By CELESTINE DUNCAN, Editor
Plants in the carrot (Apiaceae) family share the characteristic of an umbel-shaped flower head. The family includes hundreds of plants, some that are valuable vegetables and herbs, and a few that are masters in chemical warfare. Accurate identification is important for management and avoiding accidental poisoning. This article reviews distribution, identification and management of three invasive, toxic plants in the carrot family: poison hemlock (Conium maculatum), wild parsnip (Pastinaca sativa), and giant hogweed (Heracleum mantegazzianum).
All three plants were introduced to North America from Europe. Poison hemlock and wild parsnip are more widely distributed in the United States and Canada compared to giant hogweed. The plants share similar moist habitats that include stream banks, creeks, roadsides, riparian woodlands, and sub-irrigated meadows and pastures. Wild parsnip is also well adapted to drier, open sites with full sun.
Figure 1: Distribution of poison hemlock (A), wild parsnip (B), and giant hogweed (C) in the United States and Canada (USDA, NRCS Plants Database).
An umbel inflorescence is the key characteristic of these three plants. It consists of a number of short flowering stalks that spread from a common point, similar to an umbrella. Compound umbels also have a second “umbrella” of smaller flowering stems within the inflorescence. Similarities and differences between the three invasive plants are described below.
Figure 2: Identification and toxic properties of poison hemlock, wild parsnip, and giant hogweed.
- Introduced from Europe
- Biennial; sometimes perennial
- Height: up to 7 feet
- Leaves: alternate, triangular (8 to 16 inches), finely divided and smaller than giant hogweed
- Stem: hollow internodes, purple-speckled
- Flower: white; umbel
- Taproot: large, fleshy, resembles parsnip
- Reproduces by seed
- Toxin: Coniine and g-coniceine alkaloids affect the nervous system; causes death in humans and livestock
- Introduced from Europe
- Height: up to 5 feet
- Leaves: alternate, pinnately compound and branched; divided into leaflets with 2 to 5 pairs
- Stem: hollow internodes, smooth with few hairs, deeply grooved
- Flower: yellow; umbel
- Taproot: large, fleshy; up to 3 feet
- Reproduces by seed
- Toxin: Sap has furanocoumarins that cause phytophotodermatitis 1 on skin exposed to sun. Long-term damage to skin is possible
Introduced from Europe
Perennial; sometimes biennial
Height: up to 20 feet
Leaves: deeply incised and 3 to 5 feet wide, with hairy leaf ribs but hairless leaf undersides
Stem: hollow internodes with reddish-purple bumps and stiff white hairs
Flower: white; umbel
Taproot: Branched, up to 2 feet
Reproduces by seed
Toxin: Sap has furanocoumarins that cause phytophotodermatitis on skin exposed to sun. Permanent scarring may occur
Impacts to Human Health and Livestock
Poison hemlock is the most toxic of these three plants and is well known as the poison used to kill Socrates in 399 BC. All parts of hemlock (leaves, stem, fruit, and root) are poisonous to humans and livestock. Poison hemlock is sometimes confused with edible or medicinal herbs in the same family (wild parsnip, parsley, osha, and others) with resulting illness or death in humans. Poisoning in children has been reported when the hollow stems are used for whistles and straws. Although animals don't prefer hemlock, livestock poisoning occurs especially in spring when more desirable forage is lacking. Details regarding poisonous alkaloids in hemlock can be found here.
The sap of wild parsnip and giant hogweed contains toxic compounds called furanocoumarins.2 These compounds can cause serious rashes, burns, or blisters to skin exposed to the sap and then sunlight. Scars on skin from contact with the plants can last from a few months to two or more years. Blindness can occur if sap comes into contact with the eyes. Both plants pose a risk to natural area managers involved with vegetation management, hikers, and others exposed to the plant. Although roots of wild parsnip are reported to be edible, some reports indicate that furanocoumarins are also present in the roots, and consumption is discouraged.
[ 1 ] Phyto is a prefix that means plant. Photo indicates that a process involves light. Dermatitis is inflammation of the skin.
[ 2 ] Furanocoumarins are toxic chemicals that help to protect plants from pests.
Protective clothing and eye protection should always be worn when treating these toxic plants, regardless of the management method selected. Wash or dispose of protective clothing immediately following treatment, especially when physical contact with the plants occurs. Do not burn dry hemlock plants or debris, as burning may release toxins into the air.
Poison hemlock and wild parsnip can be controlled by applying Opensight® specialty herbicide at 2.5 to 3.3 ounces of product per acre (oz/A), with a nonionic surfactant. The lower rate of Opensight will effectively control wild parsnip. Apply to rosettes in spring or fall (Figure 3). Opensight is selective for broadleaf plants and will not cause injury to most grasses. Sites labeled for use include seasonally dry wetlands, non-irrigation ditch banks, rights-of-way, and many other non-crop and natural areas. Opensight can be used up to the water’s edge. Reports suggest that poison hemlock toxicity and palatability for livestock increases following herbicide application. Grazing animals should be kept away from treated plants for three weeks following application, or until the plant dies.
Giant hogweed can be controlled with selective broadleaf herbicides, including Milestone®, Opensight®, or Garlon® 4 Ultra specialty herbicides. Apply Milestone at 7 fluid ounces per acre (oz/A) or Opensight at 3.3 oz/A with a nonionic surfactant. Apply early in the growing season when plants are at the rosette to early bolt growth stage or to rosettes in fall. Apply Garlon at 2.5 to 4 quarts per acre (qt/A) in spring when plants are actively growing.
Application of Accord® XRT II and Rodeo® herbicides will also control the three plants. Both of these herbicides are nonselective and will control both grasses and broadleaf plants. Reestablishing desirable vegetation will likely be necessary following application. Apply Accord XRT II at 1.3 to 2.7 qt/A or Rodeo at 2.25 to 3.75 qt/A at the rosette growth stage in spring. Individual stems can be treated by injecting 5 mL of a 5-percent solution of Accord XRT II directly into the stem about 12 inches above the plant crown.
Individual plants can be controlled by digging or completely severing the root at least six inches below the plant crown. Garlon® 4 Ultra (25% solution) can be applied to the cut surface of large roots that remain in soil, to help ensure regrowth does not occur. Protective clothing—including waterproof gloves, long sleeve shirts and pants, and eye protection—are necessary. Disposable, waterproof coveralls taped at the wrist should be worn to protect skin from giant hogweed or wild parsnip sap.
Mowing may reduce seed production if the plant is cut at the late bud to early flower growth stage. Caution should be used since sap and plant fragments are released during mowing operations.
The poison hemlock moth (Agonopterix alstroemeriana) was first recorded in North America in New York State in 1973. It was accidentally introduced to the Northwest U.S. shortly after, and has spread rapidly from both locations. The insect does not provide effective control of poison hemlock at this time. No biological agents are available in North American for controlling wild parsnip or giant hogweed.
Follow-up monitoring and treatment is necessary to assure that mature plants and seedlings are completely controlled. Seed remain viable in soil for an extended period of time, so sites should be monitored for a minimum of five years. Restoring desirable vegetation to disturbed, infested sites will reduce the opportunity for future reinvasion.
Burch, Pat. Field Scientist, DowDuPont.
DiTomaso, JM and GB Kyser et al. 2013. Weed Control in Natural Areas of the Western United States. Weed Research and Info. Center Univ. of California. 544 pp.
Gucker, Corey L. 2009. Heracleum mantegazzianum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Online: bit.ly/techline-FEIS [2018, July 30].
How to Control Giant Hogweed. Department of Environmental Conservation. New York State. Online: bit.ly/NYDEC-controlhogweed
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Pastinaca sativa. Nancy Eckardt, Global Invasive Species Team, The Nature Conservancy. Online: bit.ly/BUGWOOD-wikiwildparsnip
Poison hemlock. University of California, IPM Program. Online: bit.ly/UCIPM-hemlock
USDA, Forest Service. Field Guide for Managing Poison Hemlock in the Southwest. Online: bit.ly/USDA-IPMhemlocksouthwest
USDA, ARS. Poison hemlock (Conium maculatum). Poisonous Plant Research Lab Poison. Online: bit.ly/USDA-poisonhemlocklogan
USDA, NRCS. 2018. The PLANTS Database (plants.usda.gov, 30 July 2018). National Plant Data Team, Greensboro, NC 27401-4901 USA.
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Milestone and Opensight specialty herbicides are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas, or rotating the treated area to sensitive crops. Consult the label before purchase or use for full details. Always read and follow label directions.
State restrictions on the sale and use of Garlon 4 Ultra specialty herbicide and Accord XRT II herbicide apply. Consult the label before purchase or use for full details. Always read and follow label directions.