Para Grass is an exotic grass that threatens wetlands across northern Australia. Introduced into Australia in the late 19th century, it is now established in coastal floodplains across the Top End between Darwin and Nhulunbuy. It is a particular problem on the floodplains of the Mary River and Magela Creek, as well as on Croker Island. A small population was recorded on Bathurst Island in the 1990s.
Para Grass is an extremely vigorous grass that spreads by producing new roots along robust branching runners. It chokes wetlands and has been recorded suppressing native plant diversity by up to 75%. At risk are a range of plants that grow in permanently or seasonally flooded wetlands, including tiny plants like bladderworts, Typhonium and ground orchids, as well as the more robust Wild Rice and Arrowleaf Monochoria. Para Grass has been used to suppress regrowth of the noxious weed Mimosa. Changes in vegetation structure and composition wrought by Para Grass render habitat unsuitable for species as diverse as Water Mouse, Howard Springs Toadlet and Yellow Chat. Many fish, including Barramundi, cannot survive in the oxygen-depleted waters Para Grass creates. Fires fuelled by Para Grass kill trees. Successive fires can eventually reduce complex vegetation communities, with a range of species and life-forms, to monospecific grassland.
The Northern Territory Parks and Conservation Masterplan lists Para Grass as a significant environmental weed in 11 out of 13 bioregions in the Top End. However, the Northern Territory Government still recommends its use as a pasture grass in wet or seasonally flooded areas in this region. These are exactly the type of habitats in which it is most invasive, and in which its use is now actively discouraged in Queensland. It is listed as a high impact weed in the Field Guide to Assessing Australia’s Tropical Riparian Zones and recognised as a Key Threatening Process to biodiversity in northern Australia.
While herbicides are not registered for the treatment of Para Grass in the Northern Territory, elsewhere spraying with glyphosate has been found to be effective. The best level of control is attained by spraying actively growing plants. Roundup Biactive® can be used to control Para Grass with no adverse effects on other wetland species. Grazing can be used to keep Para Grass in check, but not eliminate it. Another solution, used in Townsville, is to burn patches of Para Grass in the late dry season. These areas are returned to open water, allowing Wild Rice to recolonise.