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Poison Hemlock Published 1/13 In this Guideline:
Identification Biology Impact Management About Pest Notes Publication Glossary

Mature poison hemlock.

Distribution of poison hemlock in California in 2012.

For more detail, see the California Invasive Plant Council (Cal-IPC) Cal WeedMapper program, then choose Poison Hemlock from the Common Name menu.

Seed leaves and first true leaves of poison hemlock.

Mature leaf of poison hemlock.

Purple-speckled stem of poison hemlock.

Poison hemlock, Conium maculatum, is a member of the plant family Apiaceae, which contains a few important vegetable crops such as carrots, celery, and parsnip, and herbs such as parsley, cilantro, chervil, fennel, anise, dill, and caraway. It is a tall, invasive, highly poisonous weed that is sometimes mistaken for one of its crop relatives.

Poison hemlock was introduced from Europe as an ornamental plant, probably during the 1800s. It is now widely distributed in the western United States and is commonly found at lower elevations in regularly disturbed areas such as roadsides, ditch and stream banks, creek beds, and fence lines, as well as on the edges of cultivated fields. It can also invade native plant communities in riparian woodlands, floodplains of natural aquatic systems, and grazing areas, particularly pastures and meadows. Although it tends to be more competitive where moisture is abundant, it can also survive in dry sites. A map of the distribution of poison hemlock in California can be found on the Cal-IPC Cal WeedMapper Web site.

IDENTIFICATION

Poison hemlock's growth form changes during its development, which usually spans two years. Seedlings are distinctive. The very first leaves to emerge (seed leaves, or cotyledons) are simple, tapered at the base, elliptical, rounded at the tip, and have a prominently veined undersurface. The first true leaves are smooth, pale green, and triangular, with many deeply lobed leaflets arranged along both sides of a main stalk.

During the first year, growth is usually limited to a large rosette of dark glossy-green leaves that are at least 2 feet long, sheathed at the base, and divided several times along the main stalk of the leaf. The foliage of poison hemlock can resemble wild carrot, but poison hemlock lacks hairs on its leaves and stems.

During the spring of its second year, the plant develops branching erect stems that bear alternately arranged leaves. The stout, ridged stems are hollow (except at the nodes), typically grow to 6 feet tall (but can grow to as much as 10 feet tall in fertile soils), and are distinctively mottled with purple spots. Green stems and leaves lack hairs and exude an unpleasant odor when crushed. The fleshy white taproot is long and sometimes branched.

Plants bloom from spring until summer in the second year of growth and have small white flowers clustered in multiple flat to slightly convex shapes on the end of stalks. The plant typically dies after it sets seed, leaving stems that can persist long into winter.

BIOLOGY

Poison hemlock is a herbaceous plant that reproduces solely by seeds that separate from the plant when mature. Despite prolific seed production, the plant doesn't have a well-developed mechanism for long-distance seed dispersal. While water, birds, or rodents may spread some seeds, most simply drop close to the parent plant, resulting in a clumped distribution pattern in the field. Seeds are dispersed over a considerable time period, beginning in July and ending in late February.

The extended period during which poison hemlock disperses its seed contributes to its long-term survival in a particular area. About 85% of the seeds mature on the plant by mid-July, prior to dispersal. Once dispersed, these nondormant seeds can germinate almost immediately if moisture and temperature conditions are favorable. The remaining 15% of the seeds are dispersed in a state of dormancy. In addition, many nondormant seeds that remain attached to the flowers during winter can become dormant. These dormant seeds require high summer temperatures, low winter temperatures, or both before they can germinate. Once seeds break dormancy, they can germinate from late summer to early spring, as long as temperatures remain cool and the soil remains moist. Like many other weed species, poison hemlock doesn't require light to germinate.

The long dispersal period and the transition of nondormant seeds to a dormant state ensure seed germination won't be confined to a single month or season. However, poison hemlock seeds remain viable for only two to three years, unlike the long-lived seeds of most weed species.

IMPACT

In grazing areas, poison hemlock can crowd out more desirable forage species, and its toxicity causes serious livestock losses when animals feed on fresh forage, harvested silage, or, to a lesser degree, contaminated hay. Silage does little to reduce the plant's toxicity. Poison hemlock can also be found in grain fields, where it can contaminate harvested seed and invade perennial crops. In alfalfa, it poses a significant problem only in the first cutting. Subsequent regrowth of alfalfa can suppress regrowth of poison hemlock. While poison hemlock can be a major weed of roadsides and some crops, it isn't a common garden and landscape invader.

Poison hemlock is toxic to both humans and livestock, affecting the central nervous and reproductive systems. Cases of human poisoning are comparatively rare and are generally associated with children using the hollow stems as flutes or adults mistakenly confusing poison hemlock with an edible plant such as parsley, parsnip, or anise. Sensitive people may experience contact dermatitis when handling this plant.

Eight known alkaloids contribute to poison hemlock's toxicity. Environmental conditions such as soil moisture, soil type, temperature, and the season of growth can alter the plant's alkaloid composition and concentration, making it difficult to predict the degree of toxicity of a given plant or contaminated product.

Animals tend to avoid poison hemlock when other forage is available. They typically feed on the plant only when forage options are limited or when poison hemlock has contaminated green chop, silage, or hay. All classes of livestock and wildlife are susceptible to poison hemlock from ingestion, including cattle, horses, pigs, goats, sheep, elk, and turkeys. Of the domesticated animals, cattle, goats, and horses are the most sensitive. A lethal dose in horses and cattle is as low as 0.25 to 0.5% (fresh plant weight) of the animal's body weight. Sheep and pigs are somewhat less susceptible.

Symptoms of poisoning include nervousness, trembling, knuckling at the fetlock joints, ataxia, dilation of the pupils, a weak and slow heartbeat, coma, and eventually death from respiratory paralysis. These symptoms can occur within 30 to 40 minutes in horses and 1 1/2 to 2 hours in cattle and sheep.

Central nervous system toxicity in livestock usually occurs in spring when poison hemlock is among the first green plants to emerge. This is also when concentrations of some of the most potent alkaloids are at their highest. In fall, regrowth or newly germinated poison hemlock may be the last green forage available. In the western United States, this time coincides with the critical period of gestation in many animals, and ingestion can cause fetal deformity (i.e., crooked calf disease) in pregnant cattle, pigs, or goats. Winter poisonings are also common when cattle are fed harvested hay.

Some of poison hemlock's alkaloid compounds have the ability to pass into milk when animals feed on sublethal amounts of this plant, which can adversely alter the flavor and safety of milk used for human consumption.

MANAGEMENT

Most management strategies are designed to reduce the incidence of poisoning in livestock. When poison hemlock infestations are present, injury can be minimized by preventing grazing in areas where the plant is the only available forage or by removing pregnant livestock from infested areas at the most susceptible period of the animal's gestation.

It is important to prevent a small-scale infestation of poison hemlock from becoming a more significant problem. This can be accomplished by periodically inspecting the area for newly established plants. Once identified, remove individual plants by hand pulling, hoeing, or spot application of an herbicide. Wear gloves to minimize direct contact with the toxic sap. It is essential to prevent isolated plants or a small cluster of plants from producing seed. Don't burn plants or plant debris, because burning may release toxins into the air. Use certified weed-free hay in order to prevent poisoning livestock.

Mechanical Control

Hand removal is recommended for small infestations. When pulling the plants, the entire taproot should be removed to prevent regrowth. However, care must be taken with manual control to minimize soil disturbance that can encourage further germination of seeds at infested sites. Solid carpets of hemlock seedlings aren't uncommon following soil disturbance. Plowing or repeated cultivation of newly germinated plants will prevent poison hemlock establishment. In areas where cultivation isn't practical or possible, repeated mowing once the plants have bolted but before they have flowered can reduce further seed production. Routine mowing reduces poison hemlock's competitive ability, depletes its energy reserves in the taproot, and prevents seed production. Close mowing has the additional advantage of reducing the amount of toxic leaf material available for livestock grazing.

Biological Control

The European palearctic moth Agonopterix alstroemeriana is the main herbivore feeding on poison hemlock. This moth was probably introduced by accident, and poison hemlock is considered its only known host plant. The larvae live in conspicuous leaf rolls and feed on foliage, buds, and flowers in spring and early summer. The adult moths emerge in summer and can be found from June until March of the following year. Despite its widespread occurrence, the moth hasn't been shown to be an effective control agent for most infestations of poison hemlock.

Chemical Control

Although several herbicides are available for controlling poison hemlock, herbicides should be used only on seedlings or small rosettes and not on fully mature plants. In addition, it is best to handpull individual plants or small infestations, which are typical of gardens and landscapes. Herbicides such as 2,4-D, triclopyr, and glyphosate, available to both residential users and small noncommercial operations, may be a more effective option with larger infestations. In California, herbicides such as chlorsulfuron, hexazinone, and imazapyr are available to licensed applicators.

The broadleaf selective herbicide 2,4-D is most effective when applied soon after plants reach the rosette stage. Both the amine and ester formulations of 2,4-D are effective. Using 2,4-D may make poison hemlock more attractive to livestock but doesn't change its toxicity, so some caution must be exercised if using 2,4-D in grazed pastureland or in silage production.

Like 2,4-D, triclopyr is also a broadleaf selective herbicide that is most effective on smaller plants. It doesn't kill most grasses. Apply it during the seedling to rosette stage of growth.

Glyphosate is nonselective, so exercise caution to minimize injury or mortality of desirable plants that might help suppress new poison hemlock seedlings. Apply to actively growing plants before they begin to bolt. Cooler temperatures can reduce the effectiveness of glyphosate.

Chlorsulfuron is somewhat selective against broadleaf weeds and not only gives excellent preemergent control but can also provide some postemergent foliar activity on poison hemlock. Desirable grasses should be well established before application. Apply chlorsulfuron to actively growing poison hemlock plants in the rosette stage. Other preemergent photosynthetic inhibitors, such as hexazinone, give excellent control of poison hemlock. In alfalfa, herbicides should be applied when the forage crop is dormant.

Treating poison hemlock with herbicides may require repeated applications for a couple of years until the seedbank has been significantly depleted. Once the weed is under control, maintaining desirable forage species with proper pasture management, fertilization, irrigation, and drainage can effectively help prevent reinfestations.

WARNING ON THE USE OF PESTICIDES

REFERENCES

Burroughs, G. E., and R. Pyrl. 2001. Toxic Plants of North America. Ames, Iowa: Iowa State Univ. Press.

DiTomaso, J. M., and E. A. Healy. 2007. Weeds of California and Other Western States. Oakland: Univ. Calif. Agric Nat. Res. Publ. 3488.

PUBLICATION INFORMATION

Pest Notes: Poison Hemlock

UC ANR Publication 74162        

Authors: J. M. DiTomaso, Plant Sciences, UC Davis; J. A. Roncoroni, UC Cooperative Extension, Napa Co.; S. V. Swain, UC Cooperative Extension, Marin Co.; and S. D. Wright, UC Cooperative Extension, Tulare/Kings Co.

Editor: M. Fayard

Technical Editor: M. L. Flint

Produced by University of California Statewide IPM Program

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