Why is it a problem?
Giant hogweed is known to form dense patches of very tall plants, particularly along streams. Each plant can produce up to 100,000 papery seeds, which either float away or remain in the soil as a seed reserve. In ecological terms, hogweed uses these traits to create a mess, crowding out and shading other species with tall stems and huge, four-foot-wide leaves. The one bright spot is that hogweed appears to spread only by seed, not by roots or rhizomes.
In addition, hogweed sap contains furocoumarin, known for making human skin highly photo-sensitive. The risk of severe burns and possibly scarring makes hogweed of particular concern for human health. Many other animal species seem to be unaffected.