The flora of the Outer Hebrides had been described by a succession of botanists since the 18th century, however a comprehensive account of the flowering plants and ferns was not published until 1991 (Pankhurst and Mullin). As a result of extensive survey work since 2002 the distribution of the flowering plants and ferns is now well documented.
The composition and diversity of the vegetation is influenced by the climate, geology, geographical position of the islands and the island effect (the size of the islands and proximity to the mainland). This combination of factors has produced an assemblage of vascular plants that is generally described as relatively small but interesting.
The non-vascular plants (mosses, liverworts, freshwater and marine algae) whilst not as comprehensively studied are important elements in the plant biota. A number of Sites of Special Scientific Interest have been designated in recognition of the importance of their bryophyte communities or their populations of aquatic plants and algae.