Invasive Pest Parades as Botanical Cousin

Have you stumbled across the toxic and invasive giant hogweed in Michigan? Probably not. More likely you've discovered the native cow parsnip, which resembles hogweed in many ways.

As you head outdoors this summer, there are certain plants and animals to look out for. Some, like poison ivy, are fairly widespread and easy to identity. One plant pest that’s not found much in Michigan—even as it receives a lot of coverage—is giant hogweed (Heracleum mantegazzianum). It’s thought that this member of the carrot family (Apiaceae or Umbelliferae) was introduced as an ornamental because of its size and striking appearance. Despite its good looks giant hogweed contains a sap that can cause photo-dermatitis, a severe reaction in which skin that comes in contact with hogweed sap blisters when exposed to the sun. Add to this the fact that giant hogweed also resembles cow parsnip (Heracleum lanatum), a native plant in the same family, and you’ve got a recipe for ID confusion!

The flowers of giant hogweed are
umbrella-shaped and domed on top.

Cow parsnip flowers are flat on top.

Matthaei-Nichols natural areas manager Jeff Plakke says that several visitors have reported seeing giant hogweed on our properties.

“Giant hogweed has been talked about a lot recently and is intriguing because it's so big and the sap is very toxic,” says Plakke. Good news: it’s not giant hogweed, which is so far not widely spread in Michigan. “What some people are reporting is the native cow parsnip which looks similar,” Plakke adds.

One obvious distinction between giant hogweed and cow parsnip is that “giant hogweed grows much, much larger,” according to Plakke.  Cow parsnip may reach 7 feet in height; giant hogweed up to 14 feet tall. Leaves of cow parsnip can get almost 3 feet wide, while giant hogweed leaves may span as much as 5 feet.

Hogweed stems are green with
pirple splotches.

Cow parsnip stems are
green with fine white hairs.

Cow parsnip grows in a number of places at Matthaei Botanical Gardens including along the trails in the Fleming Creek floodplain and along the edge of Willow Pond. At the Arb there are colonies of cow parsnip growing along the River Road (Nichols Drive) and in the wetland boardwalk area. A large patch near School Girl’s Glen bridge has been reported numerous times over the past few years, Plakke adds.

Check out these links for more information:

A good website with pictures of both cow parsnip and giant hogweed for comparison is New York State’s Department of Environmental Conservation. 

A short video from the Invasive Species Council of Metro Vancouver explains a lot about both species.

(Photos courtesy New York State Department of Environmental Conservation.)