Giant hogweed growing in Washington, DC. The iron fence in the photo is 6 feet tall. Photo by Sandy Farber Bandier, University of the District of Columbia Cooperative Extension Service
Imagine an alien world where simply brushing up against the giant leaves of the plants that tower above you will lead to severe burns on your skin, and where getting the plants’ sap in your eyes causes blindness, and where these unpleasant plants cannot be eradicated without a three-year intensive campaign against them.
Unfortunately you don’t need to leave the eastern US to find this outlandish world of hostile vegetation. Giant hogweed is an invasive plant that goes beyond the standard practice of taking over native habitats and crowding out the sources of food and cover that our native wildlife species depend upon. They also shade out all competition creating bare earth and an erosion hazard, and are a serious public health threat.
Introduced as an ornamental plant from Eastern Europe sometime in the 20th century, giant hogweed prefers moist soils, but does well in many different environments. It is especially problematic along the banks of waterways, as its seeds can float downstream for up to three days.
It has been identified in western and central Maryland, Washington D.C., Pennsylvania, all New England states, and several other states scattered throughout the country. Most states where it’s present have active eradication programs working to completely remove it from the ecosystem.
Giant hogweed is in the carrot and parsley family, and can be confused with its cousins. Cow parsnip, Angelica, wild parsnip, wild chervil, poison hemlock, Queen Anne’s lace, and golden Alexanders are common lookalikes. Some of these plants also contain toxins, but none are as potent as giant hogweed.
In general, a mature giant hogweed plant is bigger in every respect than all of its lookalikes. It grows taller—up to fifteen feet, has bigger leaves—up to five feet wide, and has a thicker stem—up to four inches in diameter. The stem has purple blotches that surround hollow spines.
The leaves are deeply lobed, and the edges are very serrated and pointy, a bit like a huge exaggerated maple leaf. Look alike species have leaves that are smaller with shallower lobes and more rounded edges, or have compound leaves with separate leaflets. The plant reaches its tallest height when it flowers in mid-May to June after four or more years of growth. The flowers are flat-topped clusters of small white flowers about two and a half feet wide. The plant dies the same year that it flowers, although there are occasionally suckers that survive.
The sap of giant hogweed is laced with chemicals called furocoumarins. The plant probably evolved with these compounds to help protect against fungal attack, but they also serve as a potent weapon against humans and other animals. The sap is present throughout the plant, and is especially heavy in the base of the stem and in the spines on the stem. Just touching these spines can be enough to get sap on your skin.
If the sap does contact skin, these chemicals eliminate the skin’s natural protection from the sun’s ultraviolet rays, especially in the first two hours after contact. Without this protection, your skin easily burns, and even brief exposure to the sun can lead to severe injury lasting months. Increased sensitivity to sunlight often lasts for years after contact.
The moral of the story is to approach this plant with extreme caution and a full set of protective gear, including long sleeves, pants, and eye protection. Waterproof synthetic gear offers further protection from the sap. The seed heads contain 50,000 or more seeds and should be the main target of control. It is essential that seeds are not allowed to set. Seeds are viable in the soil for up to seven years, although most will have germinated after three.
Purple blotches on stems and serrated,
Control of small groups can be done with hand tools. Never use a weed whacker, as it will create uncontrollable sprays of the toxic sap. Herbicides with triclopyr or glyphosate are also an option. If at all possible, leave the control of this dangerous plant to the experts.
Giant hogweed is federally listed as a noxious weed, which means that there are regulations controlling its transportation and sales. You may also be able to get assistance in controlling the plant on your property.
If you think you’ve seen giant hogweed in Maryland, please contact the Plant Protection and Weed Management Section of the Maryland Department of Agriculture at (410) 841-5920. In Delaware, please contact Faith Kuehn at (800) 282-8685 or Faith.Keuhn@state.de.us. Visit http://1.usa.gov/hogweed for more information.