Sedges are grass-like plants widely distributed especially in the tropics and subtropics belonging to the genus Cyperus in the family Cyperaceae. The differences easiest to see between sedges and grasses are 1) that grass stems are usually but not always round in cross section, while sedge stems are more or less triangular; 2) sedge leaves are spirally arranged in three ranks while grasses have alternate leaves forming two ranks; and 3) leaf sheaths (lower part of a leaf enclosing the stem) in grasses are split whereas leaf sheaths of sedges are not split. Sedges are mostly found in moist areas such as wetlands, but are also common constituents of forest-margin vegetation. Like most tropical plants, they are sensitive to frost. Renewed attention is being paid to these plants due to their critical role in wetland ecosystems.

Genus Cyperus comprises about 650 species of which 207 are found in East Africa. Some of the commonly occurring species in the region include:

C. blysmoides (C. bulbosus var. spicatus) (Water Grass): It is a perennial sedge. It is limited to the highlands (500-1800 m) of East Africa (Kenya, Tanzania and Uganda) and it is also present in Ethiopia. It is abundant as a weed in some upland crops such as coffee and wheat. It is propagated by bulbs (they are flattened stems bearing fleshy leaves and serve as organs of storage and vegetative reproduction).

C. rigidifolius (Highland Nut Sedge):
It is a perennial sedge that is widespread in the highlands (1200-3350 m) of East Africa. It can be a problem in pastures, and for pyrethrum and other crops, especially where there is little or no cultivation. It is propagated by rhizomes (a root-like underground stem) and tubers.

C. teneristolon (C. transitorius; Kyllinga pulchella):
A perennial sedge localized in the uplands of Kenya and Tanzania. It is also present in Ethiopia. It is a weed of crops in the highlands. It is propagated by rhizomes and seed.

C. papyrus (Papyrus, Papyrus Sedge or Paper Reed):
It is a stately aquatic member of the sedge family. It is a herbaceous perennial native to Africa. This tall, robust, aquatic plant can grow 4 to 5 m (13 to 16 ft) high. It forms a grass-like clump of triangular green stems that rise up from thick, woody rhizomes. Each stem is topped by a dense cluster of thin, bright green, thread-like stems around 10 to 30 cm (4 to 10 in) in length, resembling a feather duster when the plant is young. Greenish-brown flower clusters eventually appear at the ends of the rays, giving way to brown, nut-like fruits. Papyrus sedge (and its close relatives) has a very long history of use by humans, notably by the Ancient Egyptians'. It is the source of papyrus paper, parts of it can be eaten, and the highly buoyant stems can be made into boats. It is now often cultivated as an ornamental plant.

C. papyrus grows in full sun, in wet swamps and on lake margins throughout Africa, Madagascar and the Mediterranean. In deeper waters it is the chief constituent of the floating, tangled masses of vegetation known as sudd. Papyrus ranges from subtropical to tropical desert to wet forests, tolerating annual temperatures of 20 °C to 30 °C and a pH of 6.0 to 8.5. Papyrus flowering prefers full sun to partly-shady conditions. Like most tropical plants, it is sensitive to frost. The "feather-duster" flowering heads make ideal nesting sites for many social species of birds. As in most sedges, pollination is by wind, not insects.

Mature nut-like fruits are distributed by water

C. esculentus (Yellow Nut Sedge, Earth Almond, Tiger Nut Sedge or Chufa Sedge):
C. esculentus is a light green perennial sedge growing to about 1 m in height with solitary stems growing from a tuber. The stems are triangular in section, and bear slender leaves 3-10 mm wide. The flowers of the plant are distinctive, with a cluster of flat oval seeds surrounded by four hanging leaf-like bracts (modified leaves from where flowers arise) positioned 90 degrees from each other. The plant foliage is very tough and fibrous, and is often mistaken for a grass.

Distribution and ecology
It is present in over 20 African countries. It is widespread above 500 m in Kenya, Tanzania and Uganda but most common in the highlands (1500-2100 m). It is widely distributed in arable land and irrigated areas.

Host range and damage
It can be a serious weed of coffee, cotton, groundnut, maize, pineapple, rice, sisal, soybean, sugarcane and vegetables. If left unchecked, C. esculentus can cause a yield loss of over 40% in maize and soybean.

Good use
It produces tubers which are nearly spherical, about 10 mm in diameter, dark brown in colour. Tubers are edible with a slightly sweet, nutty flavour. It is commercially grown for these tubers in some countries (e.g. Burkina Faso, Ghana, Mali, Nigeria and Spain). The tubers are quite hard and are generally soaked in water before they can be eaten, thus making them softer and giving them a better texture. They are eaten as vegetables, made into sweets or used to produce the famous "Horchata de chufas" (a creamy white drink) of the Valencia region in Spain. They have excellent nutritional qualities with a fat composition similar to olives and a rich mineral content, especially phosphorus and potassium. The oil of the tuber was found to contain 18% saturated (palmitic acid and stearic acid) and 82% unsaturated (oleic acid and linoleic acid) fatty acids. "Horchata de chufas" can be useful in replacing milk in the diet of people intolerant to lactose to a certain extent. Since the tubers contain 20-36% oil, C. esculentus has been suggested as potential oil crop for the production of biodiesel.

Dispersal: Seed and tubers are an important means of dispersal of this species.

C. rotundus (Coco-grass, Purple Nut Sedge or Red Nut Sedge):
C. rotundus 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, reddish-brown to purplish-brown. The fruit is a three-angled achene (small, dry, not opening when ripe, one-seeded).
The root system of a young plant initially forms white, fleshy rhizomes. Some rhizomes grow upward in the soil, and 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.

Distribution:
It is a species of sedge native to Africa, southern and central Europe (north to France and Austria), and southern Asia. C. rotundus is one of the most invasive weeds known, having spread out to a worldwide 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

Host range
Primary hosts: Gossypium spp. (cotton), Saccharum officinarum (sugarcane), Citrus spp., Coffea spp. (coffee), Arachis hypogaea (groundnut), Zea mays (maize), Oryza sativa (rice).
Secondary hosts: Syzygium aromaticum (clove), Cocos nucifera (coconut), Allium cepa (onion), Capsicum annuum (bell pepper), Fragaria sp. (strawberry), Agave sisalana (sisal hemp), Sorghum, Glycine max (soybean), Camellia sinensis (tea), Lycopersicon esculentum (tomato), Triticum (wheats).

Damage
C. rotundus is considered the worst weed based on its occurrence in 52 crops in 92 countries and its capacity to cause substantial yield losses. Examples of crop loss caused by C. rotundus include 35-89% in vegetables, 30% in cotton and 75% in sugarcane harvest. Even the growth of tree crops can be reduced, for example, mulberries in Japan, citrus in Israel and coffee in Kenya. Much of this can be attributed to the capacity of C. rotundus to remove nutrients from the soil and store them in its tubers, making them unavailable to crops. Adding nitrogen to a crop can actually improve the competitiveness of C. rotundus, causing even greater crop loss than where no fertilizer is added. C. rotundus undoubtedly competes with crops for water but also for light when it grows tall enough. There is evidence that extracts from C. rotundus suppress the growth of plants but it is difficult, under field conditions, to separate the effects of allelopathy (the roots releasing substances harmful to other plants) from competition.

Good use
It is claimed that C. rotundus is an important medicine in India and China and noted for its use by pharmaceutical companies to produce diuretics, anthelminthics and treatments for coughs, bronchial asthma and fever. It makes a poor fodder but has value in binding together soil. However, its negative attributes as a weed far outweigh its usefulness.

The typical life cycle of C. rotundus starts with growth of the apical bud of a tuber. As the tuber shoot extends, it swells to form a basal bulb (sometimes called a corm), usually near the soil surface, from which an aerial shoot and roots are produced. Up to three or four rhizomes develop from each basal bulb, extending 5-30 cm before turning up to form a further basal bulb and, in more mature plants, these rhizomes may form dormant tubers, without any aerial shoot. Further branch rhizomes develop, however, from lateral buds on the tuber and eventually, branched chains of rhizomes and tubers become an extensive, underground network. Meanwhile, the new shoots grow and produce flowers within 3-8 weeks of emergence if stimulated by short photoperiods of 6-8 hours. Most seeds of C. rotundus are not viable.

Tuber dormancy is high on undisturbed sites and may last for at least 7 years. A growing tuber exerts dormancy on others in the same rhizome/tuber chain, and fragmentation of the network by cultivation breaks this dormancy. Hence, cultivation stimulates the growth of C. rotundus. Large populations of shoots and tubers can develop from a single tuber; as many as 600 plants have been produced in a single year. It has been reported that 2-3 million tubers per hectare per week can be produced during active growing periods, yielding 30-40 million tubers per hectare. Forty tonnes of underground organs have been produced on one hectare in a year, giving C. rotundus a phenomenal capacity for vegetative reproduction. Most tubers are found in the upper 15 cm of soil but they can grow down to 30 cm or more in favourable conditions.

It has low tolerance of shade, a property that can be exploited in controlling this weed by crops with dense canopies. Temperature has a marked effect on the germination of tubers. Sprouting has been reported at temperatures of 13-43°C but the range varies with biotype. Desiccation kills tubers but the duration and temperature of the drying period affects this process. The critical moisture level for tuber germination seems to be in the region of 11.5-15%.

Dispersal:
Dispersal of C. rotundus occurs when tubers are moved by tillage equipment or other farm machinery. Flood water may also carry the tubers. Contaminated soil in nursery stock is an avoidable but common method of dispersal. Under natural conditions, a population of C. rotundus extends its boundary by a few metres in a year. Spread by seed is generally regarded as being unimportant.

Habitat
C. rotundus is widespread in the tropics and subtropics, growing in almost every soil type, elevation, humidity, soil moisture and pH, but not in soils with a high salt content. Its range at increasing latitudes and altitudes is limited by cold temperatures. It occurs in cultivated fields, fallow land, neglected areas, road and rail sides, banks of irrigation canals and streams, edges of woods and sand dunes. Generally, it does not tolerate shade.

This illustration shows the methods promoted on Infonet-Biovision. The methods shown at the top have a long-term effect, while methods shown at the bottom have a short-term effect. In organic farming systems, methods with a long-term effect are the basis of crop production and should of preference. On the other hand methods with a short-term effect should be used in emergencies only. On Infonet-Biovision we do not promote synthetic pesticides.

Tillage has little effect on C. esculentus when tubers are dormant during the off season: the tubers are then less susceptible to desiccation than those of C. rotundus. Conversely, when tubers have sprouted, and before new stolons and tubers are formed, it is much more susceptible than C. rotundus to disturbance and desiccation by tillage. Hence, pre-sowing cultivations prior to late sowing of crops can cause useful reductions, and inter-row cultivation in the crops (e.g. maize, soybean etc.) can also be effective at an early growth stage when the reserves of the parent tuber are newly depleted. Other measures that may help in suppression of C. esculentus include shading, improved drainage and crop rotation.

It is extremely difficult to remove C. esculentus completely from fields. It is considered an intrusive weed. This is due to the plant having a stratified and layered root system, with tubers and roots being interconnected to a depth of 50 cm or more. The tubers are connected by fragile roots that are prone to snapping when pulled, making the root system difficult to remove intact. The plant can quickly regenerate if a single tuber is left in place.

Successful cultivation depends on destroying the tubers of C. rotundus by exposing them to desiccation or by exhausting the food reserves. This can be especially effective under hot dry conditions, but requires the necessary power to disturb rhizomes and cut roots to at least 15 and preferably 20-25 cm depth. Repetition may be needed. Under moist soil conditions, a single cultivation may only serve to spread and aggravate the problem and it is essential that tillage is repeated as necessary to prevent re-establishment from fragments of the rhizome/tuber system. This could necessitate cultivating every 2-3 weeks until the crop forms a canopy to suppress further growth of the weed. In practice this consumes much time and energy and could be detrimental to soil structure

Organic mulch made from crop residues provides temporary suppression of C. rotundus but a film of 1000-gauge polyethylene is an effective barrier to growth which can be used in nurseries and high value field crops.

Solarisation with black plastic is not fully successful, as tubers deeper in the soil survive and sprout, piercing the plastic as they emerge. With clear plastic, however, the sharp shoots become leafy in the light as they emerge, and since are trapped under the plastic are killed by the high temperature.

The main phytosanitary risk is C. rotundus tubers attached to farm machinery or on the roots of crop plants moved from place to place. Introduction is limited by prohibiting the import of plants with soil around their roots, and by cleaning farm equipment before transporting to other sites.

It is questionable whether a cosmopolitan weed like C. rotundus with a huge regenerative capacity would be a good target for biological control but it is claimed that several fungi are promising candidates for classical biocontrol including Puccinia conclusa, P. philippinensis and Phytophthora cyperi and they warrant evaluation as possible mycoherbicides.

Arthropod herbivores of C. rotundus that have been investigated as biological control agents include Athesapeuta cyperi, Chaetococcus australis, Bactra minima, B. venosana and B. verutana.