Happened to brush my hand against an old nettle today, one of few left standing. Hours later my hand is still numb, for a plant that was on the verge of dying, I'd say that's pretty potent
They are a vey interesting plant. You may or may not know this but they don't sting as such. They have silica hairs that are barbed that are very sharp. When u brush against them they sick in your skin, causing the irritation.