Weed Management
For park management purposes, a weed is defined as any naturalised (established and reproducing in the wild) plant that is not native to Kakadu.
Weeds compete with native plants for light, moisture and nutrients and often do not provide appropriate food and shelter for native wildlife. Particularly invasive weeds reduce plant and animal diversity, change burning regimes, and alter the structure, function and species composition of natural ecosystems.
Kakadu remains one of the most weed free conservation areas in Australia. Only a small number of weeds found in the Park are considered invasive: mimosa, salvinia, para grass, mission grass, gamba grass, candle bush, calopo, Gambia pea, golden shower, poinciana and coffee bush. Of these, mimosa, salvinia and para grass are given priority for control because of their potential to spread over large areas.
You can play an important part in preventing the introduction of weeds and minimising their spread. Check your vehicles, trailers and equipment before entering the Park, keep to established roads and tracks, and don't enter quarantine areas.
Mimosa (Mimosa pigra)
A Central American woody shrub that under ideal conditions grows up to 4 metres tall and is highly invasive. Large infestations are on the Adelaide River floodplain, the Daly, Finniss and Mary Rivers and on the East Alligator floodplain near Oenpelli.
Factors that contribute to mimosa's success are a lack of natural enemies, a rapid growth rate, production of large quantities of easily transported viable seed, and a tolerance of drought and flood. Unchecked, mimosa forms impenetrable thickets across floodplains.
In Kakadu the threat posed by mimosa was identified early, and prompt action has meant that the Park is free of large mimosa infestations-it remains virtually an 'island in a sea of mimosa'. Controlling the plant takes considerable resources: since the 1980s four people have been employed full time in the Park to locate and destroy mimosa by pulling out or mattocking small plants and applying herbicides to larger ones.
Salvinia (Salvinia molesta)
Water Hyacinth (Eichornia crassipes)
These noxious weeds were discovered in the Magela Creek system in 1983. The water hyacinth was successfully eradicated but salvinia spread rapidly into other tributaries of the East Alligator River and onto the Magela floodplain. Despite quarantining of the area and cooperation from the public, a new infestation was found in Nourlangie Creek in 1990. During the wet season salvinia is flushed out of Nourlangie Creek into the South Alligator River.
A biological control agent, the weevil Cyrtobagous salviniae, was introduced soon after salvinia's discovery. It effectively controls the weed most years. Towards the end of the dry season the weevil population rapidly decreases because most of the salvinia has been eaten. During a poor wet season, high nutrient levels cause the salvinia to grow faster than the weevil population can regenerate. This results in a blanket of salvinia over the water, but as the weevil numbers increase the salvinia is reduced later in the year.
With the help of the CSIRO Division of Entomology, a Management Plan has been developed to closely monitor the weevil's effect. Floating booms are also used to contain salvinia, and occasionally a low-impact herbicide is used to prevent excessive build-up of the weed and reduce the chance of it spreading further.
Para grass (Brachiaria mutica)
was introduced to the area as pasture grass in the 1930s. Like mimosa, para grass can take over huge areas of floodplain, growing vigorously when grazing pressure is reduced and after burning. The grass is quickly filling in a number of Kakadu's wetlands and threatening wildlife habitats. Biological control is not an option at the moment since para grass is still being promoted as a valuable pasture grass for cattle outside the Park. Control involves pulling out small infestations and using herbicides in larger areas.
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