The Collaborative International Dictionary of English v.0.48:

leopard's bane \leop"ard's bane`\ (l[e^]p"[~e]rdz b[=a]n`) n.
   (Bot.)
   A name of several harmless plants, as Arnica montana
   (Arnica acaulis syn. Doronicum acaule), Senecio
   Doronicum, and Paris quadrifolia.

   Syn: leopardbane, leopard's-bane.
        [1913 Webster +PJC]