Skip links and keyboard navigation

Invasive Aquatic Pasture Grasses

Invasive aquatic pasture grasses, notably olive hymenachne (Hymenachne amplexicaulis), para grass (Urochloa mutica), and Aleman grass (Echinochloa polystachya) can drastically alter the structure, composition and processes of wetlands and riparian zones[11]. These vigorous plant species, introduced primarily for agricultural purposes, have spread into wetland ecosystems, outcompeting native flora, and disrupting the biodiversity of these habitats. The change in these ecosystems can reduce the environmental and social-cultural values of an area, such as opportunities for recreation and tourism, as well as cultural and aesthetic values. Ongoing management of the impacts from invasive pasture grasses on natural wetlands and their ecosystem services requires a Whole-of-System, Values-Based approach.

Hymenachne in a wetland. Photo by Gary Cranitch © Queensland Museum

Quick facts

The earliest introduction of para grass
in Queensland is documented as 1880 by Dr Joseph Bancroft, with a second introduction by The Colonial Sugar Refining Company using it in 1984 for erosion control[8]. Olive hymenachne and Aleman grass were approved for release in Queensland nearly 100 years later in the mid-1980s for use within ponded pastures and their ability to grow in deeper water[5]

Impacts to the ecological values of wetlands from olive hymenachne, para grass, and Aleman grass

Olive hymenachne, para grass, and Aleman grass can displace native vegetation in wetlands, altering the structure and composition of wetland ecosystems. Known for their rapid growth and ability to form dense vegetation stands and mats across water, they can obstruct water flow, reduce light penetration, exclude the colonisation of native vegetation at ground level, and diminish the diversity of aquatic plants[12]. These changes in wetland dynamics have cascading effects on the entire ecosystem, affecting water quality, biodiversity, and the ecological services that wetlands provide. Olive hymenachne can also hybridise with native hymenachne plants, and the hybrid can be as invasive as olive hymenachne[4]. Impacts from invasive aquatic pasture grasses to the ecological values of wetlands can include:

  • Impacts to stream and wetland hydrology[2]
  • Reduced species richness and biodiversity[11], including reduced or complete dominance over native vegetation, reduced abundance of native fish and reduced richness of turtles and birds[8].
  • Physical and/or physiological (e.g. low dissolved oxygen) barriers to fish movement[3].
  • Increased sedimentation by increasing friction and allowing particles to settle out, resulting in changes to benthic organisms and sediment buildup[1]
  • Increased organic load or smothering of open water surfaces, resulting in decreased water quality including reduced dissolved oxygen levels and fish kills[14]

It has been estimated that there can be as much as an 85% reduction in channel discharge capacity, because of invasive grass related sedimentation. Such hydrological and morphological changes to streams, caused by invasive pasture grasses, have been linked to increased flooding[1].

Impacts to native wetland animals from olive hymenachne, para grass and Aleman grass

Primarily introduced to Queensland as ponded pasture grasses to provide feed for cattle in areas where there is low productivity during the dry season, olive hymenachne, para grass and Aleman grass were selected for their hardiness and ability to grow in wet environments. Once in natural wetland environments, these grasses often form dense monocultures, out-competing native flora.

Native birds, such as Brolga, suffer declines in food resources when these grasses replace natural food sources like Bulkuru bulbs[6]. The dense matted growth form of these grasses impedes access to other resources in the water or soil, and limit edible seed and food available for birds[3]. Large monocultures affect nesting and roosting habitats for waterfowl[12], for example, when the preferred nesting sites and stable food resources for magpie geese (Oryza rufipogon and Eleocharis species) are replaced by para grass[1].

Many of Australia’s native fish require free movement within waterways and between estuaries and freshwater wetlands to maintain the various stages of their life cycles. When exotic weeds form large dense mats and stands, they can impede fish movement and disrupt migrations. High amounts of organic matter within wetlands because of exotic weeds has been found to reduce oxygen levels, resulting in area avoidance and fish kills[14][6].

Increased sedimentation of streams caused by dense stands of invasive grasses may have a smothering effect on benthic species by creating anaerobic conditions. Loss of benthic organisms can have significant negative effects on wetland food chains[1].

Olive hymenachne in a wetland. Photo by Gary Cranitch © Queensland Museum

Impacts to water quality from olive olive hymenachne, para grass and Aleman grass

Water quality can be significantly impacted by invasive aquatic pasture grasses in wetlands. Because olive hymenachne and Aleman grass can grow in deeper water than native vegetation, they can encroach further into wetlands, smothering open water surfaces and decreasing water quality[12]. This, as well as forming dense monocultures, increases the organic load in a waterbody. Plants produce oxygen during the day through photosynthesis and consume available oxygen at night. If the demand for oxygen from bacterial decomposition, microalgae, phytoplankton, and aquatic fauna, combined with the nighttime consumption of oxygen by plants, exceeds the amount generated by daytime photosynthesis, low oxygen levels can occur in the waterbody[14]. Some native fish can endure periods of low oxygen, but when this becomes chronic, they can die. When flooding brings more organic material and nutrients to the wetland and it decomposes, the oxygen levels can reduce even further, often resulting in large fish kills.

Some native fish can endure periods of low oxygen, but when this becomes chronic, they can die. When flooding brings more organic material and nutrients to the wetland and it decomposes, the oxygen levels can reduce even further, often resulting in large fish kills.

Impacts to social values of wetlands from olive hymenachne, para grass and Aleman grass

In addition to the ecological impacts of invasive pasture grasses in wetlands, social values can also be impacted. For example, impacts to wetlands from invasive pasture grasses can have flow-on effects to recreational and commercial fisheries. Recreationally and commercially valuable species of native fish have evolved to use specific habitats. If these habitats are changed dramatically, by the spread of invasive aquatic pasture grasses, there is the chance that these fish will be lost or reduced in abundance. Invasive pasture grasses can invade nursery and adult habitat areas for high value fish species such as barramundi (Lates calcarifer). Additionally, as prawns breed in response to natural flood events, disruptions to natural run-off patterns caused by invasive pasture grass may affect recruitment[13]. The modification of habitats of native invertebrates and other aquatic microfauna can impact on native fish that depend on them for food[5]>with possible impacts on the viability of commercial, recreational, and traditional fisheries.

Invasive pasture grasses are a serious problem in the sugarcane industry, increasing production costs associated with the control of the grasses and the pest species which inhabit the grasses (e.g. rats)[10]. The primary impact is the blocking of irrigation and drainage channels designed to divert water away from sugar cane crops[1][6]. In some cases, these grasses smother young cane and contaminate “seed-cane” plots, decreasing productivity through direct competition and reducing yields[3][13].

The environmental conditions caused by invasive pasture grasses in wetlands can be a major source of mosquito breeding where natural hydrological and biological processes (e.g. decreased abundance of predatory fish) cannot work effectively[7][13]. Substantial increases in mosquito abundance could increase the prevalence of mosquito-borne diseases such as encephalitis and Ross River viruses[5][3].

Invasive aquatic pasture grasses have the potential to affect traditional activities, including traditional management practices, as they can interrupt both physical and spiritual connections to Country. Infestations can restrict or prevent Indigenous hunting activities, limit the availability of traditional foods such as yams, and render habitats unsuitable for culturally significant species, including magpie geese. Physical access to sacred sites may be hindered, impacting on social wellbeing[13].

Large infestations of invasive aquatic pasture grasses can lead to loss of amenity and can significantly affect recreational activities and tourism[13]. Of concern is the potential for invasive pasture grasses to damage extensive wetlands within World Heritage listed National Parks and wetlands that attract numerous visitors each year[5].

It is important to note that olive hymenachne, para grass and Aleman grass were introduced for and are valued on private grazing land as high value fodder for cattle. The introduction of these grasses increased available green feed for cattle over the dry season[8]. This allows higher stocking rates and liveweight gains of animals at a time when native pasture quality and quantity are at their lowest[9].

Additional information


  1. ^ a b c d e , Hannan- Jones, M & Csurhes, S (2012), Invasive Species Risk Assessment, Para grass. [online], Queensland Government. Available at:
  2. ^ , Hannan- Jones, M & Webber, J (2016), Invasive Plant Risk Assessment, Aleman grass. [online], Queensland Government. Available at:
  3. ^ a b c d , Wearne, LJ, Clarkson, J, Grice, AC, Dekker van Klinke & R and Vitelli, JS (2010), 'The biology of Australian weeds 56. Hymenachne amplexicaulis (Rudge) Nees', Plant Protection Quarterly, vol. 25, no. 4, pp. 146-161.
  4. ^ Clarkson, J, Mirko, K & Evans, D (16 February 2011), 'A report of hybridisation in Hymenachne (Poaceae, Panicoideae) with description of Hymenachne x calamitosa, a new species of hybrid origin from tropical Australia', Telopea. [online], vol. 13, no. 1-2, pp. 105-114. Available at: [Accessed 29 May 2024].
  5. ^ a b c d Csurhes, SM, Mackey, AP & Fitzsimmons, L (1999), 'Hymenachne (Hymenachne amplexicaulis) in Queensland', Pest Status Review Series. [online], Land Protection, Queensland Government. Available at:
  6. ^ a b c Fisk, P (1991), Ponded Pastures as a potential weed problem.
  7. ^ McCabe, J (1991), ‘The effects of ponded pastures on wildlife and landscape management’, Proceedings of the ‘Probing Ponded Pastures Workshop'.
  8. ^ a b c Middleton, CH (1991), History of Para Grass in Queensland’, Proceedings of the ‘Probing Ponded Pastures Workshop'.
  9. ^ Middleton, CH, Murphy, K & Wildin, J (1991), Productivity of Ponded Pastures’, Proceedings of the ‘Probing Ponded Pastures Workshop.
  10. ^ Rat management in sugarcane. [online], vol. B14023, Sugar Research Australia. Available at:
  11. ^ a b Reid, B, Grice, T, Holtum, J, Nicholas, M, Wallace, J & Waltham, N (2018), Assessing the potential for controlling four invasive species (aleman grass, water hyacinth, olive hymenachne and para grass) using seawater in northern Australian coastal wetlands. [online], vol. 18/31, James Cook University and CSIRO Australia. Available at:
  12. ^ a b c Tait, J (2011), Guidelines for the Use of Grazing in the Management of Exotic Pasture Weeds in Wetland and Riparian Habitats, Wetland Care Australia, Ballina, NSW.
  13. ^ a b c d e Threat abatement plan to reduce the impacts on northern Australia’s biodiversity by the five listed grasses. (2012). [online], Australian Government. Available at:
  14. ^ a b c Waltham, NJ, Pyott, M, Buelow, C & Wearne, L (September 2020), 'Mechanical harvester removes invasive aquatic weeds to restore water quality and fish habitat values on the Burdekin floodplain', Ecological Management & Restoration. [online], vol. 21, no. 3, pp. 187-197. Available at: [Accessed 24 March 2021].

Last updated: 20 May 2024

This page should be cited as:

Department of Environment, Science and Innovation, Queensland (2024) Invasive Aquatic Pasture Grasses, WetlandInfo website, accessed 3 June 2024. Available at:

Queensland Government
WetlandInfo   —   Department of Environment, Science and Innovation