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Cyperus rotundus L. 1753
pronounced: sy-PEE-russ roh-TUN-duss
(Cyperaceae – the sedge family)
common name: Nut Grass, Purple Nut Sedge
This native of tropical Asia is a major weed in most tropical and subtropical parts of the world. It is a perennial plant that usually grows to about 30 cm in height, although it can grow much taller. In irrigated crops such as cotton it can grow as high as 80 cm. The ‘nut’ part of the common name comes from its tubers, that look a little like nuts, although botanically they have nothing to do with nuts. As with many other sedges, the leaves sprout in ranks of 3 from the base of the plant. There is a purple colouring in the first few layers of leaves at the leaf base. This colour then fades to white in successive layers. The flower stems are triangular in cross-section, 1–3 mm across. Seed heads occur at the ends of the stems and may be compact to spreading with 3–9 branches. The flower is bisexual, with 3 stamens and a 3-stigma carpel. The seeds are brown to almost black in colour, and are a little less than 2 mm in length. Seed is produced prolifically, but seedlings are only very rarely produced: the plant spreads by its tubers.
The tubers are brown and firm, and milky white inside. They are usually 1.5–2 cm long and up to 1 cm wide, varying from globular to elongated in shape. These tubers are produced on the ends of underground rhizomes, with a new plant typically producing 4–8 new tubers every 4–6 weeks. A new shoot arises from each new tuber and forms a new plant and starts the process all over again.
Nut Grass is a major weed of river systems and irrigation. It is present in most inland river systems and is a major problem in most crops there. It is especially difficult to manage in summer rainfall areas and perennial crops such as lucerne. It is highly competitive with cotton. In that crop it spreads very quickly, a single tuber at the beginning of the season can produce up to 2,000 tubers by harvest time. If not treated correctly, heavy infestations can completely suppress cotton production. It is a difficult weed to manage, but can be more-or-less controlled with strategic cultivation and glyphosate applications.
As well as competing for soil nutrients with the crops, the plant is alleopathic, its roots releasing substances harmful to other plants. Many herbicides will kill the plant’s leaves, but few have any effect on the tubers. Weed pulling in gardens usually results in the breakage of the roots, leaving the tubers in the ground to grow again. Ploughing distributes the tubers in the field, spreading the infestation. Tubers cut by the plough will still produce new plants. Not for nothing is it often termed ‘the world’s worse weed’.
Despite its bad reputation, it does have some beneficial uses. In traditional Chinese medicine, it is considered the primary qi regulating herb. Indian Ayurvedic medicine uses the plant for treating fevers and disorders of the digestive system. Arabs of the Levant traditionally use roasted tubers, while they are still hot, or hot ashes from burned tubers, to treat wounds, bruises and boils. Despite the bitter taste of the tubers, they do have some nutritional value, and are used as a famine food in parts of Africa. The tubers are an important nutritional source of minerals and trace elements for migrating birds such as cranes.
The Nutgrass Borer Bactra venosana feeds on this plant.
Information about medicinal qualities of plants, or about their use as medicines, is for interest only, and is not intended to be used as a guide for the treatment of medical conditions.
Photographs taken at Nelly Bay, 2010
Page last updated 11th November 2016