Giant hogweed is an invasive non-native plant species that is invading streamside (riparian) areas throughout North America and Europe.
French Creek likely has the oldest population of giant hogweed in western Canada and it is considered the epicenter of invasion on southeastern Vancouver Island.
Giant hogweed plants can grow up to 5 m tall and produce up to 120,000 seeds upon flowering.
Giant hogweed is a human health hazard. Contact with sap in the presence of sunlight can cause skin blisters (contact dermatitis).
The Invasive Alien Plants Program (IAPP), a program of B.C. Ministry of Forests & Range, has developed a management strategy to manage giant hogweed in the French Creek watershed in association with landowners and interested groups.
The removal of Giant Hogweed (and Knotweed) from private land is free to most property owners in the Comox Valley and Central Island* (See Removal Assistance below for more info and list of participating locations).
Examples of giant hogweed in North Vancouver (left) and at Vanier Park in Vancouver (right)
Giant hogweed (Heracleum mantegazzianum) is a member of the Carrot Family which originated in the Caucasus Mountains in west central Asia. In its native habitat it is found in subalpine meadows and forest edges. It is related to native cow-parsnip (Heracleum maximum) which occurs in streamside areas, moist forests, and subalpine meadows throughout BC.
Giant hogweed is identified by its large size (3–5 m tall when flowering), serrated leaf pattern, and seed morphology. It is sometimes confused with native cow-parsnip or seaside angelica (Angelica lucida) although the similarities are superficial.
For more photos and identification resources for giant hogweed, visit the E-Flora BC website.
Key identifying features of giant hogweed: 1) deeply lobed and toothed (serrated) leaves; 2) tall stems with extensive purple-reddish blotches; and 3) seeds with blunt ends and oil ducts greater than 3/4 the length of the seed.
Some key attributes of giant hogweed include:
Giant hogweed disperses and establishes by seed. Pieces of roots, leaves, and stems do not resprout like Japanese knotweed or English ivy. Seeds are moved by wind (1–4 m) or longer distances by water along watercourses, ditches, or storm pipes. Each plant can produce thousands of seeds per plant (up to 120,000 (!) but more typically several thousand).
The establishment of new populations over longer distances is usually by humans. Gardeners sometimes grow giant hogweed as a specimen plant and it is suspected that giant hogweed was introduced to French Creek as a garden introduction. As well, soil or plant material contaminated with seed is sometimes used for roadfill or other construction activities resulting in the establishment of new populations. Small roadside populations are often established from seeds blowing from trucks during transport of cut plants or seed heads.
Giant hogweed grows in a variety of soil types and vegetation communities (forests, beaches, wetland margins) but prefers moist areas with high-light levels such as streamside areas, forest edges, and roadsides. It can grow and flower in forested areas but it is typically smaller and produces fewer seeds.
Giant hogweed grows for 3–5 years before flowering and dying in the last year. During the first years of growth, it stores increasing amounts of energy in its roots to put into development of the flower stalk, flowers, and seeds. Many established populations consist of plants of varying ages: small first year plants, larger older plants, and fourth or fifth year plants in flower.
Repeatedly cutting the flower stalk at ground level can be used to kill giant hogweed. However, it often it develops a secondary flower stalk and inflorescence and there is also more chance of being exposed to plant sap when working with larger plants. Removing the seed head in the summer (June 30–July 15) can prevent the production and dispersal of new seeds which is sometimes a useful short-term control technique.
Giant hogweed seeds may remain in the ground more than five years although most seeds germinate in the first two years. Control efforts must include site monitoring to prevent hogweed re-establishment from seedbank material.
Giant hogweed sap contains chemicals which can cause severe dermatitis (welts, rashes, and blistering, followed by pigmented scarring) when they contact skin in the presence of sunlight (light-activated phytodermatitis**). The chemicals, called furanocoumarins, are activated by sunlight to become irritants. They are found in the leaves, stems, flowers, and roots of giant hogweed, as well as other members of the Carrot Family. Contact with plant sap can occur by brushing against broken plant parts, handling plant material, or even by touching tools or mowing equipment that was used for giant hogweed control. Children playing with the large plants, or maintenance workers or gardeners involved in vegetation control are most likely to acquire dermatitis from giant hogweed.
Giant hogweed can cause severe dermatitis if sap contacts skin and then is exposed to sunlight (left). Appropriate clothing and safety equipment should be worn to minimize the risk of exposure (right).
There are four main recommendations for minimizing the risk of being affected by giant hogweed dermatitis when undertaking control activities:
1. Wear protective clothing that covers all exposed skin and eyes when handling giant hogweed. This includes gloves that are impermeable to plant juices (thick latex gloves), coveralls or raingear, and a facemask or safety goggles. WorkSafe BC has issued a Toxic Plant Warning for workers who may come into contact with this species.
2. Use control techniques that minimize the generation, dispersal, and contact with plant sap. Use a long-handled shovel to cut plant roots, or a long-handled sickle to cut flower heads. Do not use a brushcutter or mower unless you have proper training and safety equipment because they distribute plant sap to exposed surfaces.
3. If you are exposed to giant hogweed sap, wash affected skin as soon as possible with warm, soapy water or streamwater if you are a remote area. Rinse eyes immediately. If you are unsure if all plant sap is removed, keep the area of skin covered (no sunlight) until you wash thoroughly. Consult a doctor if have extensive exposure to giant hogweed sap or if develop blisters or other signs of dermatitis; make sure you tell health care providers about your exposure to giant hogweed sap.
4. Wash all clothing and equipment thoroughly with soap and water after contact with giant hogweed. Be careful not to redistribute plant sap when washing clothing or equipment.
To view a WorkSafe BC video and download a Toxic Plant Warning brochure on giant hogweed, visit the WorkSafe BC website.
**Dermatitis from giant hogweed is sometimes referred to as phytophotodermatitis because of the combination of plant sap (phyto) and sunlight (photo) that is required. The reaction is caused by furanocoumarin chemicals (psoralens) present in plant sap in combination with ultraviolet-A light. Other plants in the Carrot Family can cause dermatitis including celery, carrots, Queen Anne’s lace, and cow-parsnip. Furanocoumarins are also present in grapefruit and limes.
The recommended control methods for landowners on private property is manual cutting of the taproot in spring or early summer or careful use of glyphosate, a herbicide commonly called Roundup®. Root cutting is the preferred method in most sites because of its effectiveness. As a secondary measure, the removal of flowering heads in early to late summer (June 30–September 15) is recommended as a low effort method for preventing further seed dispersal. However, removal of flowering heads does not kill the plant.
Giant hogweed taproots should be cut at least 10 cm below the soil surface using a sharp long-handled shovel or spade.
Instructions for manual root cutting
Like many members of the Carrot Family, giant hogweed has a deep taproot that stores energy for annual growth or flowering. The root can be 12 cm thick and 45 cm long. The large root also provides a buffer against damage to the above-ground parts of the plant from mowing or browsing. Giant hogweed can rapidly resprout forming leaves or a new flower stalk after cutting the leaves and stem.
This characteristic can also be exploited for control and we recommend root cutting as the most effective method of killing small groups (<100) of giant hogweed plants. The suggested method is to:
Wear protective clothing (see WorkSafe BC Toxic Plant Warning) that covers exposed skin and glasses or a face-shield to cover the face and eyes. Gloves are essential.
Using a sharp long-handled shovel or spade, sever the root about 10–15 cm below the soil surface. All plants should be treated, not just large plants.
Leave the above-ground part of the on-site to decompose unless there is risk that people will touch them. If possible, do not handle the plant at all and allow it to rot where it falls. If flowerheads with developing seeds are present, remove the flowerheads, put them in a plastic bag, and dispose of them in your household garbage.
Undertake one round of root cutting in early spring (April 1–May 15) and a second round in early-summer (June 15–July 15) to treat any plants that were missed or have resprouted.
Wash any tools that have been in contact with plant sap.
Monitor the site annually for 3–5 years around June 15 to ensure that new seedlings are identified and removed (root cut).
Before (left) and after (right) root cutting at a giant hogweed control site in North Vancouver.
Giant hogweed is an invasive plant that is considered both a public health and environmental hazard. It is common in streamside areas of French Creek from Coombs to the stream mouth (see map). It was introduced into the French Creek watershed before the early 1960s. French Creek likely has the oldest population of giant hogweed in western Canada and it is considered the epicenter of hogweed invasion on southeastern Vancouver Island. Giant hogweed is now found from Sooke to Courtenay. It is also found in the Lower Fraser Valley, particularly North and West Vancouver.
Distribution of Giant Hogweed in French Creek
Recent mapping (October 2007; Invasive Alien Plant Program) identified 53 sites in the French Creek watershed with giant hogweed. All mapped sites are within 50 m of the mainstem of French Creek. The giant hogweed population is found along approximately 14.4 km of stream channel. Four sites occurred in the lower developed portion of the watershed (stream mouth to the E&N rail corridor), ten sites between the E&N rail corridor and Highway 19, 17 sites between Highway 19 and Pacific Rim Highway, and 22 sites upstream of the Pacific Rim Highway in the Coombs area.
Distribution of giant hogweed sites in the French Creek watershed (click on map to enlarge)
Giant hogweed stands along French Creek
While the overall number of sites increased with increasing distance upstream from the stream mouth, the mean density of sites declined from 6–10 plants per square meter in the few sites near the mouth, to 2–5 plans per square meter in the Coombs area. It is also important to note that the existing survey likely underestimates the distribution of giant hogweed in the watershed. Small patches are likely to be missed by surveys and it also suspected that more sites may occur away from the stream corridor. We encourage landowners and stewards to report new sites using available maps (see Getting Involved for more information) or using the Report-A-Weed online invasive plant reporting tool.
B.C. Ministry of Forests & Range is looking for French Creek landowners willing to participate in controlling giant hogweed in the French Creek watershed. You can participate in the following ways:
Control giant hogweed on your property. Become familiar with the methods and safety measures required to control giant hogweed and conduct removals in late spring or early summer.
Map locations of giant hogweed in the French Creek watershed. Maps are being mailed to property owners in areas currently known to have the largest areas of hogweed. If you receive a package, please record any hogweed locations on your property on the map provided and return them as requested. If you have hogweed on your property and did not receive a map, please contact us. Alternately, you can allow B.C. Ministry of Forest & Range staff to access your property during July–August 2009 to map any hogweed on your property.
Increase public awareness about giant hogweed in French Creek. Encourage your neighbours and other landowners to participate in the program.
Project Contact Information
For more information on the French Creek Giant Hogweed Project, contact:
Do live in Central Vancouver Island or the Comox Valley and want us to remove your Hogweed?
The removal of Knotweed and Giant Hogweed from private land is free to most property owners in the Comox Valley and Central Island* (See below for list of participating locations). Please fill out our “Release of Liability for Access and Herbicide Use Agreement Form” for the removal of Knotweed or Giant Hogweed from your property.
This permission form adds you to Coastal ISC’s treatment list. It allows Coastal ISC staff to contact you and set up a suitable date to go to your property to monitor and remove the Knotweed or Giant Hogweed species either manually or with a herbicide.
*Participating locations include: Comox, Cumberland, Courtenay, Comox Valley RD, Qualicum Beach, Parksville, and the Regional District of Nanaimo (this list is subject to change). Email us if you would like to know more: firstname.lastname@example.org.
**currently being updated, some links may be out of date or not working
The Giant Hogweed Best Practice Manual: Guidelines for the management and control of an invasive weed in Europe. Published by Forest & Landscape Denmark (C. Nielsen, H.P. Ravn, W. Nentwig, and M. Wade (eds.), 2005). Download manual.
Best Management Practices for Giant Hogweed. King Country Noxious Weed Control Program, King County, Washington. Download BMPs.
E-Flora BC – Heracleum mantegazzianum. Electronic Atlas of the Plants of British Columbia (www.eflora.bc.ca). by Brian Klinkenberg (editor), Lab for Advanced Spatial Analysis, Department of Geography, University of British Columbia, Vancouver. View atlas page.
Invasive Alien Plants Program (IAPP). B.C. Ministry of Forests & Range. Visit website.
Aggressive Ornamentals – Giant Hogweed. B.C. Ministry of Agriculture and Lands. Visit website.
Field Guide to Noxious Weeds and Other Selected Invasive Plants of British Columbia. B.C. Ministry of Agriculture and Lands. Visit website.
The Biology of Invasive Alien Plants in Canada, Giant Hogweed (Heracleum mantegazzianum). Published in the Canadian Journal of Plant Science 86 (by N.A. Page, R.E. Wall, S.J. Darbyshire, and G.A. Mulligan, 2006). Download paper.
Heracleum mantegazzianum Sommier & Levier. From Biological Flora of the British Isles series published in the Journal of Ecology (by G.E.D. Tiley, F.S. Dodd, and P.M. Wade, 1996). Download paper.
Ecology and Management of Giant Hogweed. Edited by P. Pyšek, M.J.W. Cock, W. Nentwig, H.P. Ravn, 2007). Download book info.