The Tridens flavus is a type of grass that has purple and oily seeds hence its names purpletop or greasegrass.
Tridens flavus, the purpletop tridens, red top, or greasegrass, is a large, robust perennial bunchgrass native to eastern North America. The seeds are purple, giving the grass its common name. The seeds are also oily, leading to its other common name, "greasegrass". 
