What Is Poison Hemlock: Where Does Poison Hemlock Grow And How To Control

What Is Poison Hemlock: Where Does Poison Hemlock Grow And How To Control

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By: Jackie Carroll

Poison hemlock plant is one of those nasty weeds that no one wants in their garden. Every part of this noxious plant is poisonous, and its invasive nature makes it nearly impossible to control without chemicals. Let’s learn more about poison hemlock removal and the characteristics of the plant in this article.

What is Poison Hemlock?

Thanks to the imagination of mystery and gothic novel writers, most of us have heard of poison hemlock. You may have seen it without realizing what it is because of its resemblance to cultivated plants and other weeds.

Poison hemlock (Conium maculatum) is a poisonous invasive weed that has caused many accidental deaths because of its resemblance to carrots, including the wild carrot (Queen Anne’s lace). The poisonous agents in the plant are volatile alkaloids found in every part of the plant. In addition to causing death when ingested, the plant also causes a miserable dermatitis in sensitive people upon contact with skin.

Socrates drank the juice of this notorious plant to commit suicide, and ancient Greeks used it to poison their enemies and political prisoners. North American Natives dipped their arrowheads in hemlock to make sure every hit was fatal.

Where Does Poison Hemlock Grow?

Poison hemlock prefers disturbed areas where forest has been cleared. You may see it growing in livestock pastures, along roadways and railroads, in waste areas, along streambanks, and near fence rows. All parts of the plant are poisonous to livestock and humans, and it only takes a small amount to poison horses and cattle.

Poison hemlock look-alikes include both wild and cultivated carrots and parsnips. You can tell the difference between them because the tips of poison hemlock leaves are pointed while the tips of parsnip and carrot leaves are rounded. Upon close inspection, you may see purple splotches on hemlock stems, but never on carrot or parsnip stems.

Poison Hemlock Removal

You can pull up small plants along with their long taproot if the soil is damp. Kill larger plants by biological or chemical means.

The hemlock moth (Agonopterix alstroemericana) is the only effective biological agent, and it is very expensive. The moth larvae feed on the leaves and defoliate the plant.

Control the weed chemically by spraying young sprouts with an herbicide such as glyphosate. That being said, chemicals should only be used as a last resort. Organic approaches are more environmentally friendly.

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How to Manage Pests

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.


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.


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.


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.


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.


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.


Pest Notes: Poison Hemlock

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.

Technical Editor: M. L. Flint

Produced by University of California Statewide IPM Program

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All contents copyright © 2019 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

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Poison Hemlock Plant Info - Learn About Poison Hemlock Removal And Look Alike Plants - garden

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Poisonous Plant Research: Logan, UT

Poisonous Plants

Poison-hemlock grows throughout the United States. It is very toxic and sheep, cattle, swine, horses, and other domestic animals are poisoned by eating small amounts of green or dried plant. It is also extremely poisonous to humans. Poison-hemlock is sometimes confused with western water hemlock, a more deadly species, because the names are similar. Poison-hemlock is commonly called deadly hemlock, poison parsley, spotted hemlock, European hemlock, and California or Nebraska fern.

Poison-hemlock has white flowers that grow in small erect clusters. Each flower develops into a green, deeply ridged fruit that contains several seed. After maturity, the fruit turns grayish brown. Poison-hemlock starts growing in the early spring. It usually grows for 2 years, but in favorable locations it may be a perennial. Roots of poison-hemlock may easily be mistaken for wild parsnips. Poison-hemlock grows along fence lines, in irrigation ditches, and in other moist, waste places. It may be 2 to 3 meters tall. The hollow stem usually is marked with small purple spots. Leaves are delicate, like parsley, and it has a white taproot. Poison-hemlock is a biennial in the parsnip or wild carrot family.

All parts of poison-hemlock (leaves, stem, fruit, and root) are poisonous. Leaves are especially poisonous in the spring, up to the time the plant flowers. Fresh leaves are unpalatable, so livestock seldom eat hemlock when other feed is available. The toxic compounds are coniine, g-coniceine, and related piperidine alkaloids. People may be poisoned by eating any part of a hemlock plant. Often, poisoning occurs after the victim confuses hemlock root with wild parsnips, hemlock leaves with parsley, or hemlock seed with anise. Whistles made from hollow
stems of poison-hemlock have caused death in children.

Where and When It Grows
Because of its attractive flowers, poison-hemlock was brought to the United States from Europe as a garden plant. It is moving onto rangelands. Poison-hemlock is found at roadsides, on edges of cultivated fields, along creekbeds and irrigation ditches, and in waste areas.

How It Affects Livestock
Poison-hemlock ingestion frequently is fatal. Sheep may be poisoned by eating as little as 100 to 500 gm of green leaves. Cattle that eat 300 to 500 gm may be poisoned. Signs usually appear within an hour after an animal eats the plant. Animals die from respiratory paralysis in 2 to 3 hours. Convulsions, which are common in western water hemlock poisoning, seldom occur with poison-hemlock.

Skeletal deformities or cleft palate may be induced in offspring of cows, sheep, goats, and pigs that eat poison-hemlock during gestation. Susceptible stages of gestation when animals should not be exposed to this plant include 40 to 70 days in cows and 30 to 60 days in sheep, goats, and pigs. Palate and skeletal deformities in calves are indistinguishable from the lupine-induced crooked calf disease.

Signs and Lesions of Poisoning

  • Nervous trembling
  • Stimulation followed by depression Ataxia, especially lower and hind limbs
  • Salivation
  • Lack of coordination
  • Dilation of the pupils
  • Rapid, weak pulse
  • Respiratory paralysis
  • Coma
  • Death
  • Convulsions have been reported
  • Occasionally bloody feces and gastrointestinal irritation
  • Skeletal birth defects occur in calves when cows eat poison-hemlock between 40
    and 70 days gestation

    How to Reduce Losses
    Avoid stressing animals that are not recumbent. For recumbent animals, support respiration and treat with activated charcoal and a saline cathartic. Gastric lavage may be beneficial with atropine therapy to control parasympathetic signs. Animals that recover seldom show aftereffects, although pregnant animals may give birth to deformed offspring.

    Poison-hemlock may be controlled by treating plants before they begin to bud with 2,4-D plus dicamba (1 kg + 0.5 kg ai/Ac). Repeat applications may be needed. Follow all precautions for handling herbicides.

    Watching for Poison Hemlock to Return

    Once I started removing the plants, I was able to see just how invasive this plant is. Everywhere I looked there were small starts of the Poison Hemlock plant. I am not sure that I made a huge dent in the Hemlock population in the garden. I think it will be coming up for a long time. Now that we know what we are looking at, I am sure my family members will be getting rid of it frequently.

    After I was done pulling the Poison Hemlock, Poison Ivy, and assorted other invasive weeds that had taken over the garden, I made sure I cleaned up all the clippings and bagged them for the trash. Normally, in my barnyard and farm we would add the weeds to the compost pile. In the case of Poison Hemlock, composting is not recommended.

    Clean the clippers used with soap and water and wipe down with alcohol wipes after the job is done. Poison Ivy oils can get on everything and be transferred by touching the gloves and tools. I also washed all the clothing and gloves, just to be sure!

    Did I over react to finding and removing poison hemlock? I don’t know. I am an unsure gardener and when I read that a plant is poison to ingest or inhale it makes me uncomfortable. This is how I handled removing Poison Hemlock in the yard of my family member. If you have tackled this task, please share what you did in the comments.

    More about Poison Hemlock from Grow Forage Cook Ferment

    Legal status in King County, Washington

    Poison-hemlock is a Class B Noxious Weed on the Washington State Noxious Weed List that is selected for required control on public lands and public rights-of-way by the King County Noxious Weed Control Board. On private property, control of poison-hemlock is recommended but not required in King County. For more information about noxious weed regulations and definitions, see Noxious weed lists and laws.

    The King County Noxious Weed Control Board encourages all property owners to remove poison-hemlock where possible and to avoid introducing it to new landscapes.

    Identifying Toxic Plants: Conium maculatum, Poison Hemlock

    Sometimes harmless wildflowers have an evil twin lurking.

    Here in the mid-south summer wildflowers are blooming and among them lurks a deadly poison. Spotted hemlock, or poison hemlock is blooming now and it is best that people become familiar with this weed. Children love to pick wildflowers and this is a showy, large plant with lacy, frothy blooms that would definitely attract their attention. Most kids have been educated not to put strange plants in their mouths, however the toxins in this one are so potent that just the sap on their hands can make them very ill. Kids love to pick bouquets and with school turning out many will be wandering afield.

    Conium maculatum is a European native and bears a resemblance to the harmless Queen Anne's Lace (Daucus carota or wild carrot) that is also blooming now. Both sport lacy, white blossoms made up of hundreds of smaller flowers. These are called umbels and the shape bears a resemblance to an umbrella (where the name of our favorite rain-shedding tool originated.) Check out the links above for even more images of these two plants.

    It has a dark history connected with the famous philosopher Socrates, who was ordered to take the poison because his teachings were considered immoral and there was a detailed account of the effects of the toxin as it progressed through his body. Ultimately, it causes respiratory failure and it only takes a little to be fatal. Through history, there are cases of hemlock poisoning that are well-documented, even down to children perishing from making whistles from the hollow stems, so it is best to recognize this deadly plant when you see it and remove it promptly before it sets seed. It has a ferny, fluffy foliage similar to Queen Anne's Lace and its other cousins in the Apiaceae or parsley family and the flowering stalks arise in late spring from the biennial rosettes of the year before.

    The images below will point out the differences so that identification is easy. This will allow adults to remove the toxic plants from their property to prevent the young, uneducated or livestock from stumbling on them. The hemlock generally grows in damp, waste areas and favors, road ditches and creeksides. Queen Anne's Lace prefers open meadows and sunny fields. The hemlock forms a bushy, shrubby plant with thick, red spotted, hollow stems. The stems can often reach a couple of inches in diameter and the plants often tower over 6 feet tall. Queen Anne's lace is identified by solitary, airy stems sporting a single blossom, or at the most two and rarely reach more that 3 feet tall. Once you learn the difference, it is no trouble to tell them apart.

    This plant is so toxic that precautions need to be taken when you remove it as well. It is best done in spring before it blooms. The plants in the image above are along a stretch of local highway known as Sinkhole Hill (yes, that is an actual place that my nearby readers know) Wear a mask, gloves and goggles to prevent accidental contact with the sap and cut the plants to the ground. Dispose of them in garbage bags and never compost it. It may take several years to clear out a thick infestation since the millions of seeds stay viable for a number of years. You can also use a chemical herbicide, however that does not remove the poison from the dead plants, so it is best to cut and bag. Clean your shears thoroughly before removing protective gear and wash your clothes.

    Most plants encountered in the wild are relatively harmless, or at worst cause minor discomforts but this is one that is widespread throughout the world that should be treated with respect and caution. Knowledge is power, so learn what dangers lurk in your neighborhood and educate your children as well.

    Watch the video: The Most Dangerous Plants in the World

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