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Giant Hogweed

By Therese Ciesinski, GardenSMART

If you think poison ivy is a scourge of the great outdoors, meet giant hogweed, the plant that tells poison ivy, “hold my beer.” Without knowing what it was, you’d want one in your garden. A magnificent specimen with huge leaves and umbrella-sized flowers, it’s a monster in more ways than one.

Giant hogweed (Heracleum mantegazzianum) is dangerous, a toxic plant that thankfully most people won’t run across. However, everyone who spends time in the outdoors should know what it looks like, just in case. Common names include giant cow parsley, cartwheel flower, or hogsbane.

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What makes the plant dangerous is its sap, which is found in the stems, roots, leaves, flowers, and seeds. Hazardous to humans, it causes what is known as phytophotodermatitis, which makes the skin hypersensitive to ultraviolet light. Contact results in swelling, burns, blisters, itching and scarring. It can be quite painful. If the sap touches the eyes, it may cause permanent blindness.

It is a striking plant, looking like Queen Anne’s lace on steroids. (Both are members of the carrot/parsley family.) The flowers can be up to two and a half feet across. The plant can grow to 20 feet in height, with leaves five feet wide. Giant hogweed flowers in June and early July, and the sap is at its most toxic when in flower.

The plant is technically biennial, but unlike the standard definition of biennial ­– emerging in year one, blooming, setting seed and dying in year two – giant hogweed can take up to five years to get to flowering stage.

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In the U.S., it is primarily found in Washington and Oregon, and in the East from New England south to Maryland and Indiana, and west to Wisconsin. It is hardy to USDA Plant Hardiness Zone 3.

Giant hogweed prefers full sun and moist soil, so it grows in ditches and stream banks. It is also found in fields and farmland, and along fence lines. It resembles a number of weeds and ornamental plants, either in size or flower shape, and be confused with angelica, cow parsnip, and wild parsnip (all of which can also burn the skin), as well as queen Anne’s lace, elderberry, water hemlock and giant hemlock. The last two are poisonous if ingested.

If exposed to the sap, it’s advised to immediately get out of the sun. Keep the area covered and away from sunlight for at least 48 hours. Wash with soap and cold water. A reaction can occur within 15 minutes, and the full effects of exposure appear within 24 to 48 hours. Topical steroids can help, however, it’s best to see a doctor. It can take years for the burns to heal.

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Photograph by GerardM at nl.wikipedia, CC BY-SA 3.0, via Wikimedia Commons.

Giant hogweed is native to parts of Georgia (the country), Afghanistan, and southern Russia. It was originally brought to the U.S. as an ornamental in 1917, but given that a single plant can produce up to 20,000 seeds, it soon escaped cultivation and started appearing in the wild.

It tends to grow in large patches, crowding out native plants, another reason it’s on the federal noxious weed list. Individual states also have laws prohibiting it from being grown, imported, sold, or transported.

Because of giant hogweed’s invasiveness and toxicity, states have developed management plans to kill the plants when they find them, but since it seeds so prolifically, complete eradication can take years.


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