Poison Hemlock

Poison hemlock ( Conium maculatum L.) is historically considered to be one of the most toxic plants. The famous Greek philosopher Socrates is even said to have died of hemlock poisoning. It is a biennial weed that is prevalent in pastures, hayfields, damp waste areas, ditches and streambanks, and rights-of-way. All parts of the plant contain certain alkaloids, which are the toxic components, and for poison hemlock, the two principal ones are coniine and ɣ-coniceine. Seed and dry matter contain high levels of coniine, while fresh plant material contains ɣ-coniceine. The alkaloid content of the plant can vary between ecotypes, growth stage of the plant and the environmental conditions. Leaves are more toxic in springtime, whereas the fruits are toxic during fall.

Among livestock, cows are most susceptible to hemlock poisoning. It has been documented that 5.3 grams of plant material per kilogram of an animal's body weight can cause mortality. This means that roughly 8.5 pounds of plant material in fresh forage could kill a 1,600-pound animal. Common clinical signs of toxicity in animals include nervousness, trembling, staggering, and frequent urination and defecation, which can occur within 30 minutes to two hours after ingestion. Swine and sheep are less sensitive than cows to poison hemlock. In humans, accidental ingestion is the most common form of poisoning. Contact with the plant should be avoided due to the ability of plant sap to cause rashes or blisters.

Poison Hemlock Identification

Poison hemlock belongs to the family, Apiaceae (carrot family). Members of this family are characterized by prominent umbel-shaped flower clusters (formerly called the Umbelliferae family). Poison hemlock is characterized by smooth hollow stems between nodes with distinct purple spots or striations that appear as larger blotches in younger plants. Leaves are pinnately compound, glossy and are arranged alternately on the stem. It has an umbel-shaped bloom about 12 inches in diameter and white in color. Water hemlock, a closely related poisonous weed, can be distinguished from poison hemlock by the absence of purple spots on the stem. Poison hemlock has a taproot system, while water hemlock has branched tuberous roots. Water hemlock is also highly poisonous.

Being a biennial plant, poison hemlock takes two years to complete its life cycle and reproduces solely through seed. Fresh seeds can germinate under moist soil conditions during late summer to early fall or in spring, when day length reaches about 14 hours and diurnal temperatures fluctuate between 85 F and 60 F. It stays relatively inconspicuous in the rosette stage during fall and winter months and begins to grow rapidly during early spring, but it remains in the rosette stage during the first year’s growth. During the second year, it comes to bloom by late spring and set seeds by late summer or early fall. A single poison hemlock plant can produce 35,000 to 40,000 seeds.

Controls for Poison Hemlock

Given its poisonous nature, poison hemlock needs be proactively managed in pastures and hayfields. As this plant is capable of reproducing through seed produced in large numbers by each individual plant, control measures are most effective when executed early on. Given the biennial growth habit of poison hemlock, controls are effective during the rosette stage.

Mechanical control includes grubbing or hand-weeding, which is practical when the weeds are present in small numbers. Plant removal is facilitated by high levels of moisture in the soil; however, entire roots must be removed to prevent regrowth. Wear long gloves and a long-sleeved shirt while handing the plant. Tilling or cultivation is not recommended as the entire root system may not be removed and it creates a risk of inhaling toxic vapors. Mature blooms left behind on the soil also can produce viable seeds.

Chemical control is practical to manage larger populations of poison hemlock. Selective herbicides that have a label in cool-season pastures and hayfields include metsulfuron-methyl (Escort, Cimarron, generic Metsulfuron), florpyroxifen + aminopyralid (Duracor), 2,4-D + dicamba (WeedMaster), triclopyr (Remedy Ultra), and 2,4-D + triclopyr (Crossbow). Among these products, Duracor herbicide is newer in the market and found to be very effective to control this weed; however, it comes with restrictions due the extended residual nature of aminopyralid, one of its active ingredients. Primary restriction is to confine hay, manure or other treated plant materials to the farm for a period of 18 months after herbicide application. Homeowners or gardeners may apply glyphosate as a directed spray or using a hand-held weed wiper to obtain selective control of this weed. If using a weed wiper, higher concentrations (10% to 20% depending on growth stage) may be required to obtain good control. Herbicides are most effective when applied during the vegetative (rosette) stage before the plants bolt; fall or spring treatments are effective. As long as temperatures remain above 60 F for a few hours after treatment, it should be effective.

Biological control involving strains of viruses that infect members of the carrot family and phytophagous insects remain only theoretical at this point. Hemlock seeds can remain viable in the soil for up to six years, therefore repeated control may be necessary to tame heavily infested fields. Reseeding treated areas with desirable plants can reduce the chances of reinfestation by hemlock or other invasive weeds.

Author:  Rakesh Chandran, WVU Extension Weed Science Specialist 
Last Reviewed: July 2022

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