Sentence:  From the air, the bright green grassy floor of the tundra looks as smooth as a golf course fairway, but a closer inspection reveals peaty tussocks of crabgrass called <WSD>niggerheads</WSD> spaced just far enough apart for a foot to twist clumsily between. Walking on the frost-formed <WSD>niggerheads</WSD> is very much akin to walking across a gymnasium floor covered by thousands of glued-in-place softballs. 
Meaning of the word marked with <WSD> tags:  A tussock (clump of plant material), found on tundra.