Giant Hogweed (Heracieum mantegazzianum). Hogweed is a phototoxic plant. It?s clear watery sap sensitizes skin to ultraviolet radiation. This causes burns, resulting in blistering and painful dermatitis. When found in urban areas, can be thought of as a public health hazard.Native to Central Asia, introduced into the UK and Europe. Can grow to 5-7 m (15-20 ft) tall.