Giant Hogweed

Giant hogweed (Heracleum mantegazzianum) is arguably the most dangerous invasive plant in New York State, as a a significant threat to both human and environmental health. Exposure to the sap can cause serious burns for humans and livestock, and giant hogweed also has the capacity to invade natural and disturbed habitats. It prefers moist soils and partial shade, making riparian (stream bank) areas especially vulnerable to infestations. These plants crowd out native plants, which can contribute to streambank erosion. This herbaceous plant has been found throughout the state. 

Giant hogweed is aggressively managed by the NYS Department of Environmental Conservation (NYSDEC) due to the severity of its impacts. Due to this intense management and monitoring, giant hogweed populations in New York are extensively mapped. If you think you have found giant hogweed on your property, report the infestation to the NYSDEC. Please see NYSDEC’s page on giant hogweed for more information on the Giant Hogweed Control Program and where to report sightings.

When exposed to skin, giant hogweed sap causes phytophotodermatitis, a chemical burn from increased sensitivity to sunlight. The compound causing it in the sap, furanocoumarin, is produced within all the aboveground plant parts including seedlings, leaves, stems, flowers, and fruits. Furanocoumarin makes the skin extremely sensitive to ultraviolet light. If the affected area is exposed to sunlight, the incresaed sensitivity can cause severe burns, blistering, painful sores, and purplish/blackish scars. Symptoms begin between one to three days after exposure, including redness, itching, burn-like lesions, and blistering. These reactions subside within a few days, but impacted areas can remain sensitive to light for several years, resulting in the reemergence of burns, lesions, and blisters. Hyperpigmentation, or darkening of the skin, can cause dark blackish or purplish scars. If sap gets in the eyes, it may cause temporary to permanent blindness.

If exposed to giant hogweed sap, immediately wash the area with water and soap, and protect the area from sunlight. If irritation begins to appear, medical advice should be sought immediately; in some cases, hospitalization may be required. Make sure to cover exposed areas with sunscreen and clothing when going outside, to prevent re-injury. 

Because giant hogweed grows in a wide range of habitats, it’s a good idea to know how to identify it. Mature plants are very distinctive, but seedlings may be easier to overlook. There have been reports of people walking in bare feet being exposed to sap from seedlings. Giant hogweed’s large size when mature makes it attractive to children, who have been known to pick it and use the stems as swords or telescopes. If giant hogweed has been found in your area, educate those around you, especially children, on identification and avoidance of the plant. Although  giant hogweed is palatable to cattle, sheep, and pigs, there are reports of farmers being exposed to sap by touching animals with sap on their bodies. If you find a new population of giant hogweed, please contact NYSDEC’s Giant Hogweed Control Program for free removal by professionals.

Seedlings: Giant hogweed emerges in early spring in a rosette pattern from seeds or tuberous root stalks. Young seedlings are toothed and slightly lobed. As the plants mature, leaves become more deeply lobed. Seedlings generally have a more glossy-looking texture compared to some of giant hogweed’s look-alike species.

Giant hogweed seedling. Image by Leslie J. Mehrhoff, University of Connecticut via Bugwood.org.

Leaves: Giant hogweed leaves vary significantly in shape and size as the plant matures. Early leaves are deeply lobed into three sections. As the plant matures, leaves become deeply lobed and incised (note this difference in the pictures to the right between leaf on young plant vs leaf on mature plant). Leaves on giant hogweed are alternately arranged on the stem. Mature leaves can grow up to 5 feet (1.5 m) wide. Upper stem leaves are narrower and more triangular, but remain deeply cut and lobed.

Giant hogweed leaf. Image by Donna R. Ellis, University of Connecticut via Bugwood.org

Mature plant: Giant hogweed is a biannual or short-lived perennial, forming a rosette in its first year(s) and flowering the following year. In the first year,  the plant only produces leaves and dedicates most of its energy to belowground growth. Once the root system is adequately established, giant hogweed will flower the next growing season. In moist soils where giant hogweed thrives best, the rosette stage only lasts one season and will flower the next like typical biennial plants. However, in less ideal sites, the plant may not flower for as many as 5 growing seasons.

At maturity, giant hogweed reaches 15 feet (4.5 m). The ridged, hollow stems of giant hogweed are 2 – 4 inches (5 – 10 cm) across. The stem is mostly green with purple blotches throughout. At the nodes and bases of petioles, the plant has a cluster of short, white hairs.

Hogweed roots. Photo by Leslie J. Mehrhoff of the University of Connecticut, via bugwood.org.

Flower/seeds: Giant hogweed flowers in late June to early July. Flowers are arranged in a compound umbrella shape made up of thousands of small, white flowers that can measure a whopping 2.5 feet (0.7 m) in diameter. Seeds are flat-oval shaped, winged, and green to dark brown. They spread through wind and water, especially in floodplains, and can last in the soil for up to 10 years. The plant dies after setting seed; the dried stalks remain through the winter. 

Giant hogweed flower. Image by Robert Vidéki, Doronicum Kft. via Bugwood.org

Chemical control

Chemical management for this plant is critical due to the threat it poses to human health. Herbicides are especially important for use in large infestations of giant hogweed. In addition to more traditional foliar applications, herbicides can be injected into stems. Areas must be monitored for at least three years after treatment to ensure giant hogweed is eradicated from the site.

The King County Noxious Weed Control Program and the New York Sea Grant provide specific management recommendations for both chemical and non-chemical control.