United States Department of Agriculture

Field Guide for Managing

Poison Hemlock in the Southwest

Forest 

Service

Southwestern

Region TP-R3-16-16 Revised June 2017
Cover Photos

Top left: Pedro Tenorio-Lezama, Bugwood.org

Top right: Jan Samanek, State Phytosanitary Administration, Bugwood.org 

Bottom: Steve Dewey, Utah State University, Bugwood.org

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1 

Poison hemlock (Conium maculatum L.)

Carrot family (Apiaceae)

Poison hemlock is reported as invasive in Arizona and is 

listed as a noxious weed in New Mexico. This field guide 

serves as the U.S. Forest Service’s recommendations for 

management of poison hemlock in forests, woodlands, and

rangelands associated with its Southwestern Region. The

Southwestern Region covers Arizona and NewMexico,

which together have 11 national forests. The Region also 

administers 4 national grasslands located in northeastern

New Mexico, western Oklahoma, and the Texaspanhandle.

Description

Poison hemlock (synonyms: poison parsley, carrot fern, 

spotted hemlock) is a highly toxic weed introduced from 

Eurasia and northern Africa that can be fatally mistaken

for edible wild parsnip. Leaf features of poison hemlock 

distinguish it from water hemlock and giant hogweed.

Growth Characteristics

• Winter annual or biennial, herbaceous, broadleaf 

plant growing erect 3 to 8 feet tall.

• Fleshy, tuber-like, white taproot; smells like cow 

parsnip and looks like wild carrot. However,

parsnip has palmate leaves and wild carrot has hair 

on its stem and leaves.

• Early growth stage is a large rosette; stem and

flowers develop during the second year.

• Stems are green with purple spots; ribbed, freely

branched, hollow and without hair.

• Leaves are triangular (8 to 16 inches), lacey, fernlike, more finely divided and smaller than giant 

hogweed; leaf veins end at the tips of teeth, not at

notches like water hemlock.

• Small, white flowers have five parts; occurin 

umbrella-like clusters during June toAugust.

• Reproduces solely via seed; seeds are 1/8 inches 

long, barrel shaped, ribbed and look similar to

anise. One plant may produce up to 30,000 seeds 

that remain viable for 3 to 6 years.

• Poison hemlock has a rank, cat urine-like odor that

is especially noticeable in mid-summer or when

leaves are crushed.

Ecology

Impacts/Threats

Poison hemlock is a lethal plant with toxins mostly

concentrated in the seed, lower stem, and roots. Even small

portions accidentally consumed can result in respiratory

paralysis, coma, and death if treatment is not administered

within 3 hours of ingestion. Poison hemlock reduces the 

availability of quality forage for cattle and horses, 

contaminates haying operations, degrades wildlife habitat,

diminishes flora and fauna species diversity, and decreases

land value.

Site/Distribution

This weed prefers moist, shaded habitats subject to

frequent disturbance such as those found along riparian

woodlands, streambanks, and ditch banks. Once 

established, it can migrate to drier upland sites. It may 

invade pastures, rangeland, along the perimeter of 

cultivated fields, and roadsides. Poison hemlock is 

common in the eastern United States and is spreading in 

western States. 

Spread

Most seed germinates near the parent plant, thereby 

increasing stand density. Seed is easily dispersed by water, 

humans, birds, and rodents; it is also spread over long 

distances by adhering to surfaces and undercarriages of 

road vehicles and road maintenance equipment.

Invasive Features

Prolific seed production coupled with high germination

rates and 3-year seed viability make this weed highly 

competitive. Flower stems of poison hemlock persist 

through the winter, thereby allowing a longer period for 

seed dispersal (from September through February). 

Germination requirements are not specific; new plants may 

propagate any month of the year. Also, poison hemlock’s 

long taproot allows access to deeper soil moisture, which 

increases its competitiveness.
2 

Management

Persistence and a long-term commitment is a must for 

poison hemlock control. Due to abundant seed production 

and a high germination rate, early detection and eradication

of small populations before they can expand into larger 

populations is extremely important. The following actions 

should be considered when planning an overall

management approach:

• Healthy plant communities should be maintained to

limit poison hemlock infestations.

• Check hay and straw for presence of plant fragments

or seed before using them in areas without poison 

hemlock; feed certified weed-free hay or pellets to 

horses in backcountry areas.

• Detect, map, and eradicate new populations of poison 

hemlock as early as possible. Keep annual records of 

reported infestations.

• Implement monitoring and a follow-up treatment 

plan for missed plants and seedlings.

• Consider reseeding an area following herbicide 

treatment to increase competition.

Table 1 summarizes some management options for 

controlling poison hemlock under various situations. 

Further details on these management options are explained 

below. Choice of method(s) taken for poison hemlock 

control depends on numerous factors including the current 

land use and site condition; accessibility, terrain, and 

microclimate; density and extent of poison hemlock 

infestations; and non-target flora and fauna present, etc. 

Other considerations include treatment effectiveness, cost, 

and the number of years needed to achieve control. More 

than one control method may be needed for each site.

Physical Control

Since poison hemlock reproduces solely via seed

production, physical methods that destroy the top growth 

before flowering will reduce available seed. Methods that 

repeatedly stress and/or attack the root system are 

especially effective.

Manual Methods

Hoeing, digging, cutting or grubbing are effective

management options on smaller, isolated populations. If a 

weed eater is used, then cut before plants flower and repeat 

as necessary through the growing season. Poison hemlock

causes some people to break out in a rash upon contact

with skin; therefore, protective clothing (long pants, longsleeved shirts, and gloves) should be worn when working 

in areas with this weed. Also, inhalation of plant particles 

shouldbe minimized. Plant material should be bagged or 

piled and burned since it is highly toxic and could pose a 

danger to wildlife, livestock, and children.

Mechanical Methods

If using machinery tomanage poison hemlock, equipment 

should be cleaned after use to prevent the movement of 

seed into un-infested areas.

Mowing – Repeated mowing in localized situations can 

reduce poison hemlock seed production, stress its 

carbohydrate root reserves, and decrease its ability to 

compete with desirable grasses and forbs. A rotary mower

is ideal for larger populations or a hand-held weed eater is 

sufficient for smaller populations. Plants should be clipped 

close to the ground in the spring, after bolting, but before

seed set, and repeated again in late summer. Consecutive (3 

to 5) years of mowing will likely be necessary to address 

seed bank reserves.

Tillage – In cultivated settings, poison hemlock is mainly 

found along the untilled field margins. Repeated tillage can 

be an effective control option, but this may only be feasible 

in certain situations. Reseeding following tillage is highly

recommended since poison hemlock is a pioneer species

that will germinate rapidly from seed bank reserves

following disturbance. Local conditions dictate when 

reseeding should be accomplished. Typically, desired grass 

and forb seed are planted in late summer or early autumn in 

Arizona and New Mexico. Areas with suitable terrain 

should be tilled at 3-week intervals with a plow or disc to a 

depth of at least 4 inches, which should begin in spring 

when poison hemlock is in the rosette or early bolt stage.
3 

Table 1. Management options*

Site Physical Control Cultural Control Biological Control Chemical Control

Roadsides, 

fence lines, or 

non-crop 

areas

Mow close to the ground 

multiple times during 

growth season.

Combine mowing with

an autumn herbicide

application. 

Use seed, mulch, and fill 

materials certified to be weedfree. 

Clean machinery following 

activity in infested areas.

Train road crews to identify

and report infestations. 

Poison hemlock moth

may be used as a classical 

biocontrol agent if 

available; however, its 

impact is unknown. 

For ground application, use ATV, 

truck mounted, or tractor pulled 

spraying equipment. 

Wash under vehicle after 

application to prevent spread. 

Rangelands, 

pastures, or

riparian 

corridors 

Use repeat tillage only in 

areas to be reseeded.

Combine with herbicide 

spraying. 

Prescribed burn only

as part of a combined

method. 

Use seed and forage hay 

certified to be weed-free; use 

pellets for horses in 

backcountry areas.

Avoid driving directly

through infestation. 

Reseed with plants that are

desirable and will compete.

Avoid grazing infested 

pastures during the early 

spring when poison 

hemlock is the primary

green forage. 

Poison hemlock moth 

may be used as a classical 

biocontrol agent if 

available; however, its 

impact is unknown.

For extensive and dense 

infestations, use ground or aerial 

broadcast spraying. 

For sparse infestations, use 

backpack spraying or hollow-stem 

injection.

Wilderness, 

other natural 

areas, and/or

small

infestations 

Hand remove or cut 

before flowers develop; 

removal of the whole root 

is not necessary. Pile or 

remove debris. 

Use seed and forage hay 

certified to be weed-free; use 

pellets for horses in 

backcountry areas.

After passing through 

infested areas, inspect and 

remove any seed from 

animals, clothing, and 

vehicles. 

Same as above. Use backpack or hand-held sprayers 

to spot treat plants.

* Choice of a particular management option must be in compliance with existing regulations for the land resource.

Prescribed Fire

Little research has been conducted with regard to

prescribed burning as a management option for poison

hemlock. Considering the moist environment preferred 

by the plant and the limited dried fuel produced in these 

areas, it is assumed that fire is not likely to produce 

enough heat to destroy hemlock fruit and seeds. 

However, fire can be used as a means for debris 

disposal.

Cultural Control

Prevention, early detection, and plant removal are

critical for preventing poison hemlock establishment.

Land managers, the local public, and road crews should

be educated as to how to identify nonnative noxious

species so they can help report all suspected 

infestations. Seed and materials used for mulch, forage, 

or fill should be certified to be weed-free; pellets may 

be used for horses in backcountry areas. Vehicles, 

humans, and livestock should be discouraged from

traveling through infested areas; and a program to check 

and remove seeds from vehicles and livestock should be

implemented to help stop dispersal. If possible, weed 

screens should be used on irrigation water intakes in

infested areas to preventseed transportation in canals. 

Biological Control

Grazing

Poison hemlock contains eight known alkaloids,

including coniine and coniceine that are extremely toxic 

to humans, livestock, and wildlife. Consuming adequate 

quantities can result in striated muscle paralysis that 

begins in the legs and progresses upward until asphyxia 

occurs. Pigs are the most susceptible to poison hemlock, 
4 

Table 2. Classical biocontrol agent approved for poison hemlock

Species Type of

Agent Site of Attack Impact onHost

Use/Considerations for 

Release

Agonopterix 

alstroemeriana

moth Caterpillars feed on leaves, buds, stem, flowers, and 

immature seeds in the spring and early summer. Adult 

moths emerge in the summer and feed on leaves.

Unknown but may 

be limited.

Little researched.

followed by cattle, then horses, sheep, and goats. Livestock 

should not be grazed on infested pastures during the early 

spring when poison hemlock is one of the primary green 

forage plants. Livestock are more likely to consume poison 

hemlock via contaminated hay.

Classical Biological Control

The poison hemlock moth (Agonopterix alstroemeriana) 

has been approved by USDA for use as a biocontrol agent 

in controlling poison hemlock in the United States (table 

2). The moth was accidently introduced into North 

America from Europe where it feeds solely upon poison 

hemlock. The moth has established naturally in numerous 

western states including California, Utah, Colorado, Idaho, 

Oregon and mesic areas of Washington. The impact of the 

moth on poison hemlock has not been determined.

Organisms (insects, pathogens, etc.) used as biocontrol 

agents in southwestern States should be adaptable to arid 

environments and local conditions. Public, tribal, and 

private land managers may obtain biocontrol agents for 

release directly from local offices of the USDA Animal 

and Plant Health Inspection Service (APHIS) when these 

agents are available. Other sources for biocontrol agents 

include locally developed insectaries or private companies.

A permit must be obtained from APHIS before biocontrol 

agents can be transported across State lines. Regulations 

and permit applications (PPQ 526 permit forms) pertaining 

to interstate shipment of biocontrol agents can be found at

https://www.aphis.usda.gov/aphis/ourfocus/planthealth/ 

import-information/permits/regulated-organism-and-soilpermits/sa_apply/ct_plantpest_howtoapply. Although 

biocontrol agents may be collected and released internally 

in a given State without an APHIS permit, the State’s 

department of agriculture or agricultural extension service 

should be consulted for any regulations relating to 

movement of these agents within the State.

Chemical Control

Herbicides are an effective and economical way to manage

poison hemlock. However, new populations often return

within a few years of spraying from seed that is still

abundant in the soil. Anticipate the need to monitor anduse

follow-up herbicide applications for several years to

deplete the seed bank and attain long-term control.

All herbicides recommended in table 3 will control

emerged poison hemlock when properly applied. Each 

herbicide product will have different requirements and 

restrictions according to the label. Read and understand the 

label prior to any application. Consult the registrant if you 

have questions or need further detail. 

Herbicide Application

Phenoxy herbicides such as 2,4-D (amine or ester 

formulations), 2,4-DB, or MCPA are effective when 

sprayed (all at 1 to 2 lb. a.i./acre) in spring on newly 

emerged seedlings in open areas away from water. Care 

should be taken when using 2,4-D alone or in combination 

with other herbicides, as it has a restricted-use designation 

in New Mexico. Other selective herbicides to consider 

include dicamba (alone or in combination with 2,4-D) or 

aminopyralid in combination with metsulfuron. These

herbicides are most effective when sprayed during the

rosette stage in either spring or fall.

For poison hemlock on streambanks or near the water’s 

edge, the Rodeo® formulation of glyphosate or the Habitat®

formulation of imazapyr should be used as these products 

have approved aquatic labels. An aquatically approved 

surfactant should also be added to the spray mix. Both 

herbicides are nonselective; therefore, precautionary 

measures should be taken if non-target plants (including 

woody species) need to be protected. Glyphosate and 

imazapyr are best used in a spot spray or else in a nonselective spraying situation with no desirable plants
5 

Table 3. Herbicide recommendations

Common

Chemical Name

(active 

ingredient)

Product

Example1

Broadcast 

Treatment 

(rate per acre)

Spot Treatment 

(spray solution)2

Time of

Application Remarks

2,4-D ester or amine 

formulations3

several

manufacturers 

1–2 pounds a.i. 

per acre

3% Spring in seedling 

to early growth 

stage.

2,4-D is selective for many 

broadleaf species but will 

not harm most grasses.

2,4-D is formulated in 

different strengths 

depending on the 

manufacturer. Read and

follow label mixing

directions. 

Aminopyralid + 

metsulfuron methyl

Opensight

Chaparral 

2.5–3.3 ounces 

per acre

NA Spring or fall. A selective granular

herbicide for use on 

noncropland, rights-of-way,

non-irrigation ditch banks, 

natural areas, and grazed

areas in and around these

sites.

Dicamba several

manufacturers 

2–4 quarts 1–3% Same as above. Selective; affects many 

broadleaf species but will 

not harm most grasses.

Dicamba + 2,4-D3 Weedmaster 2–4 pints 0.7% + 0.5% 

NIS4

Seedling to

rosette stage.

Selective with a broad 

spectrum; may affect some 

sensitive pasture grasses

such as bentgrass and 

legumes such as alfalfa. Not 

for use near water.

Metsulfuron methyl Escort 3 ounces per 

acre

1 gram per 

gallon 

Early spring or late 

fall (at bud/ bloom 

or rosette stages).

Selective; not for use on 

irrigation ditches or near

waterways.

Best used during warm, 

moist conditions; activity 

may be delayed during cold,

dry conditions. Use 0.25% 

v/v NIS4

. 

Glyphosate Roundup 

Rodeo 

Roundup: 1.3–

2.7 quarts

Rodeo: 2.25– 

3.75 quarts

Roundup: 

1–1.5% 

Rodeo: 

0.75–1.5% 

At rosette stage. Nonselective; not soil

active. Rodeo is 

registered for aquatic

use.

Glyphosate may be used for

hollow-stem injections. For 

individual plant treatment 

with Roundup, inject 5 ml 

of a 5% solution into 

hollow stem; see label for 

specific details.
6 

Common

Chemical Name

(active 

ingredient)

Product

Example1

Broadcast 

Treatment 

(rate per acre)

Spot Treatment 

(spray solution)2

Time of

Application Remarks

Imazapyr Habitat

Arsenal

Habitat: 

2 pints + adjuvant 

Arsenal: 2–3 

pints +

adjuvant 

0.75–1.5% Pre-emergence to 

rosette stage.

Nonselective; slightly 

persistent; amino acid 

synthesis inhibitor. 

Habitat is registered for 

aquatic use. See label for 

adjuvant options. 

In addition to spray drift, 

non-target plants may be 

killed or injured by 

imazapyr through runoff, 

movement in soil, or root 

exudates from treated 

plants. 

1 Trade names for products are provided for example purposes only, and other products with the same active ingredient(s) may be available. Individual 

product labels should be examined for specific information and appropriate use with poison hemlock.

2 Spray solution is the herbicide/water ratio in a spray mix that may be used for spot treatment with backpack or hand-held sprayers. The amount of product 

applied during an annual growing season must not exceed the maximum application rate per acre as specified by the product label – refer to the product 

label for the site type and application.

3 2,4-D is a restricted use pesticide in New Mexico only. A certified applicator’s license is required for purchase and use.

4 NIS is an abbreviation for nonionic surfactant, an additive commonly recommended by herbicide labels for post-emergent foliar herbicide application.

growing beneath the weeds.

Herbicides may be applied with backpack or hand-held 

sprayers, ATV or UTV sprayers, or conventional boom

sprayers that are pulled or attached to a tractor or truck. 

Any equipment used to spray herbicide should be 

calibrated. For sparse populations, one person or a small 

team can spray poison hemlock using an individual plant 

treatment (IPT) approach. Plants may be spot sprayed 

with an adjustable spray nozzle attached to a hand-held or 

backpack sprayer to wet the foliage and stems without 

dripping. A hollow-stem injection approach may also be 

used, as described on the glyphosate label.

Management Strategies

Small, isolated infestations on otherwise healthy sites 

should be given high priority for treatment. The strategy

adopted for managing large infestations of poison 

hemlock must involve careful planning. Perimeters of 

infestations should be treated first and then worked 

toward the center. Although information is scarce,

integration of techniques such as mowing and herbicide 

should be beneficial since the impacts of combined 

control measures are often cumulative.

Revegetation through broadcast seeding or using a notill drill to increase competitive pressure on poison 

hemlock should always be considered where feasible. 

In areas where reseeding is planned, glyphosate can be 

broadcast sprayed for site preparation. Glyphosate is 

most effective when applied sequentially at about 1 

month intervals during the summer, coupled with a fall 

grass seeding. Make the first application in early 

summer (June or July) and the second about a month 

later, providing that green shoots are present. Sow 

perennial grass seed in late autumn as a dormant 

seeding (i.e., grass seedlings will not emerge until the 

following spring).

Regardless of the approach followed, poison hemlock 

typically cannot be controlled effectively in a single 

year. Complete control will likely require 3 to 6 years 

of repeated treatment; therefore, sufficient resources 

must be allocated for the area where control is 

attempted. After initial treatment, it is especially 
7 

important that sufficient resources also available to 

respray or retreat the treated area as necessary. Treated 

areas should be monitored periodically and measures 

taken to control missed plants and newly emerged 

seedlings. It is also important to monitor the return of 

desirable native plantspecies.

References and Further

Information

Castells, E, M.A. Berhow, S.F. Vaughn, and M.R.

Berenbaum. 2005. Geographic Variation in

Alkaloid Production in Conium Maculatum

Populations Experiencing Differential Herbivory 

byAgonopterix alstroemeriana. J. Chem. Ecol.

31(8):1693-1709.

Drewitz, J. 2000. Poison Hemlock (Conium 

maculatum).In C.C. Bossard et al. (eds.). Invasive 

Plants of California’s Wildlands.Univ. California

Press. Berkley, CA. Available at http://www.calipc.org/ip/management/ipcw/online.php (accessed 

July 2010)

Duncan, K., K. McDaniel and M. Renz. 2005. Chemical

Weed and Brush Control for New Mexico 

Rangelands. Las Cruces: NMSU College of 

Agriculture and Home Economics and

Cooperative Extension Service. Available at

http://aces.nmsu.edu/ pubs/_circulars/CR_597.pdf 

(accessed July 2010) 

Higa de Landoni, J. 1990. Conium maculatum L. IPCS 

INCHEM. Univesidad de Buenos Aires,

Argentina. Available athttp://www.inchem.org/ 

documents/pims/plant/conium.htm (accessed 

August 2010)

Invasive Plant Atlas of the United States. Available at

http://www.invasive.org/weedus/index.html

(accessed July 2010)

Moser, L. and D. Crisp. 2003. Conium maculatum. San 

Francisco Peaks Weed Management Area 

Coconino NF. Available at 

http://sbsc.wr.usgs.gov/research/projects/swepic/fac

tsheets/coma2sf_info.pdf (accessed July 2010)

Pitcher, D. 1989. Poison hemlock (Conium maculatum): 

Element Stewardship Abstract. Available at http:// 

www.invasive.org/weedcd/pdfs/tncweeds/conimac.

pdf (accessed July 2010)

Pokorny, M.L. and R.L. Sheley. 2000. Poison Hemlock.

Montana State University. Available at

http://msuextension.org/ publications/

AgandNaturalResources/MT200013AG.pdf

(accessed July 2010)

Thurston County (WA). 2010. Noxious Weed Factsheet: 

Poison hemlock: Conium maculatum L. Available

at http://www.co.thurston.wa.us/health/ehipm/ 

pdf/ Poison%20hemlock.pdf (accessed August

2010)

Tu, M., C. Hurd, and J.M. Randall. 2001. Weed Control 

Methods Handbook. The Nature Conservancy. 

Available at http://www.citeseerx.ist.psu.edu

(accessed August 2010)

USDA Forest Service. 2006. Weed of the Week: Poison 

Hemlock (Conium maculatum) Available athttp://

www.na.fs.fed.us/fhp/invasive_plants/weeds/ 

poison-hemlock.pdf (accessed July 2010)

USDA NRCS. 2010. The PLANTS Database. Available

at http://plants.usda.gov (accessed August 2010).

Suggested Web Sites

For information on invasive species: 

http://www.invasivespeciesinfo.gov/ 

http://www.invasive.org/weedus/index.html

For information about calibrating spray equipment: 

NMSU Cooperative Extension Service Guide A613 Sprayer Calibration. Available at 

http://aces.nmsu.edu/pubs/_a/A-13.pdf 

Herbicide labels online: http://www.cdms.net/
For more information or

other field guides, contact:

USDA Forest Service 

Southwestern Region

Forest Health

333 Broadway Blvd., SE 

Albuquerque, NM 87102

Or visit the Southwestern Region’s 

website for invasive species:

http://www.fs.usda.gov/goto/r3/invasivespecies

The use of trade or firm names in this publication is for reader 

information only and does not imply endorsement of any 

product or service by the U.S. Department of Agriculture. 

Recommendations made here for 

pesticide use are not obligatory, 

nor do they imply that discussed 

uses have been registered. All 

uses of pesticides must be 

registered by appropriate State 

and/or Federal agencies before 

they can be applied. 

CAUTION: Pesticides can be 

injurious to humans, domestic 

animals, desirable plants, and fish or other wildlife—if they 

are not handled or applied properly. Use all pesticides 

carefully and lawfully. Follow recommended practices for the 

disposal of surplus pesticides and pesticide containers.