Gamba grass (Andropogon gayanus) is a large, perennial tussock grass from tropical Africa that has become a serious invasive species globally, especially in northern Australia. It was initially introduced for beneficial purposes, such as pasture improvement and mine-site rehabilitation, but it has since spread beyond managed areas. It poses significant ecological and economic challenges, transforming landscapes and increasing fire risks.
Identifying Gamba Grass
Gamba grass is a robust perennial grass that forms dense tussocks, often reaching 4 meters in height and 70 centimeters in diameter. Its leaves are elongated, 30 to 60 centimeters long and 1.5 to 5 centimeters wide, featuring a distinct white midrib. The leaves, stems, and spikelets are often covered with soft hairs.
The flowering stems are loosely branched, displaying triangular-shaped seedheads with multiple groups of primary branches holding hairy flower spikelets arranged in pairs. The seeds are small, measuring 2-3 mm long and 1 mm wide, and range from light brown to brownish-black.
Environmental and Fire Hazards
Gamba grass forms dense stands that produce a substantial amount of biomass, significantly increasing fuel loads in infested areas. Landscapes invaded by gamba grass can carry eight times higher fuel loads compared to native grass savannas. This elevated fuel density leads to intense bushfires, which can result in the permanent loss of trees and other native plants, potentially transforming woodlands into grasslands. Such fires are also more difficult to control, posing increased risks to property, human safety, and cultural sites.
The invasive grass outcompetes native vegetation, leading to a reduction in local biodiversity. It alters soil conditions by changing nitrogen availability, increasing ammonium levels while decreasing nitrate and total inorganic nitrogen. Gamba grass impacts water cycles, using significantly more water than native grasses, which can reduce water levels in downstream wetlands and watercourses. The dense growth also reduces food and habitat for native wildlife, disrupting ecosystem balance.
Where Gamba Grass Spreads
Gamba grass thrives in dry tropical climates, adapting to areas with annual rainfall between 400 and 3000 mm, particularly those with a distinct dry season of three to nine months. It predominantly grows in full sun and tolerates only light shade, making open woodlands, disturbed areas, and roadsides ideal habitats. The plant reproduces primarily by seed, with an individual plant capable of producing up to 244,000 seeds annually if not grazed.
While 90% of gamba grass seeds fall within 5 meters of the parent plant, their light nature allows dispersal by wind, water, and animals. The spread is also aided by human activities, including the transport of contaminated hay and mud attached to vehicles and machinery. Gamba grass is most prevalent across northern Australia, including Western Australia, the Northern Territory, and Queensland, where it has infested thousands of square kilometers.
Controlling Gamba Grass
Managing gamba grass infestations requires a combination of strategies, with early detection and rapid response being important for effective control. Mechanical removal methods, such as slashing or hand-pulling, can be effective for smaller, isolated infestations, particularly before plants produce seeds. However, mechanical methods alone may not be sufficient for widespread infestations due to the plant’s ability to regrow from its root system.
Herbicide application is a widely used and effective control method, especially on young or regrowing plants. Gamba grass is most susceptible to herbicides during its flowering period. Controlled burning can also be used, particularly early in the wet season, to remove old growth and promote new shoots that are more vulnerable to herbicide treatment. Avoid burning plants with mature seeds, as updrafts can spread lightweight seeds over long distances.
Community involvement and preventative measures are also important in limiting the spread of gamba grass. This includes maintaining healthy native pastures, as good ground cover can offer some resistance against new invasions. Preventing the transport of seeds through contaminated hay, vehicles, and machinery is another important preventative action. Strict biosecurity measures, such as those under the Biosecurity Act 2014 in Queensland, prohibit the sale, giving away, or release of gamba grass into the environment.