A note from Matthaei Botanical Gardens & Nichols Arboretum natural areas manager Jeff Plakke on the difference between hogweed and cow parsnip—

A lot of people have been asking lately and the answer is: No, we do not have the exotic giant hogweed growing on our properties. What some people are reporting is the native cow parsnip, which looks similar. The obvious distinction is that giant hogweed grows much much larger. Cow parsnip can get up to 7 feet tall, giant hogweed up to 14 feet tall. Leaves of cow parsnip can get almost 3 feet wide, giant hogweed leaves up to 5 feet wide. Giant hogweed has been in the news a lot recently and is intriguing because it is so big and the sap is very toxic. So far it is not widely spread in Michigan. It’s just in Ingham County (Lansing) and Gogebic County (borders Wisconsin in the upper peninsula). 
Here’s a good website with pictures for comparison, and a good, short video on both hogweed and cow parsnip that shows/explains a lot about both species.
You can find cow parsnip growing in a number spots 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 Rd. (Nichols Dr.) and in the wetland boardwalk area. A large patch near School Girl’s Glen bridge has been reported to us numerous times over the past few years. It’s good that people are paying attention to the plants.
Spread the word and help to educate!