Martians standing amongst the Red Grass
The Red Sward is a type of Martian grass, scarlet in color. It is found mostly in fertile regions such as the Valley Dor where it is a known food source of the savage Plant Men, and the cities of red men who keep it well groomed in lawns.
The word 'sward' means lawn, turf, sod. ERB refers to the Barsoomian variety as being red in color, as in "running over the red sward".