Description and Variation

Giant hogweed (Heracleum mantegazzianum) is a member of the parsley or carrot family, Apiaceae (Umbelliferae). As its name indicates it is characterized by its size and may grow to 15 to 20 feet in height. Except for size, it closely resembles cow parsnip, Heracleum lanatum, a plant native to Washington. It is further distinguished by a stout dark reddish-purple stem and spotted leaf stalks. Stalks and stem produce sturdy pustulate bristles. The stem and stalks are hollow, stems vary 2 to 4 inches in diameter. The compound leaves of giant hogweed may expand to five feet in breadth. Each leaflet is deeply incised. Giant hogweed is a perennial with tuberous root stalks which form perennating buds each year. The inflorescence is a broad flat-topped umbel composed of many small white florets. Each inflorescence may attain a diameter of 2-1/2 feet. The florets produce large elliptic dry fruits marked with brown swollen resin canals up to 1 mm in diameter.

Economic Importance

Giant hogweed has been introduced to Europe, the United Kingdom, Canada and the United States as a garden curiosity. Because of its tenacious and invasive nature it soon becomes a pest within the ornamental garden and readily escapes. It has naturalized in many of the places where it was first introduced. Growing along streams in Ontario, on Vancouver Island, and in Great Britain it forms a dense canopy out-competing native riparian species and results in an increase in soil erosion along the stream banks where it occurs. The plant exudes a clear watery sap which sensitizes the skin to ultraviolet radiation. This can result in severe burns to the affected areas resulting in severe blistering and painful dermatitis. These blisters can develop into purplish or blackened scars. Proliferating populations in urban and suburban areas represents an increasing public health hazard.

Giant hogweed is currently on the federal noxious weed list. It is listed as a Class A weed on the Washington State Noxious Weed List. Its placement on these lists were due to its potential menace as a public health hazard. The number and size of populations in the Seattle area, where it has been observed over the past four years, continue to grow annually. Literature reviewed indicates it naturalizes readily once it escapes from ornamental gardens where it is often cultivated as a garden curiosity. Populations throughout Europe, the United Kingdom, Ontario, Vancouver Island, and in New York State are all thought to have gotten their start in this manner. Once established it crowds out other native plant species and increases soil erosion along stream banks. Giant hogweed is a tenacious perennial which is difficult to eradicate. Targeting all known populations for control will prevent the further spread of giant hogweed in Washington.

Geographic Distribution

Giant hogweed is native to the Caucasus mountains and southwestern Asia. It has been introduced to the European continent, the United Kingdom, Ontario, Vancouver Island (Canada) and New York State as a garden ornamental. From these initial sites of introduction it has escaped and naturalized. In Washington it has been reported from Thurston, Kitsap, Mason, King, Island, and Clark counties. Escaped populations in the Seattle area are known from Lake Washington Boulevard, West Seattle and the Wallingford area. Individuals have recently been noted along I-5 in Seattle and Rainier Avenue south near Renton.

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. Because giant hogweed often grows in wet areas, we are considering it to be an invasive freshwater weed.

History

Personal communication between J.K. Morton and C. Leo Hitchcock indicates Hitchcock knew of at least one population in the Seattle area during the early 1950's. This would be the earliest documented record of this species in Washington.

Growth, Development, and Reproduction

Giant hogweed is a perennial which takes several years from germination to produce the first flowering stalk. It is believed to be monocarpic, dying after first flowering and seed set. Individual plants however, may produce additional crowns which continue to flower and set seed. Seed longevity is known to be greater than seven years. Reproduction is through seed and perennating buds formed on the crown and tuberous root stalk. Abundant seed production, a persistent root stalk, and vegetative reproduction from perennating buds are cited as reasons for its capability to colonize rapidly and expand populations.

Response to Herbicides

2,4-D, TBA, MCPA and dicamba will kill above ground parts but are reportedly not particularly effective on persistent rootstalks. Glyphosate is considered the most effective herbicide and should be used cautiously around desirable species since it is nonselective. Application during bud stage and while the plant is actively growing is recommended by New York Cooperative Extension. In Great Britain, Wright, recommends application during the early growing stages (late March to early April). In wet areas RodeoÆ is the only herbicide permitted for use in Washington.

Response to Cultural Methods

Plants may be dug-out, but care should be taken to remove much of the root stalk. This can be difficult and unpleasant. Always wear protective clothing and avoid getting the sap on your skin. Mowing serves only to stimulate budding on the perennating root stalk, but might be successful if done consistently and persistently enough to starve the rootstalk.

Biocontrol Potential

Cattle and pigs are cited as possible biocontrol agents. Both eat giant hogweed without apparent harm. Trampling also damages the plant. Whether any formal investigation for phytophagous insects in giant hogweed's native range has been conducted is unknown.