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The unfriendly giant invasive plants

Hogweed looks impressive, but it can cause a number of problems for farmers and the general public.

By Jeanne Hughes

Giant hogweed (Heracleum mantegazzianum) may be described as “striking”, “majestic”, and “spectacular”. Indeed, giant hogweed is all of these things, and it is easy to understand why it is admired and cultivated, but truly, it is not a good idea to introduce this plant on your property. Many people heard about this plant for the first time during the summer of 2010 when the media caught wind of the fact giant hogweed contains a toxic sap that is dangerous to people. This sap contains chemicals that can cause severe welts, rashes, and blistering followed by pigmented scarring when it contacts skin in the presence of sunlight (a process called 'phytophotodermatitis'). Scarring can persist for as long as six years. This sap is found in the leaves, stems, flowers, and roots of giant hogweed as well as in other plants in the carrot family, although not nearly to the same degree. Contact with sap can occur by brushing against any broken plant parts, handling plant material, or touching tools or mowing equipment used during giant hogweed removal. But before proceeding too much farther, here is some background on this plant. Giant hogweed is an invasive alien plant that originated from the Caucasus Mountains in west-central Asia, where it grows in subalpine meadows and forest edges. It arrived in our province through horticulture due to its impressive size and appeal to gardeners as a specimen plant. It can grow 2-5 metres tall, has large, deeply incised and pointed compound leaves up to three metres wide, and large, flat-topped, umbrella-shaped white flower clusters up to one metre in diameter. Another identifying feature is the hollow stem with dark reddish-purple raised spots and stiff hairs (though don't get close enough to this plant to verify the hollow stem). In the Fraser Valley, giant hogweed grows in full or partial shade along streams, moist forests, and meadows. Blooming occurs from mid-May through early August. Two native plants are sometimes confused with giant hogweed: cow parsnip (Heracleum maximum) and palmate coltsfoot (Petasites frigidus). Neither of these grows to nearly the size of giant hogweed. Giant hogweed is not only a threat to human health and safety, but also to agricultural land and natural areas as it easily crowds out vegetation, reducing crop and wildlife

14 British Columbia Berry Grower • Summer 2011


Giant hogweed is not only a threat to human health and safety, but also to agricultural land and natural areas as it easily crowds out vegetation, reducing crop and wildlife habitat value.

habitat value. It is a highly competitive plant due to vigorous early-season growth, tolerance of full shade and seasonal flooding, and ability to co-exist with other widespread and aggressive invasive plant species. Relatively shallow roots do not hold the soil as well as a healthy complex of native species, and as a consequence infestations can result in increased erosion on steep terrain or along stream banks, particularly when winter dieback exposes soil to water erosion during our rainy season.

Controlling giant hogweed, as with controlling any invasive plant you don't want growing in your field or garden, is accomplished by understanding its growth cycle and then exploiting its weaknesses.

Giant hogweed disperses and establishes by seed, which are moved by wind up to four metres, or longer distances by water along streams, ditches, or storm pipes. It grows for three to five years before flowering and dying in the

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