Giant hogweed is aptly named ... causing itchy blisters and rashes that show up 8 to 48 hours after exposure and can last for weeks. Poison sumac is shrub-like and can grow up to 25 feet tall.

Another plant you really want to avoid touching is the wild parsnip. These look related to the aforementioned giant hogweed, ...

as there's also a native hogweed which can grow tall and cause an itchy rash but that is far less problematic than giant hogweed,' says Morris, who notes that this version can be left alone for ...