The Collaborative International Dictionary of English v.0.48:

flowers-of-an-hour \flowers-of-an-hour\ n.
   An annual weedy herb (Hibiscus trionum) with ephemeral
   yellow purple-eyed flowers, native to the Old World tropics
   and naturalized as a weed in North America.

   Syn: flower-of-an-hour, bladder ketmia, black-eyed Susan,
        Hibiscus trionum.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Hibiscus trionum
    n 1: annual weedy herb with ephemeral yellow purple-eyed
         flowers; Old World tropics; naturalized as a weed in North
         America [syn: flower-of-an-hour, flowers-of-an-hour,
         bladder ketmia, black-eyed Susan, Hibiscus trionum]