Most of the plants I think of, that appeared in works of fiction or rhymes, are real. And that's not what is desired.
Cedars of Lebanon (Gilgamesh)
Grapes of wrath
Truffula Trees (The Lorax by Dr. Seuss)
Red Weed (War of the Worlds by H.G.Wells)
Plants we're not sure if it was in a book:
Triffids (Day of the Triffids)