The Collaborative International Dictionary of English v.0.48:

Hercules'-club \Hercules'-club\, Hercules'-club
\Hercules'-club\, Hercules-club \Hercules-club\prop. n.
   1. (Bot.) A densely spiny ornamental tree (Zanthoxylum
      clava-herculis) of the rue family, growing in southeast
      U. S. and West Indies. [WordNet sense 1]

   Note: It belongs to the same genus as one of the trees
         (Zanthoxylum Americanum) called prickly ash.

   Syn: Hercules'-clubs, Hercules-club, Zanthoxylum
        clava-herculis.
        [1913 Webster + WordNet 1.5]

   2. A small, prickly, deciduous clump-forming tree or shrub
      (Aralia spinosa) of eastern U.S.; also called Angelica
      tree and prickly ash. [WordNet sense 2]

   Syn: American angelica tree, devil's walking stick, Aralia
        spinosa.
        [1913 Webster + WordNet 1.5]

   3. A variety of the common gourd (Lagenaria vulgaris). Its
      fruit sometimes exceeds five feet in length.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Zanthoxylum clava-herculis
    n 1: densely spiny ornamental of southeastern United States and
         West Indies [syn: Hercules'-club, Hercules'-clubs,
         Hercules-club, Zanthoxylum clava-herculis]