Giant Hogweed
Heracleum mantegazzianum
Washington State Classification
Class A — Eradication Required Statewide
It forms dense canopies outcompeting native species and increasing soil erosion.
Quick Reference
Identification
Growth Habit
Giant hogweed is a perennial (sometimes biennial) distinguished by its large size, capable of growing to a height of 15-20 feet tall. It has stout dark reddish-purple spotted/splotched stems and leaf stalks. Stalks and stem produce sturdy purplish, pustulate bristles.
Leaves
The compound leaves of giant hogweed may grow as large as five feet wide. Each leaflet is deeply cut/lobed with leaf edges being sharply toothed (incised).
Flowers
Giant hogweed has broad, flat-topped flower clusters (umbels) of many small white flowers. Each flower cluster may grow to a diameter of 2.5 feet.
Fruit & Seeds
The flowers produce large elliptic dry seeds marked with brown swollen resin canals.
Impact
It forms dense canopies outcompeting native species and increasing soil erosion. It is a public safety hazard as it exudes a clear watery sap which sensitizes the skin to ultraviolet radiation, resulting in severe burns to the affected area causing blistering and painful dermatitis. Scars can last for as long as 6 year and sensitivity to sunlight can continue beyond that.
Ecology & Spread
Habitat
Giant hogweed may colonize a wide variety of habitats but is most common along roadsides, other rights-of-way, vacant lots, streams and rivers. Please click here to see a county level distribution map of giant hogweed in Washington.
Spread Mechanisms
Reproduction
Reproduction is by seed and perennating buds formed on the root crown and tuberous root stalk. Seed production is variable, with estimates ranging from 1,500 to 100,000 per plant. Seeds may remain viable in the soil for 7+ years. Abundant seed production, a persistent root stalk, and reproduction from perennating buds are cited as reasons for its capability to colonize rapidly.
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Control Methods
Mechanical
- It’s possible to hand-pull or dig up seedlings and young plants, but it is important to avoid getting any of the sap on your skin. The toxic sap that causes the burning is contained in all portions of the plant. Always wear protective clothing and gloves. Large plants can be cut back, using loppers to avoid contact, and then dug up. Make sure to remove the root crown and as much of the roots as possible to prevent re-sprouting. Be sure to bag the flowerheads and plant parts and put into the regular trash to prevent seed spread. Check with your county noxious weed control board to see if vouchers are available to pay for disposal of this noxious weed. Mowing plants serves only to stimulate budding on the main root stalk, but it might be successful if done consistently and persistently enough to starve the rootstalk and may be effective for large infestations.
Cultural / Prevention
- Careful digging is useful to manage weed populations. However, digging can carry undesirable weed seed to the surface and foster further germination. Cut the stems at the base to remove flower and seed heads prior to seed dispersal.
Biological Control
- There are no classic biological control agents approved for giant hogweed. Cattle and pigs are cited as possible grazers of this plant. Both eat giant hogweed without apparent harm though there could be some risk of dermatitis. Check with your county noxious weed board before attempting this control method as other methods are available for small infestations.