Possible Clues:

• Plant, first thing for summer border
• It's a rush
• Battlefield lacking moor plant with triangular stems
• The southern side of the grassy plant
• Coarse grassy plant
• Plant growing in swampy ground
• The southern border is by the water
• Triangular-stemmed marsh grass
• Coarse, grasslike plant
• Plant uses sulphur, gaining slight advantage