Glyphosate, a systemic herbicide, is most effective when applied in the spring to the smaller plants, followed up by a summer application for missed plants or those that may have re-grown. Mulching the area 10-14 days after application may help to manage seedling germination. Continue to monitor in subsequent seasons for any new seedlings. For regions permitted dicamba (alone or in combination with 2,4-D) is also effective applied in the same way.

Summary

Poison hemlock (Conium maculatum) is one of the most deadly plants in North America (spotted water hemlock (Cicuta maculata) being the worst). Ingesting even a small amount of any part of this plant is often fatal.

Poison Hemlock Look-Alikes

  • Wild Parsnip (Pastinaca sativa); does not have the purple mottling on their stems.
  • Cow Parsnip (Heracleum maximum); may also have a few purple spots but their stems are deeply grooved and covered with fuzzy hairs and are usually unbranched.
  • Queen-Anne’s Lace (Daucus carota); also known as wild carrot, does not have purple mottling on it’s stems and the stems are hairy. When it’s flowers die back they fold up like a birds nest, poison hemlock just goes to seed and turns brown. If you crush the leaves of Queen -Anne’s Lace it smells like carrot where as poison hemlock has a musty unpleasant odour.
  • Wild Celery (Apium)
  • Water hemlock (Cicuta maculata); does have purple mottling and hairless stems, but unlike poison hemlock, it has a cluster of fleshy taproots at the base.
  • Coriander (Coriandrum sativum), 

Other Poisonous Plants:

  1. Deadly nightshade (Atropa belladonna); is one of the most toxic plants found in the Western Hemisphere. Children have been poisoned by eating as few as two berries, and ingestion of a single leaf of belladonna can be fatal to an adult. all parts of this plant are toxic. The young plants and seeds are especially poisonous, causing nausea, muscle twitches and paralysis; it is often fatal. The root of the plant is generally the most toxic part, however.
  2. Foxgloves (Digitalis purpurea); highly toxic to people and animals.
  3. Azaleas (Rhododendron spp.)
  4. Rhododendrons (Rhododendron spp.)
  5. Lily-of-the-valley (Convallaria majalis);
  6. White Baneberry (Actaea  pachypoda)
  7. Deadly Nightshade (Atropa belladonna);
  8. Lantana (Lantana camara); the berries
  9. Poke Weed (Phytolacca decandra); besides being mildly to highly toxic this plant can cause a rash in sensitive individuals.
  10. Monkshood (Aconitum napellus); This is a highly toxic plant. The neurotoxins, aconitine and mesaconitine can be absorbed through the skin and cause severe respiratory and cardiac problems. So do not pick or handle this plant without gloves, especially by the root.
  11. Tansy (Tanacetum vulgare); even brushing up against it can give some people a rash
  12. Chinese lantern (Physalis alkekengi); Unripe berries, leaves.
  13. Castor bean (Ricinus communis); Ricin is a toxin that is fatal to humans in extremely small doses. Just 1 milligram is a deadly amount if inhaled or ingested. It is especially concentrated in the seed coating.
  14. Yew (Taxus);
  15. Bloodroot (Sanguinaria canadensis); the flower of this plant produces a reddish toxic sap that looks very similar to blood and contains the toxin sanguinarine. If this sap comes in contact with bare skin it can cause burning. If you get a high concentration of sap on your skin it can even eat away the flesh. If ingested, it is deadly to animals.

References:

CABI Invasive Species Compendium, rev. Nov. 2019. Conium maculatum (poison hemlock), Retrieved on Mar. 7, 2021 from: Conium maculatum (poison hemlock) (cabi.org)

CRD rev. Jan. 2019. Poison Hemlock, Retrieved on Mar. 5, 2021 from: poison_hemlock__factsheet_v5.pdf (crd.bc.ca)

King County rev. Jan, 2019,  Poison-hemlock identification and control, Retrieved on March 5, 2021, Poison-hemlock identification and control: Conium maculatum – King County

Grow Forage Cook Ferment & Cocos Creations LLC. 2018, rev. Feb. 2021. Poison Hemlock: How to Identify and Potential Look-alikes. Retrieved on Mar. 8, 2021 from: Poison Hemlock: How to Identify and Potential Look-alikes (growforagecookferment.com)

Government of Canada rev. Nov. 2017. Weed Seed: Conium maculatum (Poison hemlock), Retrieved on Mar. 5, 2021 from: Weed Seed: Conium maculatum (Poison hemlock) – Canadian Food Inspection Agency

Schaefer K. 2007. PSEP Program Manager I. Iowa State University, Weed watch: Wild parsnip and poison hemlock, Retrieved on Mar.7, 2021 from: Weed watch: Wild parsnip and poison hemlock | Integrated Crop Management (iastate.edu)

USDA ARS  rev. June 2018. Retrieved on Mar. 5, 2021 from: Poison Hemlock (Conium maculatum) : USDA ARS

Woodard CA, 2008. Poison hemlock (Conium maculatum L.): biology, implications for pastures and response to herbicides. MS Thesis. Columbia, USA: University of Missouri, Retrieved on Mar. 10, 2021, from: Poison hemlock (conium Maculatum L.) : biology, implications for pastures and response to herbicides (umsystem.edu)

stelprdb5410121.pdf (usda.gov)

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