Environmental issues

Pests and weeds

Why are introduced perennial grasses considered a threat?

Why is the 'invasion of native plant communities by exotic perennial grasses' a threat to biodiversity?

Since 1788, many perennial grass species have been introduced to Australia. Some species have adverse impacts on biodiversity and agriculture and are recognised as weeds. For example, the Commonwealth Government has listed serrated tussock and Chilean needlegrass as two of 20 Weeds of National Significance. In NSW, these species and pampas grass, giant Parramatta grass, African lovegrass and giant rat's tail grass have been listed as noxious weeds for parts of NSW under the Noxious Weeds Act.

What are the impacts of some exotic perennial grasses on biodiversity?

Increasing evidence that some perennial grass species have significant adverse impacts on biodiversity has led to the listing of 'invasion of native plant communities by exotic perennial grasses' as a key threatening process. A few examples follow:

  • Coolatai grass grows vigorously, forming an almost complete monoculture and replacing native grass and wildflower species. It tolerates drought, heavy grazing and many herbicides. It has invaded large areas of grassy woodlands and native pastures in north-west NSW and is spreading rapidly in other regions.
  • Chilean needlegrass has several features which give it a competitive advantage over many native species, such as its ability to produce a large, long-living seed bank, high survival of seedlings, tolerance to drought and effective animal-borne and water-borne dispersal mechanisms for seeds.
  • Serrated tussock infests more than a million hectares in southern Australia, but has the potential to spread over a much larger area. It invades native grasslands, grassy woodlands, dry forests and rocky shrublands. Serrated tussock forms large tussocks, with individual plants capable of producing more than 10,000 seeds annually. Some seeds remain viable in the soil for more than 10 years. Mature plants droop across the ground, smothering other species.
  • Pampas grass readily tolerates saline conditions, salt spray, drought, periodic inundation, severe frosts and strong winds. It can grow on a wide range of soil conditions and light conditions, from shaded areas through to full sunlight. Pampas grass is an aggressive coloniser and can form dense stands which prevent other plants from growing. Individual flower heads produce more than 100,000 seeds, and wind may disperse seed for several kilometres.
  • Perennial grasses, such as perennial veldtgrass, pampas grass, Coolatai grass and buffel grass, produce large amounts of plant matter which dries quickly and causes fuel loads to increase. This fuel results in fire regimes that favour the spread of these perennial grasses. Hotter and more frequent fires may lead to changes in the structure of the vegetation and in some cases to local extinctions of some plant and animal species.
  • Species such as Coolatai grass, Chilean needlegrass, serrated tussock, and invasive forms of African lovegrass are undesirable in pasture because of their low palatability to stock or low nutritional value at certain times of the year.
  • On Montague Island, the proliferation of kikuyu interferes with the nesting of adult fairy penguins. Kikuyu forms dense mats across the surface of the ground and blocks burrow entrances. Consequently, some chicks die of starvation.
  • In certain situations, phalaris spreads from pastures, drainage ditches and road verges into adjacent native vegetation. Dense stands of phalaris can smother native ground plants and reduce the growth of young shrubs and trees.

Which native ecological communities and species are threatened by exotic perennial grasses?

Several endangered ecological communities are threatened by exotic perennial grasses. Examples include the following:

Aren't some exotic perennial grasses important pasture and horticultural species?

Perennial grasses such as phalaris are important components of pastures in many areas of the tablelands and higher rainfall areas of the western slopes. They are valued because of their productivity under grazing, and the amount and nutritional value of the herbage they produce. When grown in swards containing one or more complementary species, for example annual or perennial legumes or annual grasses, they also resist invasion by weed species such as vulpia and thistles.

Kikuyu is an important pasture species for dairy cattle in some coastal areas and is used widely as a lawn.

Pastures containing perennial grasses have important environmental benefits. In comparison with pastures and crops based on annual species, they reduce deep drainage (groundwater recharge) and thereby reduce the development of dryland salinity. Pastures based on perennial grasses also have reduced the rates of soil acidification.

Some of these perennial grasses will continue to be grown widely in pastures or lawns, and have the potential to spread into natural ecosystems where they can adversely affect biodiversity.

Has anyone else recognised 'invasion of native plant communities by exotic perennial grasses' as a threat to biodiversity?

The Commonwealth Government has developed a list of Weeds of National Significance, which places serrated tussock and Chilean needlegrass among the 20 weeds of National Significance in Australia. National strategies have been developed for both these weed species.

In NSW, the following species are listed as noxious weeds for parts of NSW under the Noxious Weeds Act:

  • pampas grass
  • African lovegrass
  • Chilean needlegrass
  • giant Parramatta grass
  • giant rat's tail grass.

More information

Page last updated: 10 April 2012