Maci_gumball guessed correctly! The herb was Stinging Nettle! As well as the new rule on the cat book, it is also official here: JOINT ACCOUNTS CANNOT PARTICIPATE IN THIS BOOK. I made this rule just to be fair. Read my Guess That Cat book for the reason. Anyways, here's the riddle:

I am dandelion like,
I'm found by the waterfall,
I heal cracked pads,
As well as your breathing.

~Night