This is a post from last summer, but it relates to the Jewelweed post from a few days ago. It is also a different variety of nettle..
This is Stinging Nettle (a very good edible when young). That little orange thing behind it to the right is a Chanterelle mushroom —you might not see it. I saw it but did not notice the nettle. It got my arm and hand. It burned instantly and I had hives in under a minute. I was in the middle of the woods and thought this might be a good time to see if jewelweed really worked. I thought this because if it did not work I was screwed, and the burning in my arm was similar to multiple bee stings. I a saw a clearing about 200 yards away and took off for it at a run. Sure enough there was a big patch of jewelweed. I grabbed a handful, crushed it, and rubbed it on my arm hard. The relief was almost instantaneous. I was actually more shocked than relieved. I figure I got the jewelweed on it within 5 minutes and the rash was completely gone in 8-10 min., but the itching and burning was stopped within seconds and did not return.