Common names: Bullet Grass, more...
Synonyms: Panicum airoides R.Br., more...
Species Description: Torpedo grass, Panicum repens, is a hardy perennial grass occurring widely throughout Florida but not native to North America. Shoots, up to 1 m tall, arise from sturdy, creeping or floating rhizomes. The rhizomes are pointed with torpedo-like growing tips. Aerial stems are rigid, erect to leaning, with lower portions often wrapped in bladeless sheaths. The upper leaf sheaths are mostly glabrous with hairs usually restricted to the upper margins. The ligule (thin outgrowth at the junction of the leaf and leafstalk) is a short-ciliate membrane. Leaf blades are narrow, linear, stiffly ascending to 25 cm long and 0.6 cm wide. The inflorescence is a loose open terminal panicle 7-22 cm long with with few to many branches and stakled spikelets that are 2-3 mm long and approximately 1 mm wide, with flower parts white with yellow. The small seeds are smooth and whitish (Langeland and Burks 1998, CAIP, ISSG).
Potentially Misidentified Species: Several Panicum congeners similar in appearance occur in Florida. Beachgrass (Panicum amarum), redtop panicum (P. rigidulum), and maidencane (P. hemitomon) all can be found within the IRL watershed counties). Details of the rhizomes, leaves, and inflorescence must be studied to distinguish among species.
Regional Occurrence: Believed native to Europe or Australia Panicum repens is now widely distributed throughout the tropics and subtropics from approximately 43° north latitude to 35° south latitude (Holm et al. 1977).
In the U.S. southeast, the plant can be found from Florida to the Gulf coast of Texas, and northward along the Atlantic Coast to North Carolina. The species has also been introduced to California and Hawaii, where it has become a pest plant to the sugarcane industry (Holm et al. 1977, Godfrey and Wooten 1979, Small 1933, Langeland and Burks 1998). By 1995 P. repens was reported as naturalized in 75% of Florida counties (Wunderlin and Hansen 2004). As of April, 2007, the Florida Exotic Pest Plant Council (FLEPPC) reports the species as present in 57 of 67 Florida counties.
IRL Distribution: Panicum repens occurs along the entire east coast of Florida, including the six IRL watershed counties.
Age, Size, Lifespan: Panicum repens is a perennial grass that grows to 1 m in height.
Abundance: Panicum repens is a marginal species that occurs in and around shallow water. It is found in more than 70% of Florida's public water bodies. The largest Florida P. repens infestation, nearly 6,000 ha, is the infestation occurring in and around Lake Okeechobee (Schardt 1994, Bodle and Hanlon 2001).
Reproduction: Panicum repens can reproduce sexually through flowering and seed production, though the principal means of propagation is reported to be rhizome extension and fragmentation (Holm et al. 1977).
Small P. repens fragments can give rise to dense clonal stands through vegetative growth and rhizome fragments buried as deep as 50 cm can send up new shoots (Hossain et al. 1999, Brecke et al. 2001). The lack of apical dominance also allows each node the ability to produce axillary buds along entire rhizome lengths (Wilcut et al. 1988). Hossain et al. (2001a) reports that a single culm emerging from a single rhizome bud produced approximately 23,000 rhizome buds in a single year.
Embryology: Populations are capable of flowering and producing seeds year-round, but seed abundance and viability are highly variable (Whyte et al. 1959, Peng and Twu 1979, Wilcut et al. 1988).
Temperature: Torpedo grass distribution is restricted to the tropics and subtropics. The above-ground portions of the plant are easily killed by frost (FAO undated).
Hydrology: Panicum repens is most often encountered as riparian species in wetland and marginal habitats. It grows well even after several days in standing water, but is also drought-tolerant and can be found growing in heavy upland soils (Sayer and Lavieren 1975, Holm et al. 1977, Langeland and Burks 1998). The species is adapted to areas with significant seasonal rainfall and will not survive extended hot dry seasons (FAO undated).
Salinity: Panicum repens exhibits moderate salinity tolerance. It is reported to grow on saline sands in western Zambia (Verboom & Brunt 1970).
Trophic Mode: Autotrophic (photosynthetic).
Associated Species: Panicum repens tends to produce dense monocultures capable of displacing native vegetation.
Invasion History: The original native range of Panicum repens is in dispute. The type specimen from which the species was first described by Linnaeus in 1762 was collected from Spain, although several authorities suggest the native range of the species to be Australia. The first description of this species (as P. airoides) from Australia dates to 1810, therefore it is clear that the species has been historically present in both locations.
P. repens was intentionally introduced to the southeastern United States in the late 19th century as a cattle forage grass (Tarver 1979). It was introduced to the Gulf Coast prior to 1876, at which time it was first collected outside of cultivation near Mobile, Alabama (Beal 1896; Yarlett 1996). The plant was cultivated in Florida from at least the 1920s to the 1950s, becoming common in the state during this time and planted in most southern Florida counties and also in some central and north-central counties by 1950 (Hodges and Jones 1950).
Since this initial introduction P. repens has escaped cultivation to spread into natural and agricultural lands in Alabama, Florida, Louisiana, Mississippi, North Carolina, South Carolina, and Texas. The plant has also been introduced to California and Hawaii (USDA, NRCS 2004).
Potential to Compete With Natives: The impressive vegetative reproductive capacity of Panicum repens allows it to quickly produce dense monocultures capable of displacing native vegetation, particularly in wetland and marginal habitats where the species thrives (Shilling and Haller 1989, Bodle and Hanlon 2001).
Growth, vegetative reproduction, and spread of the species in agricultural areas are facilitated by tilling and fertilization (Hodges and Jones 1950, FLEPPC 2003).
Possible Economic Consequences of Invasion: Sutton (1996) notes that worldwide Panicum repens is one of the most invasive exotic grasses of terrestrial, wetland and aquatic habitats of the tropics and subtropics. It is now listed as a Category I invasive species because of its ability to invade and alter native plant communities (FLEPPC 2005).
In addition to overtaking natural areas, P. repens invades cultivated lands and displaces more desirable forage species. Although it was originally introduced for cattle forage, torpedograss becomes tough later in the growing season and is generally neglected by livestock. Additionally, P. repens may interfere with flood control, navigation, irrigation, recreation, and turf production (Shilling and Haller 1989, Willard et al. 1998). This invasive species has been a significant culprit in Florida citrus grove infestation and has also been reported from parks, golf courses, and preserves throughout Florida (Fleming et al. 1978, Baird et al. 1983, FLEPPC 1996).
Limited management of P. repens in Florida (e.g., management in flood control systems) from 1980-1988 cost an average of $2-2.5 million per year (Schmitz et al. 1988, Schardt and Schmitz 1991).
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