Conium maculatum - L.
Other Common Names: poison hemlock
Taxonomic Status: Accepted
Related ITIS Name(s): Conium maculatum L. (TSN 29473)
Unique Identifier: ELEMENT_GLOBAL.2.133974
Element Code: PDAPI0Q010
Informal Taxonomy: Plants, Vascular - Flowering Plants - Carrot Family
Kingdom Phylum Class Order Family Genus
Plantae Anthophyta Dicotyledoneae Apiales Apiaceae Conium
Check this box to expand all report sections:
Concept Reference
Concept Reference: Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
Concept Reference Code: B94KAR01HQUS
Name Used in Concept Reference: Conium maculatum
Taxonomic Comments: Although having the common name 'poison hemlock', this plant is a member of the parsley family (Apiaceae or Umbelliferae) and not a conifer, as are the true hemlocks (Tsuga).
Conservation Status

NatureServe Status

Global Status: G5
Global Status Last Reviewed: 09Feb1985
Global Status Last Changed: 29Feb1984
Rounded Global Status: G5 - Secure
Reasons: Widespread; native of Europe, western Asia and North Africa. Brought to the United States as a garden plant. Common in New Zealand. It also occurs in Australia, South America and the British Isles.
Nation: United States
National Status: NNA
Nation: Canada
National Status: NNA (12Oct2016)

U.S. & Canada State/Province Status
United States Alabama (SNA), Arizona (SNA), Arkansas (SNA), California (SNA), Colorado (SNA), Connecticut (SNA), Delaware (SNA), District of Columbia (SNA), Florida (SNA), Georgia (SNR), Idaho (SNA), Illinois (SNA), Indiana (SNA), Iowa (SNA), Kansas (SNA), Kentucky (SNA), Louisiana (SNA), Maine (SNA), Maryland (SNA), Massachusetts (SNR), Michigan (SNA), Minnesota (SNA), Missouri (SNA), Montana (SNA), Nebraska (SNA), Nevada (SNA), New Hampshire (SNA), New Jersey (SNA), New Mexico (SNA), New York (SNA), North Carolina (SNA), North Dakota (SNA), Ohio (SNA), Oklahoma (SNA), Oregon (SNA), Pennsylvania (SNA), Rhode Island (SNA), South Carolina (SNA), South Dakota (SNA), Tennessee (SNA), Texas (SNA), Utah (SNA), Vermont (SNA), Virginia (SNA), Washington (SNA), West Virginia (SNA), Wisconsin (SNA), Wyoming (SNA)
Canada Alberta (SNA), British Columbia (SNA), New Brunswick (SNA), Nova Scotia (SNA), Ontario (SNA), Quebec (SNA), Saskatchewan (SNA)

Other Statuses

NatureServe Global Conservation Status Factors

Range Extent Comments: Conium maculatum is a native of Europe, western Asia and North Africa. It was brought to the United States from Europe as a garden plant. "Poison hemlock is common and spreading in parts of the United States and Canada, particularly on the West Coast; it is common and of some importance in New Zealand, and it also occurs in South America and the British Isles. In Australia, it occurs generally throughout the southern states but has occurred as far north as Queensland" (Parsons 1973).

Other NatureServe Conservation Status Information

Global Range: Conium maculatum is a native of Europe, western Asia and North Africa. It was brought to the United States from Europe as a garden plant. "Poison hemlock is common and spreading in parts of the United States and Canada, particularly on the West Coast; it is common and of some importance in New Zealand, and it also occurs in South America and the British Isles. In Australia, it occurs generally throughout the southern states but has occurred as far north as Queensland" (Parsons 1973).

U.S. States and Canadian Provinces
Color legend for Distribution Map
NOTE: The distribution shown may be incomplete, particularly for some rapidly spreading exotic species.

U.S. & Canada State/Province Distribution
United States ALexotic, ARexotic, AZexotic, CAexotic, COexotic, CTexotic, DCexotic, DEexotic, FLexotic, GA, IAexotic, IDexotic, ILexotic, INexotic, KSexotic, KYexotic, LAexotic, MA, MDexotic, MEexotic, MIexotic, MNexotic, MOexotic, MTexotic, NCexotic, NDexotic, NEexotic, NHexotic, NJexotic, NMexotic, NVexotic, NYexotic, OHexotic, OKexotic, ORexotic, PAexotic, RIexotic, SCexotic, SDexotic, TNexotic, TXexotic, UTexotic, VAexotic, VTexotic, WAexotic, WIexotic, WVexotic, WYexotic
Canada ABexotic, BCexotic, NBexotic, NSexotic, ONexotic, QCexotic, SKexotic

Range Map
No map available.

U.S. Distribution by County Help
State County Name (FIPS Code)
NV Carson City (32510), Douglas (32005), Elko (32007), Eureka (32011), Humboldt (32013), Lander (32015), Lincoln (32017), Lyon (32019), Storey (32029), Washoe (32031), White Pine (32033)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
15 Meadow Valley Wash (15010013)+
16 Upper Humboldt (16040101)+, North Fork Humboldt (16040102)+, South Fork Humboldt (16040103)+, Reese (16040107)+, Little Humboldt (16040109)+, Lower Quinn (16040202)+, Truckee (16050102)+, Pyramid-Winnemucca Lakes (16050103)+, Upper Carson (16050201)+, Middle Carson (16050202)+, East Walker (16050301)+, West Walker (16050302)+, Northern Big Smoky Valley (16060004)+, Little Smoky-Newark Valleys (16060006)+, Long-Ruby Valleys (16060007)+, Spring-Steptoe Valleys (16060008)+
17 Alvord Lake (17120009)+
18 Honey-Eagle Lakes (18080003)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
Basic Description: Tall biennial (sometimes perennial in favorable locations) that reproduces from seeds.
Technical Description: The following description of Conium maculatum is adapted primarily from Munz and Keck (1973) and Parsons (1973).

Conium maculatum is a tall biennial (sometimes perennial in favorable locations) that reproduces from seeds. All parts of poison hemlock are poisonous. The long, fleshy, white taproot is sometimes branched. The stout stems have longitudinal lines and purple markings. Stems are 0.5-3 m high, erect, branched, glabrous, hollow except at the nodes, and produce an offensive odor when damaged. Leaves are fern-like, opposite, glabrous, with a strong mouse-urine smell when crushed. Whole upper leaves are sessile. The lower leaf blades are 15-30 cm long and are petioled.

The flowers are small with 5 white petals, numerous in compound flat-topped umbels at the ends of stems, produced from April to early July. Bracts of the involucre are many, lanceolate, and inconspicuous. Bractlets of the involucel are many and shorter than the spreading pedicels. Rays are many, 1.5-4.5 cm long, and spreading-ascending. The sepals are obsolete. Stylopodium are depressed-conic, and styles are short and reflexed.

The seeds are 2-2.5 cm long, grey or brown, short-lived (probably not more than 6 years), broadly ovoid, flattened laterally, glabrous, obtuse, and undulate with 5 prominent ribs when dry. Oil-tubes are obscure and irregular.

The light green seed leaves of Conium maculatum are three to five times as long as they are broad. The first true leaves are smooth, blotchy green and bisected or deeply cut two or three times. All leaves have prominent veins on the undersides.

Reproduction Comments: Life Cycle: In California, "poison hemlock reproduces only from seed, both as a biennial and winter annual, and occasionally as a short-lived perennial" (Goeden and Ricker 1982).

"Seeds germinate in autumn and plants develop rapidly throughout the winter and spring. Some produce flowering stems in the first spring and die in the summer. Others remain in the vegetative stage without producing flowering stems until the second spring, thus becoming a biennial. Plants are more likely to be biennial in very moist situations. After producing seeds, the plants die in the summer ... The spread of hemlock is by seeds which can adhere to farm machinery, vehicles, agricultural produce, mud and clothing as well as being carried by water and to a limited extent wind" (Parsons 1973).

"Hemlock is capable of rapid establishment after autumn rains, particularly on disturbed sites or where little vegetation exists at the start of the autumn growing season. Once it is firmly established under such conditions, hemlock can preclude most other vegetation and established pastures" (Parsons 1973).

Habitat Comments: Conium maculatum "commonly occurs in sizable stands of dense, rank growth along roadsides, field margins, ditchbanks and in low-lying waste areas. It also invades native plant communities in riparian woodlands and open flood plains of rivers and streams in southern California" (Goeden and Ricker 1982) and other regions in the state. It is common on shady or moist ground below 5000 feet, especially in cismontane California.
Economic Attributes
Economic Comments: Conium maculatum "has found limited use in the seed trade as a fast growing, background foliage plant sold as 'winter fern,' but the attractive, lacy leaves also have a repugnant 'mousey' odor (Robbins et al. 1970). Hendrick (1972) noted that it formerly was eaten (!) as a potherb by peasants in Russia. An extract of the highly toxic, immature fruit was prescribed, sometimes with unfortunate results, as a sedative in earlier times in North America, and native Indians used the green fruit to poison their arrows" (James 1973, Goeden and Ricker 1982).
Management Summary
Stewardship Overview: Conium maculatum is a highly toxic weed found in waste places throughout much of the world. A biennial, it reproduces only from seed. Some poison hemlock seeds germinate in the fall, producing flowers until the second spring. Poison hemlock can be easily controlled with the herbicide 2,4-D. No effective biological control techniques are known, but mechanical removal (hand pulling, grubbing, or mowing) is effective if done prior to flowering.


Dr. Dick Goedon, Dept. of Entomology, University of California, Riverside, California.

Dr. Lincoln Constance, Dept. of Botany, University of California, Berkeley, CA 94720.

Charles E. Turner, USDA-ARS, Plant Protection Research, Western Research Center, 800 Buchanan Street, Albany, CA 94710. (415) 559-5975.

Jim McHenry, Agricultural Extension Office, University of California, Davis, CA 95616.

Species Impacts: Conium maculatum can be a tenacious weed particularly in moist habitats and along streams. Poison hemlock may act as a pioneer species quickly colonizing disturbed sites and displacing natives during early successional seres. The presence of C. maculatum degrades habitat quality and could indicate a management problem on an ecological preserve.

Conium maculatum is poisonous to both humans and livestock. It was probably used to poison Socrates. "Poisoning of humans has occurred after the ingestion of seeds, leaves and roots and even as a result of blowing through the hollow stems when used as whistles or pea-shooters. The seeds, however, are the most toxic part of the plant. Extracts of hemlock have been used as arrow poisons by North American Indians, and it was used medicinally for many years in treating tumors, ulcers and gout" (Parsons 1973).

Toxic Constituents: Conium maculatum "contains at least five distinct yet closely related alkaloids: coniine, N-methyl coniine, conhydrine, lambda-coniceine, and pseudoconhydrine. Of these, lambda-coniceine predominates in the plant during its vegetative growth, while coniine and N-methyl coniine increase and become predominant in the fruits with maturity. Coniine, synthesized by Ladenburg in 1886, was the first alkaloid to be synthesized. Its structure is based on a pyridine nucleus. Coniine is a colorless, volatile, strongly alkaline oil" (Kingsbury 1964).

Toxicity: "It has been shown that the predominant alkaloid in the plant changes with stage of development, and even from hour to hour, that the total amount of alkaloid varies with the stage of growth and part of plant and with geographic area, the plants from southern latitudes being held more poisonous on the average than northern-grown ones. Variability in toxicity of this kind may explain the fact that in experimental feedings of a cow in Texas, Conium was found to produce symptoms but not death at about 2 percent of the animal's weight and did not produce death even at almost 4 percent. Coniine is volatile and is lost slowly from Conium while drying. The hemlock alkaloids are present in least amount in the root. As the plant grows, they accumulate in the stem, leaves, and fruits, being greater in amount in these organs in the order listed and in each reaching a maximum just prior to maturation of the seeds. Concentrations of total alkaloids as high as 1.6 percent have been measured in the green seed" (Kingsbury 1964).

"Cases have been described in cattle with depraved appetite--an indication of a latent metabolic disorder. While animals such as goats and sheep are not very sensitive, pigs react to quite small doses with clear symptoms of poisoning. If pregnant sows survive ingestion of the plant, besides the acute symptoms, limb deformations are observed in the piglets. That coniine has such teratogenic effects has been demonstrated in cattle" (Frohne and Pfander 1983).

Symptoms: Conium alkaloids are structurally related to nicotine and function similarly. "In addition to nicotinic activity, coniine also exhibits curare-like actions, and it paralyzes the striated musculature starting at the legs and rising until finally, while still fully conscious, death takes place as a result of respiratory paralysis" (Frohne and Pfander 1983).

Treatment: Frohne and Pfander (1983) recommend "measures to prevent absorption of the poison (elicit vomiting, gastric lavage, activated charcoal), strychnine in small doses (2 mg/h), and in the case of respiratory arrest, artificial respiration."

Management Requirements: Mechanical or chemical removal of Conium maculatum is relatively easy (see Management section below), but complete eradication may be difficult due to reintroductions and the presence of viable seeds in the soil.

Conium maculatum requires active control measures or it can become dominant on a site, particularly disturbed areas such as roadsides.

Most of the following management information was obtained through personal communication with Jim McHenry, U.C. Davis Agricultural Extension (1985).

Biological Control: There are no known methods of effective biological control of Conium maculatum. The methods of using viral infection and/or phytophagous insects to control this weed need more research and experimentation.

Conium maculatum is often found infected by one or more strains of virus such as ringspot virus, carrot thin leaf virus (CTLV), alfalfa mosaic virus (AMV), and celery mosaic virus (CeMV) (Freitag and Severin 1945, Howell and Mink 1981). However, the stands of poison hemlock seem to survive in spite of viral attack. It is generally common for virus-affected plants to be more often stunted than killed. An apparent example of this phenomenon is the presence of extremely high populations of the ringspot vector, the honeysuckle aphid Rhopalosiphum conii (Dvd.), occurring on Conium.

Conium "is the only plant known to be infected by ringspot in nature. The two symptoms most useful in identifying this ringspot have been the chlorotic areas and the line patterns. They can easily be detected by the mottling of the leaves and by line and ringspot patterns. Under natural conditions the infected plants are not stunted, but often show a downward curling of the leaflets along the midrib" (Freitag and Severin 1945).

Similarly, populations of CTLV, AMV, and CeMV were isolated from Conium maculatum during a survey in southeastern Washington in 1975 and 1979. "CTLV and CeMV were the most prevalent viruses in wild carrot and poison hemlock of southeastern Washington. CTLV and CEMV were each recovered from 9% of the wild carrots and from slightly more than 20% of the poison hemlock, with 7% and 11% infected by both CTLV and CeMV, respectively ... AMV was found only at one location, infecting four poison hemlock plants" (Howell and Mink 1981).

"The incidence of CTLV and CeMV ranged from 0 to more than 80 percent. This variation appeared related to moisture availability. Where water was short through the summer, many of the second-year biennials matured and died before new plants emerged, thus decreasing the probability of virus transmission from the older to the younger weeds ... Poison hemlock, which is abundant in southeastern Washington, is considered a natural reservoir for CeMV in England (Pemberton and Frost 1974) and California (Sutabutra and Campbell 1971)" (Howell and Mink 1981).

However, as in the ringspot virus example cited above, use of these viruses as Conium maculatum controlling agents would depend upon (1) how they affect the viability of poison hemlock and (2) the feasibility of using an agent in the wild that could also adversely affect agricultural crops.

The useability of phytophagous insects to control Conium maculatum needs more experimentation. The phytophagous insect fauna of poison hemlock in southern California is largely comprised of relatively unspecialized, polyphagous, ectophagous, sap- and foliage-feeding species. Thereof, poison hemlock hosted amazingly few insect species or individuals. A clear majority, 16 of the 20 phytophagous insect species found on this weed, were rare.

"Substantial, but unquantified seed destruction by Hyadaphis foeniculi was noted at several locations, but otherwise poison hemlock suffered little insect injury. Most parts of this weed remain essentially free of deleterious insect attack. Apparently, the century since this weed was accidentally introduced into California (Robbins 1940) has provided sufficient opportunity for only very few native phytophagous insects to overcome its toxic defenses and transfer to this colonizing plant species" (Goeden and Ricker 1982).

"The larval 'anise swallowtail' usually feeds on Umbelliferae. This butterfly may be in the process of adopting poison hemlock as an additional food plant in California ... Foreign exploration for natural enemies of poison hemlock in Europe, especially in areas of Mediterranean climate for use in California, is indicated as the next step in ascertaining whether the relative trophic vacuum that this weed represents might be usefully filled by a complex of intentionally introduced, specialized natural enemies" (Goeden and Ricker 1982).

Mechanical control: Hand Pulling or Grubbing: Hand pulling works easiest with wet soils and with small infestations. When grubbing, it is not necessary to remove the entire root system since the plant is not perennial. It is best to pull or grub out the plant prior to flowering (Parsons 1973). "Follow-up cultivation is necessary to deal with any seedlings and if possible a vigorous pasture should be established to compete with any further seedling growth" (Parsons 1973). Poison hemlock remains toxic for several years after being pulled, and it is wise not to leave the dead plants where they might be eaten by wildlife or children.

Mowing: Multiple mowings close to the ground may eventually kill Conium maculatum. "Mowing or slashing of the plants just before flowering is often effective, but sometimes new growth which requires re-treatment is produced from the base" (Parsons 1973).

Chemical control: If extensive areas are covered with Conium maculatum, chemical controls are simpler and less labor intensive.

2,4-D in moderate doses does not kill grasses (except the more susceptible bentgrass). It is most effective against poison hemlock when the ester form is mixed with diesel oil to allow penetration of the leaves and stems. It can be used to hand spot (the most effective technique), or to spray larger areas. The suggested mixture is 1.5 lbs acid equivalent per acre. Mix 2 quarts of diesel oil with 1.5 lbs of 2,4-D ester and add to 100 gallons of water in a spray tank. A 100-gallon tank should cover approximately one acre.

Banvel (active ingredient Dicamba) also works on broad-leaved plants but not as effectively as 2,4-D. The suggested mixture is 1/2 to 3/4 per 100 gallons of water and a surfactant is required.

Management Research Needs: Most Conium maculatum control projects have emphasized chemical methods, and research has been primarily concerned with controlling it on rangelands. No research has been done on removing poison hemlock to restore natural ecosystems. More work needs to be done in mechanical methods and burning. Is burning an effective control measure? When and under what conditions should it be burned? When is the best time to grub out or mow? How many times do you need to mow to keep the plant from reproducing, and how low to the ground must it be mowed? How long are poison hemlock seeds viable in the wild, and what seed reserves are present? What are the chances for reinvasion of the site? Are wildlife such as deer being injured or killed by eating poison hemlock?

A study is presently starting on biological control methods in the eastern United States, but no results are yet reported (Turner pers. comm. 1985).

Population/Occurrence Delineation Not yet assessed
Population/Occurrence Viability
U.S. Invasive Species Impact Rank (I-Rank)
Disclaimer: While I-Rank information is available over NatureServe Explorer, NatureServe is not actively developing or maintaining these data. Species with I-RANKs do not represent a random sample of species exotic in the United States; available assessments may be biased toward those species with higher-than-average impact.

I-Rank: Medium/Low
Rounded I-Rank: Medium
I-Rank Reasons Summary: Conium maculatum is highly competitive, preventing the establishment of native grasses and forbs by shading and competing for space. Mechanical or chemical removal of Conium maculatum is relatively easy but complete eradication may be difficult due to reintroductions and the presence of viable seeds in the soil. Its current range includes every contiguous U.S. state except Mississippi and Florida. Its local range is apparently expanding, particularly in the West. Since this plant is toxic to livestock, landowners are interested in controlling it.
Subrank I - Ecological Impact: Low
Subrank II - Current Distribution/Abundance: High/Medium
Subrank III - Trend in Distribution/Abundance: Medium/Low
Subrank IV - Management Difficulty: Low/Insignificant
I-Rank Review Date: 15Dec2003
Evaluator: Tomaino, A.
Native anywhere in the U.S?
Native Range: Europe, western Asia, and North Africa (Pitcher 1989).

Download "An Invasive Species Assessment Protocol: Evaluating Non-Native Plants for their Impact on Biodiversity". (PDF, 1.03MB)
Provide feedback on the information presented in this assessment

Screening Questions

S-1. Established outside cultivation as a non-native? YES
Comments: Established outside cultivation (Kartesz 1999).

S-2. Present in conservation areas or other native species habitat? Yes
Comments: Invades native plant communities in riparian woodlands and open floodplains of rivers and streams in southern California (Goeden and Ricker 1982 in Pitcher 1989).

Subrank I - Ecological Impact: Low

1. Impact on Ecosystem Processes and System-wide Parameters:Insignificant
Comments: It prevents the establishment of native grasses by shading, but does not cause a system-wide reduction in light availability (Weber 2003) or apparently impact other abiotic processes.

2. Impact on Ecological Community Structure:Low significance
Comments: It is in the herbaceous layer (Weber 2003).

3. Impact on Ecological Community Composition:Moderate significance
Comments: It is highly competitive, preventing the establishment of native grasses and forbs by shading and competing for space (Weber 2003).

4. Impact on Individual Native Plant or Animal Species:Low significance
Comments: 10% of an elk population on Grizzly Island, California, died from ingesting it in 1985 (Parsons and Cuthbertson 1992 in Bossard et al. 2000).

5. Conservation Significance of the Communities and Native Species Threatened:Medium/Low significance
Comments: Invades grassland, forests, riparian habitats, and freshwater wetlands (Weber 2003).

Subrank II. Current Distribution and Abundance: High/Medium

6. Current Range Size in Nation:High significance
Comments: In every contiguous US state except Mississippi and Florida (Kartesz 1999).

7. Proportion of Current Range Where the Species is Negatively Impacting Biodiversity:Unknown

8. Proportion of Nation's Biogeographic Units Invaded:High significance
Comments: Estimate that it is present in at least half because it is present in 46 of 48 continental US states (Kartesz 1999 and TNC 2001).

9. Diversity of Habitats or Ecological Systems Invaded in Nation:Moderate significance
Comments: Invades grassland, forests, riparian habitats, and freshwater wetlands (Weber 2003).

Subrank III. Trend in Distribution and Abundance: Medium/Low

10. Current Trend in Total Range within Nation:Medium/Low significance
Comments: Poison hemlock is common and spreading in parts of the United States and Canada, particularly on the West Coast (Parsons 1973 in Pitcher 1989). Only information found is from 1973.

11. Proportion of Potential Range Currently Occupied:Insignificant
Comments: In every contiguous US state except Mississippi and Florida (Kartesz 1999). Apparently most of its potential range is currently occupied (USDA 1990).

12. Long-distance Dispersal Potential within Nation:Medium/Low significance
Comments: Reproduces only by seed, which is dispersed by water, mud, wind, animal, humans, and machinery (Pitcher 1986 in Bossard et al. 2000). Spread by wind to a limited extent (Parsons 1973 in Pitcher 1989).

13. Local Range Expansion or Change in Abundance:High/Low significance
Comments: It invades disturbed sites, so assume local range expanding since, disturbed sites are expanding.

14. Inherent Ability to Invade Conservation Areas and Other Native Species Habitats:Medium/Low significance
Comments: Commonly occurs along roadsides, field margins, ditchbanks, and low-lying waste areas; it also invades native plant communities (Pitcher 1989). It does best in disturbed areas where soil is moist with some shade but it isalso able to form stands in dry, open areas (Parsons 1992 in Bossard et al. 2000).

15. Similar Habitats Invaded Elsewhere:Medium/Low significance
Comments: Has spread throughout the US, Canada, Australia, New Zealand, and South America (Parsons 1992 in Bossard et al. 2000). It was brought to the US sometime in the 1800's as a garden plant (Goeden and Ricker 1982 in Bossard et al. 2000).

16. Reproductive Characteristics:Moderate significance
Comments: A single plant may produce up to 38,000 seeds (Weber 2003). Seed can remain viable for up to three years (Bossard et al. 2000). It has no means of vegetative reproduction (Bossard et al. 2000). No allelopathic effects are known (Serpa 1989 in Bossard et al. 2000).

Subrank IV. General Management Difficulty: Low/Insignificant

17. General Management Difficulty:Low significance
Comments: Mechanical or chemical removal of Conium maculatum is relatively easty but complete eradication may be difficult due to reintroductions and the presence of viable seeds in the soil (Pitcher 1989).

18. Minimum Time Commitment:Low significance
Comments: Because seed may germinate up to three years after dispersal, a third year of mowing may be necessary (Baskin and Baskin 1993 in Bossard et al. 2000).

19. Impacts of Management on Native Species:Medium significance/Insignificant
Comments: Assume that there are some impacts. The primary control methods (mechanical and chemical) would likely have some collateral affects. Hand pulling is best for wet soils and small infestations (Pitcher 1989).

20. Accessibility of Invaded Areas:Low significance/Insignificant
Comments: Invades grassland, forests, riparian habitats, and freshwater wetlands (Weber 2003). Since this plant is toxic to livestock (Weber 2003), private landowners would probably welcome control efforts.
NatureServe Conservation Status Factors Edition Date: 31May1989
NatureServe Conservation Status Factors Author: Don Pitcher [86-01-14]
Management Information Edition Date: 31May1989
Management Information Edition Author: DON PITCHER [86-01-14]
Element Ecology & Life History Edition Date: 31May1989
Element Ecology & Life History Author(s): DON PITCHER [86-01-14]

Botanical data developed by NatureServe and its network of natural heritage programs (see Local Programs), The North Carolina Botanical Garden, and other contributors and cooperators (see Sources).

  • Bossard, C.C., J.M. Randall, and M. Hoshovsky. (eds.) 2000. Invasive Plants of California's Wildlands. University of California Press, Berkeley, CA.

  • Fairbairn, J. W. 1970. The alkaloids of hemlock. P.361-368 in V.H. Heyward (ed.), Biology and chemistry of the Umbelliferae.

  • Fairbairn, J. W. and A.A.E.R. Ali. 1968. The alkaloids of hemlock (CONIUM MACULATUM L.). III. The presence of bound forms in the plant. Phytochemistry 7:1593-1597.

  • Fairbairn, J. W. and S.B. Challen. 1959. The alkaloids of hemlock (CONIUM MACULATUM L.). Distribution in relation to the development of the fruit. Biochemistry Journal 72 (4): 556-61.

  • Fairbairn, J.W. 1968. The alkaloids of hemlock (Conium maculatum L.), IV: Isotopic studies of the bound forms of alkaloids in the plant. Phytochemistry 7:1599-1603.

  • Freitag, J. H. and H. P. Severin. 1945. Poison-hemlock-ringspot virus and its transmission by aphids to celery. Hilgardia 16(8) :389-410.

  • Frohne, D. and H. J. Pfander. 1983. A colour atlas of poisonous plants. Wolfe Science, London. 291 pp.

  • Goeden, R. D. and D. W. Ricker. 1982. Poison hemlock, Conium Maculatum, in southern California - an alien weed attacked by few insects. Annals of Entomological Society of America 75: 173-176.

  • Hendrick, U. P., ed. 1972. Sturtevant's edible plants of the world. Dover Pub., New York. 188 pp.

  • Howell, W. E. and G. I. Mink. 1981. Viruses isolated from wild carrot and poison hemlock. Plant Diseases 65:277-279.

  • James, L. F. and A. E. Johnson. 1976. Some major plant toxicities of the western U.S. Journal of range management 29: 356-363.

  • James, Lynn F. 1984. Poisonous plants: The hemlocks. Rangelands. 6(3):128-129.

  • James, W. R. 1973. Know your poisonous plants. Naturegraph Publishers, Healdsburg, Ca. 99pp.

  • Jepson, W. L. 1951. Manual of the flowering plants of California. University of California Press, Berkeley.

  • Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.

  • Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T., and C.A. Meacham. Synthesis of the North American Flora, Version 1.0. North Carolina Botanical Garden, Chapel Hill, N.C.

  • Kingsbury, J. M. 1964. Poisonous plants of the U.S. and Canada. Prentice-Hall, Inc., Englewood Cliffs, NJ. 626 pp.

  • Leete, E. and N. Adityachaudhury. 1967. Biosynthesis of the hemlock alkaloids. II. The conversion of -coniceine to coniine and -conhydrine. Phytochemistry 6:219-223.

  • Muenscher, W. C. 1975. Poisonous plants of the U.S. Collier Books, New York. 277pp.

  • Munz, P.A., and D.D. Keck. 1973. A California Flora and Supplement. University of California Press, Berkeley, CA. 1905 pp.

  • Parsons, W. T. 1973. Noxious weeds of Victoria. Inkata Press, Ltd., Melbourne, Australia. 300 pp.

  • Pemberton, A. W. and R. R. Frost. 1974. Celery mosaic virus in England. Plant Pathology 23: 20-24.

  • Pitcher, D. 1989. Element Stewardship Abstract for Conium maculatum. The Nature Conservancy. California Field Office. San Francisco, California.

  • Robbins, W. W. 1940. Alien plants growing without cultivation in California. California Agricultural Experiment Station Bulletin 637:1-128.

  • Robbins, W. W., M. K. Bellue, and W. S. Ball. 1970. Weeds of California. State of California, Department of Agriculture.

  • Robbins, W.W., M.K. Bellue, and W.S. Ball. 1970. Weeds of California. State of California, Department of Agriculture. 547 pp.

  • Roberts, M. F. 1971. The formation of -coniceine from 5-ketooctanal by a transaminase of CONIUM MACULATUM. Phytochemistry 10:3057-3060.

  • Roberts, M. F. 1974. An S-adrenasyl-1-methionin; conciine methyltransferase from CONIUM MACULATUM. Phytochemistry 13: 1847-1851.

  • Roberts, M. F. and Richard T. Brown. 1981. A new alkaloid from South African CONIUM species. Phytochemistry 20:447-449.

  • Roberts, M.F. 1975. Y-coniceine reductase in Conium maculatum. Phytochemistry 14: 2393-2397.

  • Simpson, B. H. 1975. Plant toxicities in sheep on a property with unusual weed control problems. Proceedings 28th New Zealand Weed and Pest Control Conference, pp. 92-94.

  • Stephens, H.A. 1980. Poisonous Plants of the Central U.S. The Regents Press of Kansas: Lawrence, Kansas. 165 pp.

  • Sutubutra, T. and R. N. Campbell. 1971. Strains of celery mosaic virus from parsley and poison hemlock in california. Plant Disease Reporter 55: 328-332.

  • The Nature Conservancy. 2001. Map: TNC Ecoregions of the United States. Modification of Bailey Ecoregions. Online . Accessed May 2003.

  • Tyler, H. A. 1975. The swallowtail butterflies of North America. Naturegraph Publishing, Healdsburg, CA. 192 pp.

  • USDA Agricultural Research Service. 1990. USDA Plants Hardiness Zone Map. Misc. Publ. Number 1475.

  • USDA. 1980. Plants poisonous to livestock in the western States. USDA Agricultural Information Bulletin No. 415. 90pp.

  • Vorobyov, N. E. 1960. On some biological properties of biennial weeds of the Danube stepp. Ukrainsky Botonichny Zhurnal 17(5) : 43-49.

  • Weber, E. 2003. Invasive plant species of the world: a reference guide to environmental weeds. CABI Publishing, Cambridge, Massachusetts. 548 pp.

Use Guidelines & Citation

Use Guidelines and Citation

The Small Print: Trademark, Copyright, Citation Guidelines, Restrictions on Use, and Information Disclaimer.

Note: All species and ecological community data presented in NatureServe Explorer at were updated to be current with NatureServe's central databases as of November 2016.
Note: This report was printed on

Trademark Notice: "NatureServe", NatureServe Explorer, The NatureServe logo, and all other names of NatureServe programs referenced herein are trademarks of NatureServe. Any other product or company names mentioned herein are the trademarks of their respective owners.

Copyright Notice: Copyright © 2017 NatureServe, 4600 N. Fairfax Dr., 7th Floor, Arlington Virginia 22203, U.S.A. All Rights Reserved. Each document delivered from this server or web site may contain other proprietary notices and copyright information relating to that document. The following citation should be used in any published materials which reference the web site.

Citation for data on website including State Distribution, Watershed, and Reptile Range maps:
NatureServe. 2017. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. Available (Accessed:

Citation for Bird Range Maps of North America:
Ridgely, R.S., T.F. Allnutt, T. Brooks, D.K. McNicol, D.W. Mehlman, B.E. Young, and J.R. Zook. 2003. Digital Distribution Maps of the Birds of the Western Hemisphere, version 1.0. NatureServe, Arlington, Virginia, USA.

Acknowledgement Statement for Bird Range Maps of North America:
"Data provided by NatureServe in collaboration with Robert Ridgely, James Zook, The Nature Conservancy - Migratory Bird Program, Conservation International - CABS, World Wildlife Fund - US, and Environment Canada - WILDSPACE."

Citation for Mammal Range Maps of North America:
Patterson, B.D., G. Ceballos, W. Sechrest, M.F. Tognelli, T. Brooks, L. Luna, P. Ortega, I. Salazar, and B.E. Young. 2003. Digital Distribution Maps of the Mammals of the Western Hemisphere, version 1.0. NatureServe, Arlington, Virginia, USA.

Acknowledgement Statement for Mammal Range Maps of North America:
"Data provided by NatureServe in collaboration with Bruce Patterson, Wes Sechrest, Marcelo Tognelli, Gerardo Ceballos, The Nature Conservancy-Migratory Bird Program, Conservation International-CABS, World Wildlife Fund-US, and Environment Canada-WILDSPACE."

Citation for Amphibian Range Maps of the Western Hemisphere:
IUCN, Conservation International, and NatureServe. 2004. Global Amphibian Assessment. IUCN, Conservation International, and NatureServe, Washington, DC and Arlington, Virginia, USA.

Acknowledgement Statement for Amphibian Range Maps of the Western Hemisphere:
"Data developed as part of the Global Amphibian Assessment and provided by IUCN-World Conservation Union, Conservation International and NatureServe."

NOTE: Full metadata for the Bird Range Maps of North America is available at:

Full metadata for the Mammal Range Maps of North America is available at:

Restrictions on Use: Permission to use, copy and distribute documents delivered from this server is hereby granted under the following conditions:
  1. The above copyright notice must appear in all copies;
  2. Any use of the documents available from this server must be for informational purposes only and in no instance for commercial purposes;
  3. Some data may be downloaded to files and altered in format for analytical purposes, however the data should still be referenced using the citation above;
  4. No graphics available from this server can be used, copied or distributed separate from the accompanying text. Any rights not expressly granted herein are reserved by NatureServe. Nothing contained herein shall be construed as conferring by implication, estoppel, or otherwise any license or right under any trademark of NatureServe. No trademark owned by NatureServe may be used in advertising or promotion pertaining to the distribution of documents delivered from this server without specific advance permission from NatureServe. Except as expressly provided above, nothing contained herein shall be construed as conferring any license or right under any NatureServe copyright.
Information Warranty Disclaimer: All documents and related graphics provided by this server and any other documents which are referenced by or linked to this server are provided "as is" without warranty as to the currentness, completeness, or accuracy of any specific data. NatureServe hereby disclaims all warranties and conditions with regard to any documents provided by this server or any other documents which are referenced by or linked to this server, including but not limited to all implied warranties and conditions of merchantibility, fitness for a particular purpose, and non-infringement. NatureServe makes no representations about the suitability of the information delivered from this server or any other documents that are referenced to or linked to this server. In no event shall NatureServe be liable for any special, indirect, incidental, consequential damages, or for damages of any kind arising out of or in connection with the use or performance of information contained in any documents provided by this server or in any other documents which are referenced by or linked to this server, under any theory of liability used. NatureServe may update or make changes to the documents provided by this server at any time without notice; however, NatureServe makes no commitment to update the information contained herein. Since the data in the central databases are continually being updated, it is advisable to refresh data retrieved at least once a year after its receipt. The data provided is for planning, assessment, and informational purposes. Site specific projects or activities should be reviewed for potential environmental impacts with appropriate regulatory agencies. If ground-disturbing activities are proposed on a site, the appropriate state natural heritage program(s) or conservation data center can be contacted for a site-specific review of the project area (see Visit Local Programs).

Feedback Request: NatureServe encourages users to let us know of any errors or significant omissions that you find in the data through (see Contact Us). Your comments will be very valuable in improving the overall quality of our databases for the benefit of all users.