If the List of Curiosities thread is correct:
Lady's mantle: 60 LP/h (300/5)
Uncommon snapdragon: 66 LP/h (1000/15)
Thorny thistle: 23 LP/h (555/24)
I threw a few thorny thistles away after the one I was studying took hours and wasn't even 10%. It's not bad on mental weight but that doesnt help it any.