Introduction:
Blady grass (Imperata cylindrica)
Native to Asia, cogon grass (Imperata cylindrica) is common in the humid tropics and has spread to the warmer temperate zones worldwide. Cogon grass is considered to be one of the top ten worst weeds in the world. Its extensive rhizome system, adaptation to poor soils, drought tolerance, genetic plasticity and fire adaptability make it a formidable invasive grass. Increases in cogon grass concern ecologists and conservationists because of the fact that this species displaces native plant and animal species and alters fire regimes.
Description:
Although Imperata cylindrica can have leaf blades of up to 1.5 m tall in conditions of good soil moisture and fertility, the majority of its biomass occurs below ground comprising greater than 60% of the total biomass. Cogon grass is stemless erect perennial growing in loose to compact tufts with slender flat linear-lanceolate leaves arising from the rhizomes. The scabrous leaves are 4 to 10 mm wide with prominent white midribs that are slightly off center. The leaves may be 15 to 150 cm tall, depending on habitat, with narrow sharp points.
Occurs in:
Agricultural areas, coastland, natural forests, planted forests, range/grasslands, riparian zones, ruderal/disturbed, scrub/shrublands, urban areas, wetlands.
Habitat description:
Imperata cylindrica occurs in a wide range of habitats, including degraded forests, grasslands, arable land, and young plantations within tropical and subtropical climates with 75 to 500 cm of annual rainfall. It can be found growing in almost all eco-types from the dryest flatwoods to the
Blady grass (Imperata cylindrica) Distribution in North America shown in Green.
margins of permanent bodies of water. Cogon grass has invaded areas from highly xeric, upland sites to fully shaded, mesic sites. It invades sandhills, flatwoods, hardwood hammocks, sand dunes, grasslands, river margins, swamps, scrub, and wet pine savanna communities. It is known to occur from latitudes 45°N (Japan) to 45°S (New Zealand) and from sea level to over 2,000 m elevation.
Cogon grass typically does not invade closed forests unless they are degraded for agriculture or lumbering. It is very successful in areas that are frequently burnt, overgrazed, or intensively cultivated and rapidly colonizes such disturbed sites. A high root-rhizome to shoot ratio provides I. cylindrica a substantial source of regeneration following cutting or burning. Its rhizomes are very resistant to heat and breakage and may penetrate soil up to 1.2 m deep, but generally they only reach the top 0.15 m in heavy clay soil and the top 0.4 m in sandy soils. Its capabilies of recovery and colonization after fire allow it to take advantage of slash and burn forestry practices.
A fast-growing species, I. cylindrica thrives in areas of minimal tillage, such as orchards, lawns, and roadsides. It does not generally survive regular, deep tilling. While cogon grass is tolerant of a wide variety of soil conditions, including variations in fertility, organic matter, and moisture, it grows best in relatively acidic soils (pH 4.7). Temperature markedly affects shoot and rhizome growth, with increased growth occurring at 29º/23ºC (day/night). Temperatures of -4.5°C or lower for exposure periods of 24 hours were found to be lethal to rhizomes. While in general rhizomes do not exhibit extreme cold hardiness, stands of cogon grass may survive temperatures as low as –14ºC.
Management information:
Blady grass (Imperata cylindrica) close up.
Preventative measures: A Risk assessment of Imperata cylindrica for the Pacific region was prepared by Pacific Island Ecosystems at Risk using the Australian risk assessment system. The result is a score of 19 and a recommendation of: reject the plant for import (Australia) or species likely to be a pest (Pacific).
Chemical control: Herbicides may temporarily control I. cylindrica foliage up to 12 to 24 months. Without the re-establishment of desirable species, viable rhizomes will eventually allow cogon grass to re-infest the area. Controlling rhizomes with herbicides is a difficult task. A combination of glyphosate applied at 2.8 kg ai ha_1, followed by fertilization and reseeding with Bermuda grass (Cynodou doctylon) showed that rhizome reserves were sufficient for cogon grass to recover from both treatments. In another study, cogon grass recovered after imazapyr was applied at 2.24 kg ai ha_1 in a 2-year-old loblolly pine plantation. Even these relatively high herbicide application rates there is still a remnant population of viable rhizomes that has the potential to re-infest the treated site. Further research is needed to integrate herbicide usage, which provides short-term control and a ‘‘window for re-establishment’’, with bio-control using desirable, yet highly productive plant species for long-term control.
Recommendations to control cogon grass in the southeastern USA are to treat infestations in autumn (May through to October) with glyphosate and/or imazapyr herbicides. These recommendations are consistent with a wide range of studies conducted. Fluazifop is also an effective option. Guidelines for herbicide control are detailed by Demers and colleagues (2008).
Integrated management : Shade, repeated herbicide application, and mechanical control have all been used to control I. cylindrica . An integrated approach that combines burning, tillage (mechanical control) and chemical applications provide the best approach for cogon grass management. Cogon grass should first be burned or mowed, preferably in summer, to remove excess thatch and older leaves. Subsequent regrowth (of one to four months) will reduce rhizome biomass and allow herbicides to target actively growing leaves which maximises herbicide effectiveness. Once control of cogon grass has been achieved planting of desirable vegetation should occur as quickly as possible to prevent reinvasion.
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