Giant Hogweed (Heracleum mantegazzianum) – In the first image I am standing below the plant looking up at the underneath of this massive bloom. I am 6′ 2″ tall, and these can grow up to 5m (over 16ft) tall. I was also on higher ground, the plant virtually in the river. A word of warning … do not touch these giants because they will seriously get you with burns, blisters and rashes. The sap contains furocoumarin, which if it gets on the skin makes it extremely sensitive to sunlight (phytophotodermatitis). A person can suffer phytotoxicity for months or even years, so admire this giant from a distance.