Potential Wetland Invasive Species Threats

Giant Hogweed

There are two counties in Wisconsin with known populations of giant hogweed, Iron and Manitowoc. The known populations in Manitowoc County are just east of Kiel in the Town of Schleswig. This is far too close to Calumet County to ignore.

Caution! If sap from giant hogweed's leaves or stems gets on skin in the presence of sunlight, it can cause a severe, blistering burn that appears a day or two after exposure. Wild parsnip and cow parsnip can cause a similar reaction, called phytophotodermatitis.

Giant Hogweed Leaf with GPS Unit for Scale

Giant hogweed leaf with GPS unit to show scale

Giant hogweed is striking due to its huge size. In the first year of growth, plants form a rosette of compound leaves that are 1 to 5 feet wide. Plants flower in their second year or later. Flowering plants are 8 to 15 feet tall, topped with a group of broad, dome shaped umbels of white flowers. The bottoms of leaves also have coarse, dense hairs and the flower stem shows distinct mottling. The plant produces thousands of seeds and dies after flowering. American cow parsnip, common in Calumet County, is smaller, only 3 to 7 feet in height and has a less lobed leaf structure and non-mottled flower stems.

Giant Hogweed Leaves Have Jagged Edges

Giant hogweed leaves have jagged edges

Giant hogweed disperses by seed, naturalizes easily and is found in disturbed areas such as empty lots and woodland edges. It flourishes in areas with moist soils and some shade, like the moist, shady edge of a woodlot. It can be especially troublesome along stream banks where it crowds out native vegetation, leads to soil erosion and readily disperses downstream by seed.

Because of the danger from its sap, cover all skin and protect eyes if working to eradicate this plant. Do not allow giant hogweed to go to seed.

For additional information, see this Giant Hogweed pamphlet (PDF) or visit the Department of Natural Resources (DNR) website.

Giant Hogweed Flower Spike in Seed

Giant hogweed flower spike in seed