Just got back from camping at Yellowstone Lake state park and there were tons of cow parsnip and wild parsnip. These plants cause severe burns if they contact your skin then are exposed to sun and the signage about them was not placed as close to the trail heads as it should have to warn hikers and bikers. I didn't notice them as much by the camp sites, but the trails were FULL of them. Long pants, long sleeves, and don't go off the paths.
@Philreedshikes I didn't personally take any photos of them because I was trying to keep the kids in the center of the path, but they are easy to spot when blooming because of the distinctive flowers and the large height of up to 4'. The yearlings though are hard to spot because they are low to the ground and have no flowers, only the leaves give them away. A friend of mine got severely burnt removing these from his lawn years ago, which is the only reason I know about them.
Here are some photos from the DNR. The yellow ones are wild parsnip, the white are cow parsnip. They have different leaves, but similar flowers and both will burn. Burns take a day or two to show up which makes it even worse. There is a similar, but much worse plant called giant hogsweed that can be 15' tall, but I have never seen any of them. I think those are more common in the northeast and northwest. There have been some reports of giant hogsweed in the midwest great lakes area, but those could have just been cow parsnip.
It is apparently, a native North American plant and common except in the South East. It is actually edible if peeled and good fodder for many animals including bears. The other catch apart from the peel, is that it is easily confused with poison hemlock which is potentially deadly ...so probably best not to eat it.
Apparently it is the clear sap that causes the nasty blisters. It is UV activated so most likely to cause problems on sunny days if you wack through it. Giant Hogweed, which is related but non native, has the same issue.
Another poisonous plant to avoid commonly found along the trail in S California and the PCT is Poodle Dog Brush which can give you a poison oak like rash. Apparently some people have confused it with Marajuana and smoked it putting themselves in the hospital...so don't do that.
Good advice Diesseldorf. Well taken.
But then there's this:
I'm hoping to harvest some when they dry out.
As Friar Laurence says in Romeo and Juliet, "For nought so vile that on the earth doth live
But to the earth some special good doth give..."
@TomIrvine Well there will be plenty to harvest, though I won't be the one doing that! I don't think they should be removed, but I do think the trailheads deserve some proper warning signs as most people don't know the dangers of wandering off the path when the woods are full of what look like just nice flowers.
@OldGuyot ouch to the guy who smoked the poodle dog bush! I hope he recovered.