Conium maculatum L.

Common Name: Poison-hemlock

Synonyms and Other Names:

Poison-hemlock, deadly hemlock, poison parsley, poison fool’s parsley, cigue maculee, cigue tachetee

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Identification: Poison hemlock  is a tall biennial (sometimes perennial in favorable locations), which typically forms a basal rosette in the first year of growth. It has leaves and white flowerheads resembling those of parsnips, carrots, and water hemlock. All parts of poison hemlock are poisonous. The long, fleshy, white taproot has a main stem with characteristic light red spots and a disagreeable smell; sometimes branched. Water hemlock does not have the same main taproot and stem; instead, water hemlock has a branching, tuberous root stalk similar to that of a dahlia plant.  

Stems are 0.5-3 m high, stout, erect, branched, glabrous, hollow except at the nodes, have longitudinal lines and purple markings 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. All leaves have prominent veins on the undersides.

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 it 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. 


Size: 6-10 feet

Native Range: Europe, western Asia and North Africa.

Map Key
This map only depicts Great Lakes introductions.

Great Lakes Nonindigenous Occurrences: Widespread in the Great Lakes by 1843. 

Table 1. Great Lakes region nonindigenous occurrences, the earliest and latest observations in each state/province, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Conium maculatum are found here.

State/ProvinceFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
IN199020083Little Calumet-Galien; St. Joseph; Upper Maumee
MI1843201833Black-Macatawa; Boardman-Charlevoix; Carp-Pine; Cheboygan; Clinton; Detroit; Great Lakes Region; Huron; Kalamazoo; Keweenaw Peninsula; Lake Huron; Lake Michigan; Lake Superior; Lone Lake-Ocqueoc; Lower Grand; Manistee; Maple; Muskegon; Northeastern Lake Michigan; Northwestern Lake Huron; Pere Marquette-White; Pine; Raisin; Saginaw; Southcentral Lake Superior; Southeastern Lake Michigan; Southwestern Lake Huron-Lake Huron; St. Clair; St. Clair-Detroit; St. Joseph; Thornapple; Tittabawassee; Upper Grand
NY189420083Northeastern Lake Ontario; Oneida; Seneca
OH200820086Black-Rocky; Blanchard; Lake Erie; Sandusky; Southern Lake Erie; Western Lake Erie
PA200820081Lake Erie
WI200820082Manitowoc-Sheboygan; Southwestern Lake Michigan

Table last updated 12/10/2023

† Populations may not be currently present.

Ecology: Poison hemlock is common on shady or moist ground below altitudes of 5000 ft and prefers high-nitrogen soils (Vetter 2004). It commonly occurs in sizable stands of dense, rank growth along roadsides, hiking trails, field margins, ditches, and in low-lying waste areas. It has also been reported to invade native plant communities in riparian woodlands and open flood plains (Goeden and Ricker 1982).

It reproduces only from seed, both as a biennial and winter annual, and occasionally as a short-lived monocarpic perennial (Baskin and Baskin 1990). Plants are more likely to be biennial in very moist environments. Seed production may range from around 1,500 to 39,000 seeds per plant, about 80% of which are viable (Woodard 2008). Seeds also lack dormancy restriction, and may be capable of germinating without a dormancy period (Woodard 2008). Flowering typically occurs from April to early July, peaking in some places in May (Baskin and Baskin 1990). Seeds ripen by mid to late summer, after which the plant dies.

Conium maculatum is spread by seed dispersal; seeds can adhere to farm machinery, vehicles, agricultural produce, mud, and clothing, as well as be carried by water and wind (to a limited extent) (Parsons 1973). In the U.S., most seeds are dispersed from mid-September to December (Baskin and Baskin 1990). 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 prevent the growth of most other vegetation (Parsons 1973).

Conium maculatum produces piperidine alkaloids and is considered one of the most poisonous plants worldwide. Plants grown in the northern hemisphere typically contain less piperidine alkaloids than those grown in the south hemisphere (Pitcher 2004). Two alkaloids, γ-coniceine and coniine, are particularly abundant and account for much of its toxicity (López et al. 1999). Only six other species (Aloe spp.) are known to produce piperidine alkaloids; thus, C. maculatum often possesses a novel attribute and potential advantage during invasions (Castells and Berenbaum 2006, Castells and Berenbaum 2008).

The relative and total concentrations of alkaloids as well as their distribution may vary with the developmental stage of the plant and its environmental conditions. Increased herbivory (e.g., by Agonopterix alstroemeriana—see Management) may also increase alkaloid production. For instance, Castells et al. (2005) found that populations from Washington and New York tended to allocate 2-4 times more N to alkaloid production than Illinois populations, which suffered from much lower levels of herbivory.

Means of Introduction: Deliberate introduction as a garden plant (USEPA 2008)

Status: Established across most invaded regions. Conium maculatum is in an early phase of invasion in northeastern Illinois, where populations have only been detected in disturbed roadside habitats (Granberg et al. 2009).

Great Lakes Impacts:
Summary of species impacts derived from literature review. Click on an icon to find out more...


Conium maculatum has a moderate environmental impact in the Great Lakes.
Recent research indicates that C. maculatum is more tolerant of heavy metal contaminants relative to native species, which may explain its ability to colonize disturbed habitats and displace natives during early successional stages. However, Granberg et al. (2009) also found it capable of inhabiting soils of diverse characteristics, suggesting that it is capable of spreading into natural areas.

Poison hemlock is a highly toxic weed found in disturbed areas throughout much of the world. Most vertebrates suffer from the toxic effects of this species, while very few invertebrates appear to inhabit or eat C. maculatum, even after being established in the U.S. for two centuries (Castells and Berenbaum 2008). The effects of C. maculatum on livestock are well-known, but corresponding effects on wildlife are less commonly studied. However, poisoning has been documented in elk, rabbits, rats, and some birds (Forsyth and Frank 1993, López et al. 1999, Vetter 2004).
Conium maculatum is highly competitive and often grows taller than native species, shading and competing for space and nutrients with grasses and forbs (Pitcher 2004). It can be particularly competitive in soils with high nitrogen concentrations, where it rapidly utilizes nitrogen and outgrows other vegetation early in the growing season (Mamolos and Veresoglou 2000). It may have adverse effects in moist habitats and along streams. The presence of C. maculatum degrades habitat quality and could indicate a management problem on ecological preserves (Pitcher 1989). While there are no specific cases of Conium maculatum outcompeting native species in the Great Lakes region, given its pervasive competitive nature, it is probable that this species also affects such natives.

Conium maculatum has a moderate socio-economic impact in the Great Lakes.
All plant parts are poisonous to humans and livestock; however, the seeds are the most toxic part of the plant (López et al. 1999). Poisoning of humans has occurred following ingestion of seeds, leaves, and roots, and as a result of blowing through the plant’s hollow stems (e.g., when used as whistles or pea-shooters). The conium alkaloids found in C. maculatum are volatile and can cause toxic reactions when inhaled. Symptoms can include temporary skin reactions (hyperpigmenation, blisters, or burning sensation); decreased muscle control; gastro-intestinal symptoms; nervous system symptoms; and death from respiratory failure, if exposure is large/prolonged (Centre for Aquatic Plant Management 2004, Mitich 1998, Vetter 2004). Human deaths have occurred from harvesting and consuming the roots as wild carrots or parsnips, but the frequency of cases in the Great Lakes region is unknown. (See Other Resources for more information on symptoms and possible treatment options.)

Animals of agricultural importance are also affected by the toxicity of C. maculatum, including cows, horses, goats, sheep, swine, and poultry (turkeys, chicken, and quails) (Centre for Aquatic Plant Management 2004, Frank and Reed 1990, López et al. 1999). Pathways for ingestion include live plants in pastures, dried plant material in hay, or inclusion of seeds in grain (López et al. 1999, Panter et al. 1992). Ingestion of plant matter results in acute toxicity characterized by increased salivation, tremors, ataxia, depression, and respiratory distress, possibly leading to respiratory failure with high or prolonged doses (López et al. 1999, Vetter 2004). Cows and pigs may also experience temporary blindness after ingestion (Panter et al. 1992). Acute symptoms often onset quickly, within a few hours for some species and in as little as 10 to 60 minutes for others (López et al. 1999). Animals that suffer from chronic toxicity during critical stages of pregnancy often give birth to young with mild to severe skeletal malformations, including cleft palates, arthrogryposis, scoliosis, and palatoschisis (Keeler and Balls 1978, López et al. 1999, Panter et al. 1992, Vetter 2004). If the initial poisoning is not lethal, livestock animals can recover if future ingestion of C. maculatum is avoided (Frank and Reed 1987, López et al. 1999). Cattle, pigs, goat, and elk should be kept away because they exhibit a preference to continue eating C. maculatum even after the initial exposure (López et al. 1999, Panter and Keeler 1989). The extent of such impact in the Great Lakes region is unknown.

Conium maculatum can outcompete desirable forage species (OLA and MAFF 2002). In addition to its status as a serious pasture weed in the U.S. and other countries, it is also known to infest cereal and vegetable crop fields, as well as orchards (Mitich 1998).

Alkaloids may be excreted through the milk of poisoned cattle, which can pose a threat to nursing animals or be a food safety concern if the milk is intended for human consumption (Panter and James 1990, Vetter 2004).
Conium maculatum is capable of hosting several disease-causing agents (e.g., Xylella fastidiosa, celery mosaic virus, carrot thin lead virus, and alfalfa mosaic virus) that could spread to surrounding plants from agricultural fields or vineyards (Howell and Mink 1981, Woodard 2008).

There is little or no evidence to support that Conium maculatum has significant beneficial effects in the Great Lakes.
Historically, extracts of hemlock have been used as arrow poisons by North American Indians, and medicinally in treating tumors, ulcers, and gout (Parsons 1973). Hemlock products were imported for the U.S. drug industry before World War I (Mitich 1998). However, its medicinal importance is ultimately limited by the narrow distinction between therapeutic and toxic levels of administration (Vetter 2004). Coniine hydrobromide, derived from C. maculatum, is used as an antispasmodic (Penn Veterinary Medicine 2012). Ultra-diluted natural Conium remedies used in India were tested to see if this genus had inhibitory effects on breast cancer. These remedies caused cell cycle delay/arrest and apoptosis of the two breast cancer cell lines tested (Frenkel et al. 2010). Mixtures of alkaloid containing water-alcohol extracts from C. maculatum and salicylic acid inhibit symptoms of inflammation (exudation, pain, fever) to the same extent as conventional non-steroidal anti-inflammatory treatments (Nesterova et al. 2009). Nesterova et al. (2009) believe that these substances could be an alternative treatment for pain caused by various inflammatory conditions. Conmaculatin, a piperidine alkaloid found in C. maculatum, exhibited strong antinociceptive (reduction of pain sensitivity) activity in mice; however, doses over 20 mg/kg were lethal (Radulovic et al. 2012). This compound also does not seem to affect locomotor skills (Arihan et al. 2009).
Coniine serves as an effective insecticide against aphids and blowflies (Mohammed 1999). Conium maculatum also contains the alkaloid gamma-coniceine, which displays antifeedant properties against Deroceras reticulatum (Muller), a field slug (Birkett et al. 2004). This species is native to Western Europe, but has been introduced to Michigan, Ohio, and Ontario (White-McLean 2011).

Management: Regulations (pertaining to the Great Lakes region)
Conium maculatum is a well-established invasive species in Ohio and has been designated as a prohibited noxious weed (ODNA 2000, USDA NRCS 2012). The possession, transportation, transfer, or introduction of poison hemlock is restricted in all Wisconsin counties except: Crawford, Dane, Grant, Green, Iowa, Lafayette, Richland, Rock, and Sauk (WIDNR 2010). The Robert W. Freckman Herbarium of the University of Wisconsin lists an eradication notice for C. maculatum (Herbarium 2012). However, as of 2007 it was not illegal to sell poison hemlock in Wisconsin (Annen 2007). C. maculatum is listed as a noxius weed under Ontario's weed control act.

Regionally, the Great Lakes Indian Fish & Wildlife Commission (GLIFWC) classify this species as capable of causing moderate to severe ecological impacts and/or having limited effective control options available (Falck and Garske 2003).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Physical or chemical removal of C. maculatum individuals is relatively easy, but complete eradication may be difficult due a viable seed bank and reintroductions. Management efforts may need to be continued for several years to be effective (J. McHenry pers. comm. in Pitcher 1985).

The poison hemlock defoliating moth (Agonopterix alstroemeriana) was accidentally introduced to the U.S., but it is now being investigated as a potential biocontrol agent because of its monophagous (feeding on a single food source) association with C. maculatum (Castells and Berenbaum 2006). Hemlock moth larvae feed on the young stem tissue, flowers, and seeds (Forest Health Staff 2006). High densities of A. alstroemeriana have been effective drivers of plant mortality in C. maculatum stands in the western U.S., where several hundred larvae have been reported from a single plant. However, as a chemical defense, alkaloid production appears to increase with A. alstromeriana herbivory, potentially driving surviving populations to higher levels of toxicity over time (Castells et al. 2005). Furthermore, A. alstroemeriana was found to be targeted by a predatory wasp (Euodynerus foraminatus) in Illinois, suggesting that the effectiveness of biocontrol may be lessened in the Midwest and other locations where E. foraminatus exerts top-down pressure on A. alstroemeriana (Castells and Berenbaum 2008). Although A. alstroemeriana is widespread in the U.S., larvae may still be difficult to obtain for biocontrol purposes (Castells and Berenbaum 2006).

Trichoplusia ni, the cabbage looper, is a generalist lepidopteran that is found throughout the US and occasionally feeds on C. maculatum. Overall growth of T. ni is not stunted, but larvae raised on diets enriched with the piperidines found in C. maculatum develop slower. A prolonged larval stage makes T. ni more vulnerable to predators and could reduce overall biocontrol capabilities (Castells and Berenbaum 2008).

Papulio poluxenes, black swallowtail butterfly, will lay eggs on C. maculatum, but a study conducted in central New York found low larvae survivorship (Feeny et al. 1985).

Conium maculatum is capable of being infected by multiple viruses, including ring spot virus, carrot thin leaf thin virus (CTLV), alfalfa mosaic virus (AMV), and celery mosaic virus (CeMV) (Howell and Mink 1981). However, viral infections appear to stunt growth rather than cause mortality, diminishing their potential for biocontrol (Howell and Mick 1981, Pitcher 2004). Another disadvantage to using these types of biological control agents is the potential for them to escape in neighboring habitats, especially agricultural fields (J. McHenry pers. comm. in Pitcher 1985).

Care should be taken in handling this toxic plant; it is recommended that gloves are worn (Pitcher 2004). If any body part comes into contact with any part of the plant, be sure to wash it thoroughly (OLA and MAFF 2002). Hand pulling or digging out the taproot are effective method of control for small populations, especially when the soil is moist (WIDNR 2008). Mowing close to the ground is another option of mechanical control if the blade is close to the ground (WIDNR 2008). A dust mask should be worn for protection to avoid inhaling toxins while mowing (King County 2011). In both cases, efforts are most effective if completed before the plants flower and multiple follow-up efforts should be taken to prevent regrowth or new growth (Parsons 1973, Pitcher 2004, WIDNR 2008, Woodard 2008). Poison hemlock remains for several years after death and should be removed where there is a risk of consumption by livestock, wildlife, or children (Pitcher 2004).

Effective control of large infestations may require chemical agents (WIDNR 2008, King County 2011). Chemical control of rosettes (before flowering) is a common form of management for C. maculatum because it is such a prolific seed producer (Forest Health Staff 2006, Woodard 2008). Application of herbicides early in spring when sprouts are just emerging may also result in effective control, but if C. maculatum has a large presence in the seed bank, multiple applications may be needed (Forest Health Staff 2006).

Several herbicides—including chlorsulfuron, hexazinone, imazapic, glyphosate, metribuzin, metsulfuron, picloram, triclopyr, terbacil, imazapic plus glyphosate, and metsulfuron plus 2,4-D plus dicamba—were found to be the most effective chemical agents (Forest Health Staff 2006, Jeffery and Robinson 1990, King Country 2011, OLA and MAFF 2002, Pitcher 1989, Woodward 2008). The surrounding plant community should be surveyed prior to selecting a herbicide. The effectiveness of all herbicides declines over time, so multiple applications are recommended from spring to fall (Woodard 2008).

Extra care should be taken when using herbicides near desired wild vegetation or agricultural crops. Foliar herbicides applied with a wick will minimize damage to other nearby plants if C. maculatum is growing amongst favorable vegetation (Jeffery and Robinson 1990, OLA and MAFF 2002). Glyphosate and metsulfuron is not recommended for use near croplands (Monsanto 2007, WIDNR 2008). In a study conducted by Jeffery and Robinson (1990), hexazinon, metribuzin, and terbacil controlled poison hemlock with damaging alfalfa when applied while the alfalfa was still dormant. For more in-depth instructions on the use of 2,4-D, glyphosate, and/or metsulfuron, refer to the Pacific Northwest Weed Management Handbook (Prather et al. 2011).

See also:

Midwest Invasive Plant Network - Invasive Plant Control Database

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label directions.

References (click for full reference list)

Author: Cao, L., J. Larson, L. Berent, and A. Fusaro

Contributing Agencies:

Revision Date: 7/28/2022

Citation for this information:
Cao, L., J. Larson, L. Berent, and A. Fusaro, 2023, Conium maculatum L.: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI,, Revision Date: 7/28/2022, Access Date: 12/10/2023

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.