Cyperus rotundus (coco-grass, purple nut sedge, red nut sedge) is a species of sedge (Cyperaceae) native to Africa, southern and central Europe (north to France and Austria), and southern Asia.
It is a perennial plant that may reach a height of up to 40 cm. The names "nut grass" and "nut sedge" (shared with the related species Cyperus esculentus) are derived from its tubers that somewhat resemble nuts, although botanically they have nothing to do with nuts.
As in other Cyperaceae, the leaves sprout in ranks of three from the base of the plant. The flower stems have a triangular cross-section. The flower is bisexual and has three stamens and a three-stigma carpel. The fruit is a three-angled achene.
The root system of a young plant initially forms white, fleshy rhizomes. Some rhizomes grow upward in the soil, then form a bulb-like structure from which new shoots and roots grow, and from the new roots, new rhizomes grow. Other rhizomes grow horizontally or downward and form dark reddish-brown tubers or chains of tubers.
Despite the bitter taste of the tubers, they are edible and have a nutritional value. The plant is known to have been eaten in Africa in famine-stricken areas.
Cyperus rotundus is one of the most invasive weeds known, having spread out to a world-wide distribution in tropical and temperate regions. It has been called "the world's worst weed" as it is known as a weed in over 90 countries and infests over 50 crops worldwide. In the United States it occurs from Florida north to New York and Minnesota and west to California and most of the states in between.
Its existence in a field significantly reduces crop yield, both because it is a tough competitor for ground resources, and because it is allelopathic, the roots releasing substances harmful to other plants. Similarly, it also has a bad effect on ornamental gardening. The difficulty to control it is a result of its intensive system of underground tubers, and its resistance to most herbicides. It is also one of the few weeds that cannot be stopped with plastic mulch.