When I type "sprig" into the inventory search, it returns Icon of Spriggan, Emblem of Spriggan, etc., but not "Sprigs of Rookswort," etc., even though that is what I see when I mouse over a stack of more than one Rookswort, Rubeweed, or Gandlevery in my inventory.
P.S.
If you drop all but one sprig, it loses its sprigginess. Apparently sprigs only happen in little posses.