The Collaborative International Dictionary of English v.0.48:

bridalwreath \bridalwreath\ n.
   a shrub (Spiraea prunifolia) having copious small white
   flowers in spring.

   Syn: bridal wreath, Saint Peter's wreath, St. Peter's wreath.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Spiraea prunifolia
    n 1: shrub having copious small white flowers in spring [syn:
         bridal wreath, bridal-wreath, Saint Peter's wreath,
         St. Peter's wreath, Spiraea prunifolia]