Two plants, both perennial monocots, are commonly known as nut grass or nutgrass. They are related, but distinct. Cyperus esculentus is also known as chufa sedge, yellow nutsedge, tigernut sedge, and earthalmond. Cyperus rotundus is variously called coco-grass, purple nutsedge, and red nutsedge.
Sedges, Not Grasses
Despite the common name nut grass, both species are in fact sedges, which only superficially resemble grasses or rushes. Sedges are identified by their triangular stems, with leaves growing spirally and arranged in three ranks. Grasses have alternate leaves in two ranks, and their stems are generally round. Botanically, the sedge family is the Cyperaceae, while the grass family is the Poaceae.
Wherever they are found, both are regarded as invasive or noxious weeds. C. rotundus is classified as an invasive weed in more than 90 countries, which has led to it being called "the worst weed in the world." C. rotundus is native to Africa, southern and central Europe, and southern Asia. C. esculentus is found in the warm temperate to subtropical regions of the Northern Hemisphere. C. esculentus has three varieties found in the U.S.---hermannii, leptostachyus, and macrostachyus.
Above ground, C. esculentus and C. rotundus are quite similar. They are most easily distinguished by their below-ground features. C. esculentus has one tuber (the part commonly called the nut), which is basically round and edible, with a pleasantly nutty flavor. C. rotundus has a connected system of more elongated or sausage-shaped tubers or nuts, which are hard and bitter.
The tubers of C. esculentus are highly nutritious, and are rich in potassium and phosphorus. They contain from 20 to 36 percent oil, which is about 18 percent saturated and 82 percent unsaturated. In "Domestication of Plants in the Old World" (Oxford University Press, 2000) authors Daniel Zohary and Maria Hopf describe C. esculentus as "among the oldest cultivated plants in Ancient Egypt." The tubers are also grown for food in several parts of Africa.
The tubers of C. rotundus are used by primitive tribes only as a emergency food source. These tubers are also allelopathic---they emit a substance in the soil that deters the growth of other plants and can prevent the germination of seeds.