I'll take a guess.  Fiddleheads grow in wetlands, along rivers,etc.  Another broad leafy plant that grows in that type of areas is skunk cabbage.  I don't know if this is what you touched but it does contain fixed oils and volatile oils.  Interestingly enough it does have slight narcotic influences.  I found in one of my books that used externally it eases pain. Look up a picture of skunk cabbage and see if it matches.

Another one I'm thinking could certainly be nettles but usually they can be quite tall and is more of a summer problem.