Australian National Botanic Gardens

Environmental Weeds in Australia

Introduced plants are an acute and insufficiently appreciated ecological problem. On a national scale, populations of the most invasive species are expanding. Plant species not native to Australia now account for about 15% of our total flora. About half of them invade native vegetation and about one-quarter are regarded as serious environmental weeds or have the potential to be serious weeds. The largest proportion of environmental weeds are horticultural species that have escaped from cultivation. Almost all of Australia's native vegetation has been, or is likely to be, invaded by exotic species that could result in changes to the structure, species composition, fire frequency and abundance of native communities. Those species of greatest concern include Rubber Vine (Cryptostegia grandiflora), Blue Thunbergia (Thunbergia grandiflora), the semi-aquatic grasses Hymenachne (Hymenachne amplexicaulis) and Aleman Grass (Echinochloa polystachia), Para Grass (Brachiaria mutica), Giant Sensitive Plant (Mimosa pigra) and Athel Pine (Tamarix aphylla).

Some significant species:

Name & habit	Key community/habitat affected	Nature of impact or threat	Photo
Prickly Acacia   Acacia nilotica  small tree/shrub	Mitchell grasslands	Replaces perennial Astrebla spp. with annuals or bare soil and is a long-term threat to the Mitchell grass biome; converts grassland to shrubland	1. 2.
Para Grass   Brachiaria mutica  semi-aquatic	Wetlands and streams in the wet-dry and wet tropics and sub tropics	Planted for ponded pasture but spreading into non-target areas destroying waterbird breeding habitats and choking tropical streams; replaces native vegetation	.
Buffel Grass  Cenchrus ciliaris  groundcover	Moist 'refuges' and river banks in the arid zone	Threatening keystone habitats by displacing native vegetation and altering the fire regime; likely to reduce fauna resources	1. 2.
Bitou Bush  Chrysanthemoides monilifera ssp. rotundata  shrub	Range of coastal systems: foredune, heath, littoral rainforest; range of coastal and sub-coastal systems.  NSW, Qld, Vic.	Displaces native vegetation with unknown effects on fauna
Boneseed   Chrysanthemoides monilifera ssp. monilifera  shrub	Range of coastal systems: foredune, heath, littoral rainforest; range of coastal and sub-coastal systems.  NSW, Vic, SA, WA.	Displaces native vegetation with unknown effects on fauna	.
Rubber Vine   Cryptostegia grandiflora  shrub/vine	Gallery and other riparian communities in the wet-dry tropics; dry rainforest (vine thickets)	Smothers trees and shrubs and shades out the ground layer; destroys riparian vegetation including gallery forests threatening associated fauna; forms impenetrable thickets in Queensland's Gulf river systems	1. 2.
Water Hyacinth  Eichhornia crassipes  aquatic	Standing surface waters especially where nutrient levels are high; occurs in all mainland States but particularly tropics and sub-tropics	Aggressively invades open water with potential for very rapid growth; still spreading in Australia despite extensive control measures; alters aquatic ecosystems
Aleman Grass  Echinochloa polystachia  semi-aquatic	Wetlands in the wet-dry and wet tropics; grows in water up to 2 m	As for Para Grass; recent introduction and not yet widespread but larger than para grass with greater potential for damage	1.  2.
Reed Sweetgrass   Glyceria maxima  semi-aquatic	Margins of creeks, rivers and ponded areas up to 1 m deep; temperate species eastern States	Used as a pasture or ornamental plant but is spreading to non-target areas; chokes the habitat	.
Hymenachne  Hymenachne amplexicaulis  semi-aquatic	As for para grass but can grow in water up to 2 m	Recently introduced as ponded pasture species, so not yet widespread, but has potential to modify tropical wetlands totally if not controlled
Giant Sensitive Plant  Mimosa pigra  small tree/shrub, semi-aquatic	Disturbed areas especially flood plains in the wet-dry tropics	Totally displaces native species leaving bare mud if removed; spreads by floods	1. 2.
Bridal Creeper   Myrsiphyllum asparagoides  creeper	Spreading through wide range of habitats in southern Australia	Smothers ground and shrub layers
Parkinsonia   Parkinsonia aculeata  small tree/shrub	Ephemeral wetlands and riparian communities in the wet-dry tropics	Invades mesic habitats and seasonal wetlands threatening waterbird habitats of continental significance
Mission Grass  Pennisetum polystachion  groundcover	Dry forests and woodlands of the wet-dry tropics	Displaces native sorghum changing the fire regime, which potentially reduces recruitment potential of woodland species of high conservation significance
Mesquite   Prosopis spp.  small tree/shrub	Semi-arid and arid riparian and other communities; Mitchell grasslands	Similar to prickly acacia but has a wider range of soil tolerances	1. 2.
Salvinia  Salvinia molesta  aquatic	Stationary and slow-moving water bodies, especially where nutrient levels are high; all mainland States and Territories	Aggressively invades open water with potential for very rapid growth; still spreading in Australia despite extensive control measures; alters aquatic ecosystems	1.
Athel Pine  Tamarix aphylla  small tree	Dryland river systems; currently small infestations	Displaces native trees; salinises soil; changes hydrology and geomorphology; reduces fauna resources	.
Blue Thunbergia  Thunbergia grandiflora  vine	Tropical lowland rainforest in far north Queensland, especially along watercourses	Vigorous vine rapidly spreading and smothering native vegetation to the canopy; infestation in early stages	.
Japanese Kelp   Undaria pinnatifida  marine kelp	Near-shore habitats along east coast of Tasmania	Spreading at rate of 10 km per year with potential to spread along southern coastline

Sources:

Humphries, S.E., Groves, R.H., and Mitchell, D.S. (1991) 'Plant Invasions of Australian Ecosystems. Kowari 2' (Australian National Parks and Wildlife Service: Canberra.)

Environment Australia, (1996) 'State of the Environment - Australia', CSIRO Pubishing, Melbourne.