A rotary lawn mower has one or more grass cutting blades that are rotatable in opposite directions about a vertical axis or axes. Each blade is double edged having front and rear cutting edges on each end of the blade. Each blade end has a front sloped face inboard of the front cutting edge and a rear sloped face inboard of the rear cutting edge. These faces create air flow within the cutting chamber when cutting grass regardless of the direction of rotation of the blade. The direction of rotation of each blade is reversed to switch between discharge and mulching modes of operation. A flow director has a curved vertical guide wall that guides the grass clippings through a discharge opening in the discharge mode and that guides the flow of grass clippings away from the discharge opening in the mulching mode.