Bear's hogweed (Heracleum), also known as bear's paw, is a plant genus in the umbelliferae family (Apiaceae).
All parts of the plant, especially the sap, are poisonous. The juice triggers a phototoxic reaction, especially when exposed to sunlight. These phototoxic reactions are particularly pronounced in the giant hogweed (Heracleum mantegazzianum) (Wikipedia).
On the way we saw a lot of hogweed.