The Collaborative International Dictionary of English v.0.48:

Flag \Flag\, n. [From Flag to hang loose, to bend down.]
   (Bot.)
   An aquatic plant, with long, ensiform leaves, belonging to
   either of the genera Iris and Acorus.
   [1913 Webster]

   Cooper's flag, the cat-tail (Typha latifolia), the long
      leaves of which are placed between the staves of barrels
      to make the latter water-tight.

   Corn flag. See under 2d Corn.

   Flag broom, a coarse of broom, originally made of flags or
      rushes.

   Flag root, the root of the sweet flag.

   Sweet flag. See Calamus, n., 2.
      [1913 Webster]