What is giant hogweed?
How big is it? How fast does it grow? Is there any time of the year (as with garlic mustard) that you should leave it alone? What causes the blisters? Does it cause permanent scarring?
Calhoun County Michigan
This page from the Oregon Department of Agriculture will answer most of your questions: Giant Hogweed (Heracleum mantegazzianum). This is a serious noxious weed and should be controlled when possible. It is toxic at all times of the year and in all stages of plant growth. The blister can lead to permanent scarring.