Paspalum distichum
(knotgrass)

P. distichum is a fast-growing rhizomatous grass of wet areas. It has become a major weed of rice and many other crops, as well as occurring in uncropped wetlands in both its native and introduced regions. Its introduction to Europe, Asia and the Pacific is not well documented but apparently occurred many years ago. New records are reported in e.g. Indonesia, Spain and Croatia, suggesting that it continues to spread in countries to which it has been introduced.

P. distichum has almost world-wide distribution in tropical and subtropical regions where temperature and moisture conditions favour its C4 growth habit. Distribution is throughout North America, except Canada; Mexico, Central America, and all of South America; southern Europe; southern former USSR; the Middle East; the Indian subcontinent; South-East Asia; China; Japan; Korea Republic and Korea Democratic People's Republic; the Philippines; Australia; New Zealand; and the Pacific Islands. It occurs in southern and North Africa, but not in East, West or Central Africa.

It is far from clear just where P. paspalum is truly native. It is generally accepted that it is native in N. and S. America and introduced to Europe and most of Asia. PIER (2009) suggests that it is native in the Pacific area, though introduced to Hawaii, Australia and New Zealand. Other sources suggest it could also be native in southern Africa and in Australia.

There are highly significant risks of further spread of P. distichum into regions not already infested – especially northern sub-sahelian Africa, which has so far escaped invasion. The conditions, especially in West Africa are likely to be very favourable and the risks to the important rice growing areas there and in East Africa are high. Although spreading mainly vegatatively once established, P. distichum does produce abundant fertile seed which could be transferred as a contaminant of other crop seeds, or in hay or forage. As P. distichum has shown useful characteristics as an aid to soil conservation or as a livestock feed, there could be misguided deliberate introduction.

According to Häfliger and Scholz (1980)P. distichum is found in wasteland areas, rotation crops, perennial crops and aquatic habitats, but not too frequently on grassland.

P. distichum is a non-submerged aquatic plant commonly occurring in streams and alluvial flatlands in the tropics and subtropics, and throughout the world. It populates still or moving water to a depth of one metre or more; it may also be a problem in merely irrigated conditions. It is a C4 plant (Mesleard et al., 1993) adapted to semi-aquatic environments (Datta et al., 1979) and is not cold tolerant.

P. distichum has the capacity to bind soil in streams which are subject to erosion in the tropics, and provides grazing in flat coastal regions and stream beds. It is a valuable pasture grass on alluvial flats (Chase, 1929; Bor, 1960). Attempts have been made to select strains with high palatability and limited regenerative capacity (Ikeda et al., 1988). P. distichum has a capacity for explosive growth which restricts its use for binding soil in intensively cultivated areas (Li, 1981). The weed spreads readily by seed and vegetative reproduction, by its numerous stolons and rhizomes, to become a persistent weed (Smiley, 1922; Wilkinson and Jacques, 1979; Huang et al., 1987). The percentage of hemicryptophytes in the ruderal (riverbank) environments of North Africa is increasing and becoming a significant problem (Le-Floc'h, 1991).

P. distichum occurs in ditches and wet, but rarely brackish, areas as far north as Washington, USA, and as far south as Argentina and Chile; also on temperate coasts of the Eastern Hemisphere. Most references to this species occurring in saline coastal areas are attributable to confusion with P. vaginatum, but Leithead et al. (1971) describe both species and comment that P. distichum ‘grows primarily on fresh water marshes and occasionally on brackish marshes. Tolerates moderate salinity and standing water’. It is a serious weed in areas where irrigated crops such as rice or cotton are the main crops in rotation. Thus, it is an important weed in the natural floodplains and wetlands in India, and in the paddy fields of Japan, China and the Philippines. However, it can proliferate in drier conditions provided abundant water is present for at least part of the growth cycle. It can infest orchards, asparagus plantings, or vineyards which are irrigated. It is found in highland pastures in Thailand in years of heavy rainfall, but is considered as a tropical lowland pest in this region (Falvey and Hengmichai, 1978; Datta et al., 1979). The plant is predominantly lowland, rarely found at altitudes greater than 300 m (Robbins et al., 1951; Townsend et al., 1968). In Europe it has come to dominate riparian and wet alluvial plains in Portugal (Bernez et al., 2005) and in Greece (Stroh, 2006).

Genetics

Hexaploid (2n=60) and tetraploid (2n=40) populations are common (Kadono, 1985; Shibayama, 1988; Echarte et al., 1992), and Echarte et al. (1992) have documented the existence of pentaploids (2n=50) and several hyperpentaploids (2n=52, 54, 57 and 58) in Argentina. Bor (1960) also records 2n=48.

P. distichum exhibits considerable variability, thought to result from both the heteroploidy shown by the species, and the existence of genetic differences among individuals of the same cytotype (Echarte et al., 1992).

Reproductive Biology

P. distichum has a high capacity for asexual reproduction. Under suitable conditions, almost every node of the aerial shoots (stolons) and rhizomes are capable of rooting and producing new plants. Under optimum conditions, P. distichum can elongate at a rate of 15-20 cm per week (Noda and Obayashi, 1971). Okuma et al. (1983) reported that within 2-3 years of planting overwintering stems of P. distichum on the bank of a stream, the entire surface of the stream was covered with a floating mat of the weed.

The weed also produces many seeds. Seed production by P. distichum has been estimated at 100 seeds per panicle and was about 100,000/m² in a river studied by Okuma and Chikura (1984). About 5-10% of the seeds produced were viable and the maximum, minimum and optimum temperatures for seed germination were 40, 20 and 30°C, respectively. Even at the optimum incubation temperature, 60% of viable seeds remained dormant. Exposure of seeds to 16-h daylength at optimum temperatures of 28-35°C increased germination from 14 to 40% (Huang and Hsiao, 1987). Pre-chilling seeds at 6°C and a 2-h heat treatment at 40°C had no effect on germination, and treatment with gibberellic acid (GA3) had little effect. Treatment of the seeds with oxidants including sodium hypochlorite, hydrogen peroxide and concentrated sulphuric acid indicated that seed dormancy was imposed mainly by seed coverings, including the hull and seed coat membranes. Seeds play an important role in the winter survival of P. distichum in regions where temperatures approach the critical limit for the species (Shibayama, 1988). 

In Argentina, Echarte et al. (1992) confirmed that P. distichum is an out-crossing species, though in China Ma GuoHua et al. (2003) concluded that the tetraploid P. distichum could be considered to be a facultative apomict reproducing by apospory. Quarín and Burson (1991) likewise concluded that it reproduces by aposporous apomixis and Srivastava and Purnima (1990) refer to it as an obligate asporous apomict with only aposporous embryo sacs. Seeds mature 20-30 days after flower emergence (Manuel et al., 1979). 

Physiology and Phenology

Single node segments of P. distichum sprout faster from stems than from rhizomes, and the rooting of shoots on contact with the soil stimulates new shoot production (Manuel and Mercado, 1977). Stolons root readily at the nodes, and elongation rates of up to 15-20 cm per week have been observed at high temperatures (Noda and Obayashi, 1971). On average, the maximum, optimum and minimum temperatures for sprouting and rooting of P. distichum segments are 40, 30-35 and 10°C, respectively (Huang et al., 1987). Shoots of P. distichum collected in different seasons differed in their sprouting and rooting responses at different incubation temperatures, providing a possible survival mechanism for the species. The rate of sprouting, rooting and the early growth of both single-node shoot and rhizome segments increased with incubation temperature up to 30°C, and then declined at 40°C. Growth was poor at 10°C. Single-node stem segments sprouted faster in a 16-h light regime than in darkness. Rooting was optimum in the dark at low temperatures; and in the light at high temperatures. Stems of P. distichum are kept in reserve by apical and bud dominance, to replace those destroyed under adverse conditions; a further survival mechanism for this species. Water, nutrient and light levels which favour photosynthesis and reduced transpiration are favourable to axillary bud growth (Liu et al., 1991).

P. distichum flourishes at high temperatures and is tolerant of high moisture conditions (Mesléard et al., 1993), as is characteristic of many C4 species.

Environmental Requirements

P. distichum is quite sensitive to shade. Plants kept at short daylengths (8 h) have smaller leaves and internodes than those kept at longer daylengths which tend to prolong maturation; plants in all treatments flower in 14-16 days. Deep (10-15 cm) water levels delay flowering. 

Response to climatic conditions in New Zealand has been studied by Campbell et al. (1999). As a C₄ plant it showed less tolerance to cold conditions than C₃ species.

P. distichum flourishes in wet soils but these are generally aerobic rather than anaerobic (Green and Brock, 1994). Tolerance of salinity is somewhat uncertain. There are many references to the occurrence of the weed in coastal areas, but possible confusion with the salt-tolerant P. vaginatum makes many of these references unreliable. Leithead et al. (1971) do however describe both species and comment that P. distichum ‘grows primarily on fresh water marshes and occasionally on brackish marshes. Tolerates moderate salinity and standing water’. 

Relatively few natural enemies have been recorded for P. distichum and none have been proposed as potential biocontrol agents. The most significant natural enemy is the ergot fungus Claviceps paspali, infection with  which can lead to serious toxicity to livestock.

Animal feed, fodder, forage

• Fodder/animal feed
• Forage

Environmental

• Amenity
• Erosion control or dune stabilization
• Host of pest
• Land reclamation
• Revegetation
• Soil conservation

P. distichum is sometimes confused with Paspalum vaginatum but is distinguishable by the slightly more turgid spikelets, a glume and sterile lemma which are not papery, a more pronounced midrib, and a glume which is at least obscurely pubescent (Chase, 1929). P. distichum is also distinguished by the unusually variable size of the lower glume. P. vaginatum is almost always associated with saline or brackish conditions, especially near coasts, but also in saline soils inland (e.g. Arabia), whereas P. distichum does not generally occur under these conditions.

Many other species of Paspalum can occur as weeds. One of the most widespread of these is a complex of closely-related taxa which include P. commersonii and P. orbiculare but which are usually known as P. scrobiculatum (qv). This is widespread in Asia and Africa. It is not likely to be confused with P. distichum as it has a tufted habit whether spreading or erect, annual or perennial, but without runners or rhizomes. The inflorescence also differs in having spikelets almost round in outline.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

SPS Measures

P. distichum is listed as an invasive alien plant in Europe (EPPO, 2009), requiring member countries to minimise the risks of introduction, and of spread within their countries.

Control

Cultural control and sanitary measures

Methods used to control P. distichum differ depending on whether the crops in which the weed is growing are produced under flooding or irrigation. Farmers in the Philippines consider P. distichum and Echinochloa spp. to be the most difficult weeds to control (Fajardo and Moody, 1990). Herbicides are expensive and are used sparingly, but acceptable levels of control can be achieved due to the high seeding rates, high fertilizer rates, and the use of integrated cultural practices. Flooding and deep burial inhibit the growth of P. distichum as oxygen is required for sprouting from stem cuttings centering around the joints. No sprouting was observed from severed creeping stems of P. distichum which were buried in puddled soil or submerged in water. Most sprouting occurs from overwintering stems lying above the water; almost none occurs from submerged overwintering stems. Flooding reduces the competitive advantage of P. distichum with respect to rice (Manuel and Mercado, 1977; Manuel et al., 1979). P. distichum is sensitive to shade (Manuel and Mercado, 1977; Manuel et al., 1979; Ito et al., 1987) and higher crop seeding rates, closer distances at transplanting, and the use of taller, more competitive cultivars are, therefore, potential control measures.

The effects of no-tillage systems on weed development in a variety of crops was investigated by Gong and Li (1991). The underground vegetative portions of P. distichum tend to be shallow, with aggressive shoot development. No-tillage systems are not feasible in areas heavily infested with P. distichum and minimum tillage is, at the very least, necessary for satisfactory yields of semi-dwarf rice (Datta et al., 1979; Bernasor and Datta, 1988).

In some regions nitrogen levels are increased by encouraging Azolla growth and P. distichum has emerged as a major weed under such regimes (Janiya and Moody, 1987; Krock et al., 1988).

In the Keoladeo National Park, India, P. distichum was apparently suppressed by the grazing of water buffalo. After removal of these animals, the weed expanded but was later brought under some degree of control by the grazing of wild pigs, geese and deer (Middleton, 1998). 

Physical/mechanical control

Thorough preparation of the land during the 30 day period prior to transplanting, augmented with handweeding, provides effective control of P. distichum in the Philippines (Rahman, 1992). Increasing tillage frequency from one to three harrowings substantially reduces stands of the weed (Bernasor and Datta, 1988). In fields infested by seeds, overwintering rhizomes and stems, cultivation or diskings which normally eradicate small seedlings, only help to propagate the weed by cutting the stems and rhizomes into small pieces. Ploughing often promotes the dominance of plants with bulbs, stolons or rhizomes (Hsiao and Huang, 1989a). However, deep ploughing is an effective method of control (Noda and Obayashi, 1971), as the depth of emergence is less (3.25 cm on average) than that of other perennial plants of the Cyperaceae found in similar environments (Okuma et al., 1983). Moisture conditions are important for the success of mechanical disturbance: under wet conditions control may depend on deep burial, whereas under seasonally dry conditions, shallow tillage may suffice.

Samantha et al. (1995) emphasise the need for two hand weedings to control P. distichum in rice in Bangladesh. 

Solarisation has been used with some success in Spain (Dalmau et al., 1993). 

Chemical control

P. distichum tolerates most of the popular pre- and post-emergent herbicides. Resistance to post-emergent herbicides is attributed to anatomical features such as: thickening of the cell membranes of the epidermis, cortex and central portion; poor development of vascular bundles, and no air capacity in the central portion; and the accumulation of starch granules in the cortex and central regions (Noda and Obayasi, 1971). The accumulation of starch is partly responsible for the regenerative ability of isolated segments of the stem or rhizome. Glyphosate is the most widely used herbicide for the control of P. distichum (Okuma and Chikura, 1985; Manuel et al., 1979). Thiobencarb (Shad, 1986; Singh, 1992), molinate (Srinivasan and Subbian, 1991), haloxyfop or quizalofop (Rahman and Sanders, 1991), pretilachlor (Llorente and Evangelista, 1990), butachlor (Pradhan and Chettri, 1987; Bajwa et al., 1985), fluazifop (Okuma and Chikura, 1985), sethoxydim (Parker, 1982), paraquat (Manuel et al., 1979) and fluroxypyr plus triclopyr (Schultz, 2005) can also provide effective control, depending on the crop. Metolachlor, oxyfluorfen, prodiamine, and pendimethalin were most effective in leeks (Gilreath et al., 2008). Nicosulfuron is moderately effective in maize (James and Rahman, 1997). 

Newer herbicides for rice include fenoxaprop (Reddy et al., 2000), bispyribac-sodium (Wang Qiang et al., 2000; He JinHao and Ma ZhaoJiang, 2000), pyanchor (pyribenzoxim) (Zhou XiaoJun et al., 2000), bensulfuron methyl and prosulfuron-ethyl (Sumiyoshi, 2000). 

Glufosinate is also effective in conjunction with transgenic glufosinate-tolerant rice varieties (Yu LiuQing et al., 2005). 

The use of pre-planting herbicides can reduce the need for extensive land preparation. The 30-day preparation period required to effectively control P. distichum can be reduced to 15 days when combined with glyphosate treatment. The combined use of low-tillage practices and herbicide treatment is increasing in popularity in areas where labour costs are high.

Biological Control

There are currently no biological control methods for P. distichum. The weed is an alternative host for many of the fungi, viruses, bacteria and nematodes that afflict crops in which the weed occurs.

12/08/2009 Updated by:

Chris Parker, Consultant, UK