Giant hogweed
(Heracleum mantegazzianum)
Giant hogweed reaches up to 8-20’ when in flower and has hollow, ridged stems covered in coarse white hairs and reddish-purple mottling. It is an herbaceous biennial or monocarpic perennial that grows as a low-lying bushy rosette for at least the first year. It will generally bolt in the second year based on maturation, producing 8-20’ tall. The plant dies after seed dispersal.
Other names for this plant include:
- Common names: giant cow parsley, cartwheel-flower
Classification in Wisconsin: Prohibited
- Ecological Threat
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- Invades roadsides, empty lots and woodland edges. It can crowd out native vegetation due to fast growth rates.
- Prefers moist areas with some shade, particularly along stream banks, where it can lead to soil erosion and disperse seeds downstream.
- Sap from leaves and stems can cause a phytophotodermatitic reaction on the skin when exposed to sunlight, consisting of severe burns and blisters.
- Identification
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Leaves: Compound, 1-5’ wide, palmate, deeply lobed, and pointed. The undersides of the leaves are covered in coarse white hairs, and the leaf stalk also has purple mottling. Immature rosette leaves often not lobed, resembling large violet leaves.
Flowers: Large umbels, up to 20” wide across its flat top, with many white, 5-petal flowers that bloom from May-July.
Fruits & seeds: Fruits are two winged mericarps containing one flattened, oval to elliptical seed. Plants produce approximately 20,000 seeds with a germination rate of more than 50%. Seedlings germinate after exposure to cold temperatures.
Roots: Large, deep taproot.
Similar species: American cow parsnip (Heracleum lanatum; native) is 3-7’ tall with non-mottled flower stems and pinnately divided leaves (Is it Giant Hogweed or Cow Parsnip). Great angelica (Angelica atropurpurea; native) is smaller with a smooth, purplish stem, spherical umbel and pinnately compound leaves. Glade mallow (Napaea dioica; native) is 3-6’ tall with leaves 4-12” with 5-9 deep lobes that are coarsely toothed. Poison hemlock (Conium maculatum; invasive) has similar stem and coloring but is hairless and 3-10’ tall.
- Distribution
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See the reported locations of giant hogweed in Wisconsin.
Do you know of other populations? Please send us a report.
- Control
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CAUTION: Sap exposure from leaves and stems onto natural skin can cause a phytophotodermatitic reaction of severe burns and blisters.
Mechanical: Small populations can be hand-dug. Use a sharp shovel to cut 1-2” below the soil surface before the seeds are set. Burn flowering heads or bags for landfills.
Chemical: Glyphosate (LR), 2.5% a.i., triclopyr, or metsulfron methyl (1.5oz per gallon) is effective as foliar sprays or on cut stems.
- Resources
- Sources for content:
- Czarapata, Elizabeth; Invasive Plants of the Upper Midwest: an illustrated guide to their identification and control. The University of Wisconsin Press. 2005. Pg. 136-137
