Thursday, February 14, 2008

Thursday's Invasive Threat to Illinois: Giant Hogweed

Giant hogweed introduced into North America in the 1900’s. Previous to the positive identification in Illinois, this weed has been found in 14 other states including Connecticut, Indiana, Maine, Maryland, Massachusetts, Michigan, New Hampshire, New Jersey, New York, Oregon, Pennsylvania, Vermont, Washington, and Wisconsin. It is listed as a federal noxious weed and is illegal to bring into the United States or bring across state lines. It has been planted as an ornamental in the United States and may have also been brought into the counry for its fruit which is used as a spice (golmar) in Iranian cooking.

Giant hogweed is a biennial or perennial herbaceous plant which grows to a height between 8 and 15 feet tall. It can have a taproot or fibrous root system. Its stems are hollow, between 2 and 4 inches in diameter, with dark reddish-purple splotches and coarse white hairs. Leaves are compound, lobed, deeply incised and may grow up to 5 feet in width. Flowers appear in mid-May through July. Characteristic of the carrot family, the flower heads are umbrella-shaped, up to 2 ½ feet in diameter across a flat top with numerous small flowers. The plant produces flattened, oval shapes seeds about ⅜ inch long and tan with brown lines.

Giant hogweed is commonly confused with several other plants. Cow parsnip, poison hemlock, and angelica are commonly mistaken for giant hogweed. The two sites listed below offer descriptions and comparisons between giant hogweed and look-a-like plants.

Hogweed Look-Alikes
Giant Hogweed and Similar Species

Environmental and Public Health Hazards
Due to its highly competitive and aggressive nature, giant hogweed is likely to naturalize in many areas where it was first introduced. Plants do well in many habitats can quickly dominate an area.

This plant poses not only an environmental, but also a public health hazard. Its clear, watery sap is capable of causing photodermatitis. The plant sap produces painful, burning blisters or even painless red blotches that may turn purple over time and persist for a year or more. For an adverse reaction to occur, the skin must come in contact with the sap, must be moist (perspiration), and then exposed to sunlight.

Mowing, cutting, or weed whacking are not recommended control strategies due to the plants large perennial roots system and its ability to set new growth. These measures also increase the chance of homeowners coming in contact with the plant’s sap.

Digging up the plants is one of the most effective non-chemical methods of control of giant hogweed. Protective clothing should be worn when working near this plant, including protective eye gear, rubber clothes, long sleeves, and pants. Multiple efforts may be required to completely control hogweed with this method. Cut the plants below the ground and try to remove tap roots to prevent regrowth. Continue to monitor the area for emerging plants.

Applying glyphosate is an option for chemical control. This is a systemic herbicide that is absorbed by the leaves and will move into the root system to prevent regrowth the following year. This is a broadspectrum herbicide, be careful to avoid spraying any desirable plants. The optimum time for application is during the summer months when the plant is green and actively growing. The herbicide will take time to work, so it may take up to a week for symptoms to appear. Repeated applications may be necessary. There are several home-use glyphosate products available. Please follow all label instructions.

For more information and photos

Invasive Species.Org Fact Sheet
Species Profile from InvasiveSpeciesInfo.Gov
Giant Hogweed in Connecticut
Giant Hogweed in Massachusetts

Photos are property of


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