Giant Hogweed

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As plants are finally growing vigorously, I wanted to send out a warning to many gardeners to be cautious of giant hogweed, a noxious plant that fights back when you try to eradicate it.

Currently, it’s found in Oregon and Washington, as well as a number of Eastern and Midwestern states and provinces. The scary part about giant hogweed is the sap will cause blistering (and we’re talking very severe blistering) when it’s exposed to the sun and a little moisture (such as sweat) while on the skin. It can cause permanent scars, and if you rub your eyes, potential blindness.

I always wonder how these nasty invaders end up on this continent. Giant hogweed is a native of the Caucasus region of Eurasia (around the former Soviet Republic of Georgia), but was planted as an ornamental plant in a garden in New York in 1917. Who would plant such a thing? Even I, a total plant geek, wouldn’t think of putting in something that has flesh burning sap. From there, the story is the same. It took off. Now it’s a big (literally) problem.

Identifying Giant Hogweed

Giant hogweed (Heracleum mantegazzianum) looks a lot like cow parsnip (Heracleum lanatum). The greatest difference between the two is size. Giant hogweed can grow up to 15 ft. tall. Cow parsnips are half of that. But there are other features to consider. The flower heads on cow parsnip might be a foot wide where the giant hogweed will be twice that size. The seed heads of the cow parsnip are more heart shaped, while the giant hogweed seeds are ovals. And the stems of the giant hogweed have purple splotches with course white hairs. The ones on cow parsnip have finer hairs and are green.

And while it resembles cow parsnip closer than anything else, it can often be mistaken for angelica, wild parsnip or poison hemlock (another bad actor to have in the landscape). The bottom line is unless you are absolutely positive you know what you’re dealing with, don’t touch it! Call someone from your local Extension office to take a look. If it is giant hogweed, your best bet is to call in an expert who will tackle it in full personal protective equipment. This is definitely not a do-it-yourself weed removal.

How to Kill Giant Hogweed

Giant hogweed sends down a significant taproot, but it is susceptible to root cutting to kill the plant. Its strength is in the thousands to tens of thousands of seeds it produces in a season. If you suspect you have giant hogweed growing on your property it also can be killed with Roundup. This is a case where you might want to bring out the big guns to prevent it from ever going to seed.

But, once again, I have to stress never touch the plant. If you do, wash the area immediately with lots of soap and water and stay out of the sun for at least 48 hours. If you want to be grossed out, search for images of the blistering caused by this plant. You’ll gain a lot of respect for it.

Giant hogweed is a pervasive problem in many states, and unfortunately, itseems to be increasing throughout the country. Everyone needs to keep an eye out for this plant, notify officials of its presence, and have it eradicated immediately. And never, ever think it’ll be a pretty addition to your garden!

Thankfully, I’ve never seen giant hogweed in person, so the photo is courtesy of the USDA APHIS through bugwood.org.

Meet Amy Grisak

Amy is a freelance author and photographer in Great Falls, MT who specializes in gardening, foods, and sustainable agriculture. She provides information on every kind…

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