Pennisetum polystachion
(mission grass)

P. polystachion is a vigorous annual or perennial grass growing to over 1 m height, producing large numbers of seeds with limited dormancy. It is a natural invader of disturbed ground, typically occurring in fallow land in its native region in Africa. It has been widely introduced to Asia, Australia and Central and South America, both deliberately, as a potential forage plant, and accidentally. In some of these areas, especially Thailand and northern Australia, it has flourished, spreading along roadsides into both agricultural and natural habitats, sometimes in national parks and nature reserves, reducing species diversity and greatly increasing the risk of damaging fires. It is listed as a Federal Noxious Weed in USA and as a Class B and C Noxious Weed in the Northern Territory, Australia.

Annual
Biennial
Grass / sedge
Herbaceous
Perennial
Seed propagated

There is some uncertainty over the original native range of P. polystachion. Many sources assume natural distribution in Asia as well as Africa. USDA-ARS (2008), however, indicates that P. polystachion sensu stricto is native to Africa only. In India Bor (1960) indicates it is ‘only an introduction’. The status of the perennial form ‘P. setosum’ is equally uncertain as USDA-ARS (2008) fails to identify a native range, which may perhaps include South America.

The risk of introduction appears to be moderately high. In the USA, P. polystachion has been repeatedly intercepted by border clearance personnel from the USDA as a contaminant of imported commodities. It has been intercepted in miscellaneous cargo from Mali, imported seeds from Chad and straw packing from West Africa (Westbrooks, 1989). Smither-Kopperl (2007) notes that in the years 2005 and 2006, P. polystachion was among the 3 most commonly intercepted noxious weeds at the air, sea and land borders of Washington State, USA. P. polystachion is listed as a Federal Noxious Weed in the USA (Westbrooks, 1989), and is listed as a Class B and C Noxious Weed in the Northern Territory, Australia (Parsons and Cuthbertson, 1992).
 

P. polystachion occurs in farmlands, grasslands, upland tropical hills and croplands including perennial crops, especially after forests have been cleared. It is also found in waste places. It is a troublesome weed of plantation crops in Africa, India, upland South-East Asia, Indonesia and the Philippines. It has been grown as an ornamental in Sri Lanka and the Pacific Islands, and as a pasture species in India. It is cultivated in northern Australia. It has been grown experimentally in the Royal Botanic Gardens in Sydney, New South Wales, Australia, and in northern New South Wales, but rarely becomes established outside the tropics (Holm et al., 1977; Hafliger and Scholz, 1981; Westbrooks, 1989; Parsons and Cuthbertson, 1992). It can become dominant in fire climax or subclimax savannahs when soil fertility has declined (Holm et al., 1977). In Fiji, it has invaded hilly, former forest lands that will not support livestock (Parham, 1955), up to 1000 m altitude, also in stream beds and roadsides (PIER, 2008). In Hawaii it colonises dry to mesic exposed areas in agricultural areas and disturbed forest, mostly lowland but to over 2000 m altitude (PIER, 2008).

In addition to the crops listed, many other annual and perennial crops are affected, together with pasture, turf and forestry species.

Genetics

Renno et al. (1995) and Schmelzer and Renno (1999) report a range of ploidies in P. polystachion. The sexually reproducing diploid form, known from Burkina Faso, has 2n=18. Apomictic tetrapoid forms are also common in Africa (2n=36), with pentaploid (2n=45) and hexaploid, (2n=54) forms less common. A wider range of ploidy levels is recorded from hilly regions of West Africa. ‘P.subangustum’ is also recorded as diploid, tetraploid and pentaploid, while the perennial ‘P.setosum’ form is known only as a hexaploid in Africa and also in Brazil (Techio et al., 2002), but pentaploid in Bolivia (Norrmann et al., 1994). Genetic variability is greatest in the diploid forms but some does also occur in the polyploids.

Reproductive Biology

Both annual and perennial forms reproduce almost exclusively by seed. The perennial form, ‘P.setosum’, is distinguished mainly by being a tufted or shortly rhizomatous perennial but also by being less branched (Bor, 1960). In the Philippines, it was found that a single plant can produce up to 65 tillers each with eight panicles, resulting in 330,000 seeds. The first tiller emerged 39 days after transplanting or after formation of the fourth or fifth leaf and after application of fertilizer. Percentage germination of dehulled seeds in the dark was consistently lower than in the light up to 77 days after harvesting. Germination of freshly harvested seed is inhibited by darkness, blue and far-red light, and exposure to very high temperatures. When kept in darkness, the seeds did not start germinating until 7 days after harvest whereas in the presence of light, 31% of the seeds germinated when sown 1 day after harvesting, confirming that the seeds are light sensitive. The importance of light was confirmed in Malaysia by Ismail et al. (1994) who showed an increase from 24% germination in the dark to 59% in the light, and an optimum temperature for germination of 35^°C; also by Kiatsoonthorn and Tjitrosemito (1992) who obtained an increase from 2% in the dark to over 40% in the light. Exposure to very low temperatures completely suppressed germination (Fernandez, 1980).

Laboratory experiments in Thailand revealed that the percentage germination of P. polystachion seeds (collected from Kao Kaelae, Thailand) after 1 month of storage was low (27.2%), but after 2-4 months was >90%. Percentage germination declined after 4 months storage to 28% after 12 months. Seeds collected in December had a higher rate of germination than those collected in November or January. Seeds with a diameter of 0.64-0.76 mm also exhibited higher germination rates than those with a diameter of 0.56-0.64 mm. Fertile seed production in spikes gradually decreased from 62.4% in the early part of January to 3.4% in the middle of April. However, the number of spikelets gradually decreased from approximately 175 per culm in the early part of January to 150 per culm in the middle of February, and then increased again to 225 per culm in the middle of April (Kiatsoonthorn, 1991). One study in Australia suggests that seed longevity is very limited, perhaps due to lack of dormancy, such that after a year less than 1% of viable seeds remained (Setterfield et al., 2004).

Phenology

In northern Australia, the seeds of P. polystachion germinate at any time after the opening rains of the wet season (October-November). Rapid growth follows seedling establishment, and flowering commences in late January or February and continues into the early dry season. Growth ceases in June or early July in the southern hemisphere (Parsons and Cuthbertson, 1992).

The first visible sign of the reproductive stage is elongation of the internodes of the primary tiller. The flag leaf forms 18 days after onset of reproduction, followed 2 days later by emergence of the panicle from the flag leaf. The spikelets ripen 28 days after onset of reproduction (Fernandez, 1980).

Environmental Requirements

P. polystachion is a tropical grass, well adapted to soils of low fertility, and occurs mainly in grasslands on sandy soils where native (Weber, 2003). The plant is moderately tolerant of shade. In Thailand, growth was almost normal under only 40% full light, but was severely reduced at only 10% full light (Kobayashi et al., 2003).

Few natural enemies of P. polystachion have been recorded. Waterhouse (1993) lists those records there are, which include species of gall midges, Cecidomyiidae, from Africa and India. Some of these are known pests of pearl millet (P.glaucum), but others may be host specific. He also lists records of fungal pathogens, some may also be host specific but many of them attack crop plants.

Animal feed, fodder, forage

• Fodder/animal feed
• Forage

Environmental

• Boundary, barrier or support

Medicinal, pharmaceutical

• Traditional/folklore

In the field, P. polystachion is a vigorous, robust, tussock-forming grass (in clumps) that grows from 2 to 3 m tall. It is characterized at maturity by a spike-like seedhead.

P. polystachion is spread mostly by its highly viable seeds. However, the grains alone cannot be used for positive identification (Holm et al., 1977). Shipments of crop seeds from countries or localities that are infested with P. polystachion should be closely examined upon arrival in non-infested areas. Infested shipments should be thoroughly cleaned to minimize further spread.

P. polystachion and P.pedicellatum both belong to a section of the genus Pennisetum which is characterized by a tridentate lower lemma, decurrent sharp-angled wings on the main axis, and an upper floret that is harder in texture than the lower floral bracts. In addition, the inflorescence of both species is a spike-like panicle and the spikelets are subtended by plume-like bristles. However, P. polystachion can be distinguished from P. pedicellatum by having a single, sessile spikelet in the involucre, and the lower glume minute or small. In P.pedicellatum, there are up to 5 spikelets, at least one of them on a short pedicel. Also the bristles are densely woolly, giving the inflorescence a thicker, softer texture, and the lower glume is 1-3 mm long. Some other species may also cause confusion, especially in Africa.

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.

Prevention

P. polystachion is spread mostly by its highly viable seeds. However, the grains alone cannot be used for positive identification (Holm et al., 1977). Shipments of crop seeds from countries or localities that are infested with P. polystachion should be closely examined upon arrival in non-infested areas. Infested shipments should be thoroughly cleaned to minimize further spread.

 Control

As with other annual weeds, the primary concern in control of P. polystachion is to prevent further seed production and to eliminate the seed bank in the soil. Due to successive germinations and the risk of erosion, repeated cultivation is not a practical control method. Instead, a combination of cultivation, herbicides and, in pastures, careful grazing management provides the best results. The grass is also susceptible to fire (Gupta and Trivedi, 2001).

Cultural control and sanitary measures

In rubber plantations in Thailand, shading by the legumes Canavalia ensiformis, Lablab purpureus and Mucuna pruriens provided enough shading to greatly suppress P. polystachion (Kobayashi et al., 2003).

Physical/mechanical control

Seed production can be prevented by mowing or cutting of plants before flowering (Watson, 1986). Isolated plants can be hand pulled or dug out.

Chemical control

The range of herbicides suggested includes glyphosate, glufosinate, paraquat, fluazifop-butyl, haloxyfop-methyl and imazapyr. All are most effective on young plants, or on regrowth following slashing. Longest control is provided by imazapyr, but use of this compound is limited to very few crops. Working with the perennial form, ssp.setosum, good control of young plants is possible with fluazifop-butyl, fenoxaprop-ethyl, glyphosate, glufosinate and paraquat. Faiz (1999), working in rubber in Malaysia also had good results with glyphosate, imazapyr and glufosinate; also with MSMA, but paraquat was less effective. On established plants, best results were obtained with imazapyr, metsulfuron and haloxyfop.

In soyabeans in Brazil, good results are obtained with various combinations of imazethapyr, imazaquin and sethoxydim with bentazon, fomesafen or clethodim; also with haloxyfop-methyl, propaquizafop, fenoxaprop-P-ethyl and fluazifop-P-butyl. These are often combined with a mineral oil and/or surfactant (Barros et al., 1992a,b). Miller (2006) describes that thick stands of P. polystachion in pastures may be treated with glyphosate using a rope-wick applicator, as soon as the weed overtops young pasture plants. In cotton, maize, or sorghum, pre-emergent or post-emergent directed applications of atrazine, ametryn, diuron, fluometuron or paraquat are effective (Parsons and Cuthbertson, 1992).

Recently, P. polystachion in tea plantations in Sri Lanka has been treated by application of diuron on plants that are less than 45 cm high before emergence of inflorescence or flowering. Surfactants may also be used. Phytotoxic effects are seen within 1 week of spraying and complete kill is observed in 2-3 weeks, but the herbicide will not kill mature plants in flower. Glyphosate can also be used for control but is expensive; dalapon is ineffective (Watson, 1986).

10/03/2008 Updated by:

Chris Parker, Consultant, UK