The Collaborative International Dictionary of English v.0.48:

Dogbane \Dog"bane`\, n. [Said to be poisonous to dogs. Cf.
   Apocynaceous.] (Bot.)
   A small genus of perennial herbaceous plants, with poisonous
   milky juice, bearing slender pods pods in pairs.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dogbane
    n 1: any of several poisonous perennial plants of the genus
         Apocynum having acrid milky juice and bell-shaped white or
         pink flowers and a very bitter root