C. difformis  is a sedge which is listed in Holm's list of the world's worst weeds, being a problem especially in rice, sugarcane, tea and maize. It is a dominant weed in direct-seeded rice when it occurs in high plant densities; forms dense mats of vegetation in the young crop and can cause rice yield losses of 12-50%. It is native to the tropics of the Old World, but is spreading well outside its native range. It has a relatively short generation period of as little as 4 to 6 weeks from seed to seed. It can spread along waterways and grow in disturbed sites, lake margins and on river banks.

Cyperaceae (the sedge family) has a cosmopolitan distribution and plays a dominant role in wetland vegetation. Based on recent molecular phylogenetic studies, the family Cyperaceae consists of two main clades, corresponding to the subfamilies Cyperoideae and Mapanioideae (Simpson et al., 2003; Muasya et al., 2009). In the Cyperoideae, two clades stand out because of their extraordinary species diversity: (1) the clade corresponding to the predominantly temperate tribe Cariceae (approximately 1950 species); and (2) the clade corresponding to the mainly tropical tribe Cypereae (approximately 1120 species). Together, they cover nearly three-fifths of the species diversity in Cyperaceae (Larridon et al., 2013). Cyperus difformis was first described by Linnaeus (1756). It originated in the Old World tropics and is one of approximately 950 species in the genus Cyperus (Stevens, 2012).

C. difformis varies in height from 6 to 80 cm. The stems are smooth, triangular, slightly winged and 0.7-3.0 mm thick. The roots are numerous, fibrous and reddish. The leaves are smooth (or slightly scabrid on the midrib and margin), flat, linear, 5-25 cm long or often two-thirds of the plant height, 2-6 mm wide, sometimes reduced to sheaths. Sheaths are tubular, united, green to reddish-brown and without leaf blades at the base.

The inflorescence consists of dense, globose, umbellate heads, simple or compound, 5-15 mm in diameter, with 10-60 stellately spreading spikelets. The inflorescence is rather loose, simple or compound, subtended by 1-4 leaf-like bracts, one of which can be up to 25 cm long. The umbel rays are 1-5 cm long, some long and some long peduncled.

Spikelets are linear to oblong-linear, compressed but slightly swollen, obtuse, 2.5-8 mm long, 0.8-1.25 mm wide, 6-30 flowered. Glumes are 0.6-0.8 mm long, obovate, pale-yellowish to dark reddish-brown with yellow or white margins and a green midrib ending in a short mucro. Stamens 1-2. Style 3-branched. Achenes are 0.6-0.8 mm long and 0.3-0.4 mm wide, triangular, obovate-elliptic, yellowish-brown or pale-brown, minutely papillose. These descriptions are based on Haines and Lye (1983) and Holm et al. (1977).

C. difformis is native to the Old World (subtropical and tropical areas). Currently it is widespread throughout southern Europe, Asia, Central America, North America, South America, Africa and the islands of the Indian and Pacific oceans. It is mainly a weed of the tropics and subtropics but can be found from latitudes 45°N to 35°S (Holm et al., 1977).






 

The history of introduction of C. difformis in the New World is not well known. Because this species behaves as an agricultural weed, it is highly probable that it was introduced as a contaminant of crop seeds (Holm et al., 1979; USDA-ARS, 2014). In the Caribbean, it was collected in the 1960s in Puerto Rico and the Dominican Republic (US National Herbarium).  

The risk of introduction of C. difformis is high. This species has been listed as one of the world's worst weeds, being a problem especially in rice, sugarcane, tea and maize (Holm et al., 1979). It can grow in swamps, lake margins, and along the edges of rivers and canals. Because the species spreads along major waterways and in rice production areas, its likelihood to invade new habitats remains high.

C. difformis is readily distinguished by its dense, globose heads composed of many radiating spikelets. However, as a seedling or non-flowering plant, it closely resembles many immature, annual Cyperus spp. In a key to seedling Cyperaceae, Kostermans et al. (1987) use the coleoptile, seed, first leaf blade and first leaf sheath as diagnostic characters. Unlike C. iria, C. difformis releases no aroma when crushed.

C. difformis is usually found on flooded or very wet soils, open soggy grasslands, pools (but not in deep water) and riverbanks, often associated with C. halpan and C. iria, where it is usually scattered but often becoming dominant. It prefers fertile soils but can also grow on poor sandy or clay soils (Kostermans et al., 1987). It is one of the commonest weeds of paddy or flooded rice but it has also been reported as a weed of upland rice and crops such as bananas, sugarcane, tea and maize (Holm et al., 1977). In general, C. difformis can be found growing in grasslands on mountain slopes, in shallow water, water margins, lake margins, riversides, swamps, wet places in grasslands, along trails, and in rice paddy fields. 

C. difformis is an annual sedge, sometimes behaving as a perennial. It propagates from seeds (achenes or nutlets) which are produced in large quantities. In Italy, Jacometi (1912) reported that one plant could produce 50,000 seeds, with about 60% germination. Such fecundity enables C. difformis seedlings to become established at high densities, enabling it to rapidly cover the ground and become the dominant vegetation. In tropical climates, the plant can flower and produce seeds throughout the year, provided that sufficient moisture is present.

C. difformis is a weed of flooded rice, even where there is good water control, being capable of germinating under water. Civico and Moody (1979) note that this species has exceptional tolerance of flooding, greater than that of C. iria.

Studies in India have shown that chromosome numbers in C. difformis (n=16) do not vary but the plant exhibits marked phenotypic plasticity which is characteristic of individuals belonging to different populations/clones (Bir et al., 1992).

The ability of C. difformis to complete its vegetative and reproductive life cycle within one month makes it a very competitive weed. It is a not a particularly tall plant but it can have a high biomass per hectare. It competes with crops mostly for nutrients and water, rather than for light.

Like rice, C. difformis has the C3 photosynthetic pathway, a condition which favours growth in submerged soils.

There are no known natural enemies of C. difformis other than those pests and diseases of rice for which it is an alternative host. For further information, see Economic Impact.

C. difformis is rated by Holm et al. (1977) to be a serious weed of rice in the United States, Australia, Italy, Japan, Madagascar, Taiwan and the Philippines, and a principal weed of rice in several European and African countries. In California, this plant has become resistant to rice-field herbicides. In Asian rice production, where herbicides are not used, this weed may account for 60-70% of the total biomass of the rice field. It is difficult to separate the competitive effects of C. difformis from those of other components of the weed flora, but 12-50% reductions in rice grain yields have been caused by this weed (Ampong-Nyarko and DeDatta, 1991). The rate of appearance of C. difformis and tiller number are the main factors causing yield loss in early rice (Yu, 1992). The costs of controlling C. difformis, whether manual, mechanical or chemical, are significant.

C. difformis is a host for several pests of rice. Meloidogyne incognita, a major root-knot nematode in southern Nigeria, can grow and reproduce on C. difformis (Atu et al., 1988). Eggs of the rice stem borer, Diopsis macrophthalma, have also been found on this weed in southern Nigeria (Alghali, 1979). The node-feeding black bug, Scotinophara latiuscula, an occasional pest of wetland rice fields in the Philippines, can develop on C. difformis (Barrion and Litsinger, 1987). The brown planthopper, Nilaparvata lugens can use C. difformis as a temporary food plant (Chu and Yang, 1984). Larvae of the rice leaf-folder, Marasmia patnalis, can survive for a short period on C. difformis before migrating to the rice crop (Joshi et al., 1985). Pathogens of rice that have been reported on C. difformis include Puccinia conclusa (Dube et al., 1979), Sarocladium oryzae (cause of sheath rot) (Balakrishnan and Nair, 1981) and Thanatephorus sasakii (cause of sheath blight) (Bandara and Nadaraja, 1979).

C. difformis can impact natural wetlands and native vegetation swamps and flooded areas. Having heavy seed production and massive seedling densities, this rapid growing weed can quickly form dense colonies smothering native vegetation (Holm et al., 1979; USDA-NRCS, 2014). 

The experienced eye can detect the seeds of C. difformis as a contaminant of crop seed

The principles of good weed management in rice, such as those advocated by Ampong-Nyarko and DeDatta (1992), are applicable to C. difformis in rice and in other crops. These include the need to prepare clean seedbeds, prevent seed production, establish a healthy and vigorous crop and avoid contamination of crop seed at harvest.

C. difformis is susceptible to many of the usual methods of weed control in rice and other crops. These include hand-pulling, manual and mechanical tillage and trampling in puddled fields. Flooding of rice fields to a depth of 20 cm strongly suppresses the growth of C. difformis (Williams et al., 1990) but with the continuous use of the more usual shallower flooding, C3 weeds, including C. difformis, become dominant (Ampong-Nyarko and DeDatta, 1992).

Biological Control

At present there appears to be little scope for biological control of C. difformis, but good suppression of growth is possible where a thick mat of Azolla pinnata is established on the water surface of irrigated ricefields (Janiya and Moody, 1984).

Chemical Control

Many herbicides are approved for use in rice but their use is dictated by the conditions used to grow the crop, e.g. irrigated, rainfed lowland, upland and deepwater. C. difformis is susceptible to the following herbicides commonly used in rice: bensulfuron, bentazon, bifenox + 2,4-D, butachlor, butralin, 2,4-D, MCPA, oxyfluorfen, pendimethalin, piperophos + dimethametryn, pretilachlor + safener (e.g. fenclorim), propanil, thiobencarb and thiobencarb + 2,4-D.

Other rice herbicides active against C. difformis are cinmethylin and chlomethoxyfen (Ampong-Nyarko and DeDatta, 1991). Pyributicarb is reported to have excellent activity against C. difformis (Tsuzuki, 1990). Paraquat and glyphosate can both be used as non-selective, post-emergence herbicides against C. difformis, such as for land preparation using zero tillage. Resistance to bensulfuron has been reported in the USA and Australia (Heap, 1997).

Integrated Weed Management

Integrated weed management is recommended for cost-effective weed control.