flower-of-an-hour

from The Century Dictionary.

• noun The bladder-ketmia, Hibiscus Trionum, the flower of which is open only in mid-day.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun annual weedy herb with ephemeral yellow purple-eyed flowers; Old World tropics; naturalized as a weed in North America