The Collaborative International Dictionary of English v.0.48:

Matweed \Mat"weed`\, n. (Bot.)
   A name of several maritime grasses, as the sea sand-reed
   (Ammophila arundinacea) which is used in Holland to bind
   the sand of the seacoast dikes (see Beach grass, under
   Beach); also, the Lygeum Spartum, a Mediterranean grass
   of similar habit.
   [1913 Webster]