Thatch grass (Hyparrhenia rufa)

Assessment ID: 1086
Jurisdiction: North Territory
Methodology: Northern Territory Weed Risk Management System - terrestrial
Management area: Darwin Region

Notes: Technical report available at: https://environment.nt.gov.au/media/docs/rangelands/weed-management-publications/wra-report-thatch-grass-updated-logo.pdf

Status: PUBLISHED
Last updated: 26/8/2025 12:01:15 pm AEST

Questions answered: 39 / 47

Weed Risk: 182.53 (High)
   Invasiveness (raw/adjusted): 8.00 / 6.664
   Impacts (raw/adjusted): 11.00 / 5.786
   Potential distribution (raw/adjusted): 9.00 / 4.734

Feasibility of Control: 391.45 (Very high)
   Control costs (raw/adjusted): 7.00 / 6.363
   Current distribution (raw/adjusted): 10.00 / 10.000
   Persistence (raw/adjusted): 8.00 / 6.152

Result: Prevent entry, contain regional spread

Invasiveness

A1. What is the ability of the plant to establish amongst intact native environments?
Answer: High

Comments: H. rufa and M.minutiflora are able to invade otherwise intact native-dominated savanna ecosystems (D'Antonio & Vitousek 1992). Found on disturbed ground and roadsides (Smith 2002). Invaded habitats: grassland, savanna, disturbed sites (Weber (2003). Easily establishes in tropical areas, and aggressively invades natural areas (Global Invasive Species Programme 2003). Habitat: dry to mesic roadsides, disturbed areas (Pacific Island Ecosystems at Risk 2006, Smith 1979). Grows as a roadside weed (Jacobs & Wall 2007). Invasion of native grasslands by these grasses has been documented in Brazil, Colombia, and Venezuela where Hyparrhenia rufa (Jaragua) and Melinis minutiflora (Molasses grass) have displaced native pasture grasses such as Trachypogon plumosus….Ecophysiological studies demonstrate that these grasses tolerate frequent defoliation better than native grasses. Hyparrhenia has also invaded Central American woodlands and pastures (D'Antonio & Vitousek 1992). Jaraguas remarkable aggressiveness and self-seeding ability is demonstrated by its capacity to compete with native savanna grasses (Parsons 1972). In Venezuela, four African grasses are the most prominent invaders: ….Hyparrhenia rufa (Nees) Stapf. in lowland savannas with poor soils and marked dry seasons. The successful encroachment of the alien grasses generally took place only in the wetter and/or more fertile habitats of the savanna. In more stressful sites, the indigenous community persists (Baruch 1996). More references available at: https://environment.nt.gov.au/media/docs/rangelands/weed-management-publications/wra-report-thatch-grass-updated-logo.pdf.

A2. What is the reproductive ability of the plant?

(a) Time to seeding
Answer: 1 year or less

Comments: No specific information.

(b) Annual production of viable seed /m2 or /plant
Answer: High

Comments: The grass produces seeds abundantly (Weber 2003). It produces abundant viable seed from which it is easily established (Skerman & Riveros 1990).

(c) Vegetative reproduction
Answer: None

Comments: Root stocks can also be planted (Skerman & Riveros 1990).

A3. Do propagules of the plant have properties that allow them to be dispersed long-distance by natural means?

(a) Flying animals (birds, bats)
Answer: No

Comments: No specific information.

(b) Other wild animals
Answer: Yes

Comments: No specific information.

(c) Water
Answer: No

Comments: No specific information.

(d) Wind
Answer: Yes

Comments: Seeds are dispersed by wind (Smith 2002). The seed are able to disperse on the wind after fires and germinate well in these conditions (Starr et al. 2003).

A4. How likely is long-distance dispersal by human-assisted means?

(a) Deliberate spread by people
Answer: Unlikely

Comments: Hyparrhenia rufa was introduced to the Northern Territory to determine its performance as an improved pasture plant in pasture trials (Cameron et al. 1984). A number of C4 African grasses, most importantly, Hyparrhenia rufa …were brought in to support grazing in savanna regions and in cleared forests (Central and South America) (D'Antonio & Vitousek 1992). Outside of Brazil, it has perhaps reached its maximum development in the drier western side of Central America, especially in Guanacaste and in Nicaragua where it is recognized as the base of a substantial live-stock industry (Parsons 1972). Commonly cultivated throughout the tropics for cattle fodder (Starr et al. 2003).

(b) Accidentally by people and vehicles
Answer: Common

Comments: Spread is also by vehicles and machinery eg. road graders (Smith 2002). It is dispersed by seed with uncommon ease (Parsons 1972). Seeds with long bristles are capable of catching on people…that walk past the plant (Starr et al. 2003).

(c) Contaminated produce
Answer: Occasional

Comments: No specific information.

(d) Domestic/farm animals
Answer: Occasional

Comments: Seed with long bristles are capable of catching on … animals that walk past the plant (Starr et al. 2003).

Impacts

B1. What is the plant's competitive potential?
Answer: High

Comments: It forms dense swards ... that displace native grasses and forbs, preventing the establishment of other species and transforming native savannas into species pure stands (Weber 2003). It outcompetes and smothers other weeds, and – since it is a fire-adapted species – it readily replaces native plants after fires (Global Invasive Species Programme 2003). Especially in the drier areas, where the dry season lasts five months or more, it has held a strong competitive advantage (Parsons 1972). Jaraguas remarkable aggressiveness and self-seeding ability is demonstrated by its capacity to compete with native savanna grasses (Parsons 1972). The opportunistic water use of alien grasses contrasts with that of native grasses and probably contributes to the higher competitive potential of the former in the seasonal tropical savannas (Baruch et al. 1985). Some African grasses, such as … H.rufa, have allelopathic effects which act either directly, retarding the growth of potential woody competitors or indirectly through secretion of antibiotics that suppress the growth of nitrifying bacteria (Marinero 1964, Boughey et al. 1964, cited in Baruch 1996. The higher growth rates and nutrient concentration of the introduced grasses (including H.rufa) are consistent with their ability to establish rapidly, compete successfully for resources, and displace T.plumosus from moist, fertile sites. Conversely, the slower growth rate, lower nutrient concentrations and superior water relations characteristics are consistent with the capacity of T. plumosus to resist invasion by introduced grasses in poorer sites (Baruch et al. 1985). It competes successfully with weeds and smothers them (Skerman & Riveros 1990).

B2. What is the plant’s potential to modify the existing fire behaviour and alter the fire regime?
Answer: Some potential

Comments: After fires, H.rufa and other fire adapted non-native grasses, dominate the understory, which in turn adds to the fuel load, frequency and size of future fires. Most native plants are not well suited to fire and eventually few remain (Starr et al. 2003). May carry wildfire (Smith 1979 cited in Pacific Island Ecosystems at Risk 2006). All four of these grasses (including H.rufa) burn readily and resprout rapidly following fire; the consequent grass-fire interaction is thereby capable of maintaining cleared forest land as a derived savanna or grassland, preventing succession back to forest. In Central America, Hyparrhenia has received the most attention from ecologists, since when it is not heavily grazed it forms tall, dense stands that burn readily and intensely. In contrast, fires in comparable sites dominated by native grasses are patchy and less intense. H. rufa-fueled fires can burn into successional and even intact tropical dry forest and represent a serious threat to preservation of this ecosystem in Guanacaste National Park in Costa Rica and elsewhere (D'Antonio & Vitousek 1992). Even more than the other exotic grasses in the New World tropics, jaragua appears to be aided by fire. After a number of years, it may tend to weaken and eventually be invaded by other species unless regularly burned. Stockmen, who know this well, fire jaragua ranges each year in the dry seaon and graze it rather closely during the rainy period to avoid it becoming rank and fibrous with progressively less nutritive value (Parsons 1972). More references available at: https://environment.nt.gov.au/media/docs/rangelands/weed-management-publications/wra-report-thatch-grass-updated-logo.pdf.

B3. What is the plant’s potential to restrict the physical movement of people, animals, vehicles, machinery and/or water?
Answer: Low

Comments: Forms dense closed stands nearly 3 m tall (Baruch 1996).

B4. What is the plant’s potential to negatively affect the health of animals and/or people?
Answer: None

Comments: Ndyanabo (1974) recorded 0.84 percent total oxalic acid in the dry matter, but no toxicity (Skerman & Riveros 1990).

B5. Does the plant potentially have negative effects on natural and cultural values?

(a) Reducing habitat quality for native animals?
Answer: High

Comments: The decrease in species richness and structural heterogeneity accompanying the displacement of the indigenous savanna would lead to a loss of diversity and reduced persistence of the native animal populations that depend on native plants for nourishment and refuge (Medina 1993 cited in Baruch 1996).

(b) Threatened species or communities
Answer: More than one

Comments: The native partridge pigeon (Geophaps smithii smithii) could be impacted by thatch grass as it requires open patchy grasslands (J. Woinarski, NT Biodiversity Conservation, pers. comm. 2007). The brush-tailed rabbit-rat (brush tailed tree-rat) Conilurus penicillatus could be impacted by thatch grass as it requires open patchy grasslands (J. Woinarski, NT Biodiversity Conservation, pers. comm. 2007).

(c) Sites of natural or cultural significance
Answer: More than one

Comments: Kakadu National Park and the Tiwi Islands are both areas of conservation significance that would be adversely affected by Hyparrhenia rufa (Harrison et al. 2009, J. Woinarski, NT Biodiversity Conservation, pers. comm. 2007).

B6. Is the plant presumed to have negative effects on environmental health?

(a) Soil chemistry/stability
Answer: Yes

Comments: The higher biomass per unit area of the communities dominated by alien grasses (including H. rufa) indicates that the amounts and rates involved in nutrient cycling should be higher than in the native savannas. It is possible that the high nutrient requirements of alien species deplete the soil reserves and there is also a potential for nutrient loss from the large standing dead biomass exposed to rainfall leaching and to volatilization by fire in the communities dominated by alien grasses (Baruch 1996).

(b) Water quality
Answer: No

Comments: No specific information.

(c) Hydrology
Answer: Yes

Comments: Although detailed studies are not available, it is conceivable that the establishment of communities dominated by alien grasses can alter the water balance of the areas through increased evapotranspiration rates, rainfall interception, and decreasing soil water recharge and runoff in virture of their higher leaf are index, deeper root system and higher transpiration rates than native savanna communities (Baruch 1996).

Potential distribution

C1b. What is the Climatch suitability score (which indicates the proportion of the NT environment that is suitable for the plant)?
Answer: 4

Comments: The CLIMATCH model used by the NT Weed Management Branch predicts that 41% of the Northern Territory is climatically suitable for Hyparrhenia rufa (NT Weed Management Branch 2007). A common roadside weed in Queensland. First introduced into the Northern Territory in the 1960s and is present in the Darwin/Palmerston area (Smith 2002). A weed of roadsides in Queensland and now is becoming established in the Darwin region of the Northern Territory (Smith 2001). The altitude range is reported as up to 2,000 m (6.562 ft) in Colombia. Minimum rainfall requirements are reported as 600-1,400 mm (24-55 in) annually and it is said to be rather drought tolerant (Skerman & Riveros 1990).

C2. How many broad habitat types in the NT will the plant potentially naturalise in (up to 5)?
Answer: Three

Comments: The broad vegetation types that Hyparrhenia rufa will potentially naturalise in are: * Tropical riparian areas * Tropical open forests/savanna woodlands Of these, the favoured vegetation type is tropical open forests/savanna woodlands (NT Weed Risk Management Committee 2007, Rossiter-Rachor et al. 2012).

C3. What is the potential of the plant to occur throughout its favoured habitat in the NT (as identified in question 2)?
Answer: Some

Comments: Hyparrhenia rufa has the potential to occur through some of its favoured habitat (NT Weed Risk Management Committee 2007).

Control costs

A1. How detectable is the weed?

(a) Distinguishing features
Answer: Sometimes distinct

Comments: An erect, densely tufted perennial or occasionally annual grass 1-2.5 m tall. Leaves flat, elongate, 30-60 cm long, 2-8 mm wide, narrowed at base, very rough margins. Flower 20-40 cm long, pairs of racemes on sinuous stalks. Racemes c.2 cm, reddish-brown. Spikelets, mostly 5-7 in each raceme, 3-44 mm long, flattened from the ack, covered with rusty-brown hairs. Awn 15-20 mm long with 2 bends, twisted, red-brown, sparsely covered with stiff hairs (Smith 2002). A very variable perennial from 60-240 cm high. Panicle loose and narrow up to 50 cm long, with slightly spreading or contiguous racemes with shortly hairy or nearly glabrous spikelets 3.5-5 mm long. The rusty brown hairs on the spikelets and the racemes terminally exserted from the spatheoles distinguish it from H.filipendula and H.hirta. The flowering stems have little leaf. The sheaths of the leaves enclose about half the length of each internode, giving the culm a banded appearance (Skerman & Riveros 1990).

(b) Active growth period
Answer: 4-8 months

Comments: No specific information.

(c) Height at maturity
Answer: 0.5 - 2 m

Comments: Erect annual or perennial to c. 2m high (Jacobs & Wall 2007). H.rufa, a bunch grass, forms dense closed stands nearly 3 m tall (Baruch 1996). A very variable perennial from 60-240 cm high (Skerman & Riveros 1990). Culms erect; 30–250 cm long (Clayton et al. 2006).

A2. What is the general accessibility of infestations at the optimum treatment time?
Answer: High

Comments: No specific information.

A3. How expensive is control of the weed in the first year of targeted control, for an infestation that has reached maximum weed density?

(a) Chemical costs
Answer: Low

Comments: No specific information.

(b) Labour cost
Answer: Low

Comments: No specific information.

(c) Equipment cost
Answer: Medium

Comments: No specific information.

A4. What is the general community perception of this weed within the region?
Answer: Low

Comments: No specific information.

Current distribution

B1. What is the current pattern of the species’ distribution across the weed management region?
Answer: Restricted

Comments: No specific information.

Persistence

C1. How long will it take to reach the maintenance period?
Answer: Medium

Comments: No specific information.

C2. What is the minimum time period for reproduction of sexual or vegetative propagules?
Answer: <1 years

Comments: No specific information.

C3. What is the maximum longevity of sexual or vegetative propagules?
Answer: 2-5 years

Comments: No specific information.

C4. What is the threat of reinfestation from outside the management region?

(a) Long-distance (>100m) dispersal by natural means
Answer: Occasional

Comments: Seeds are dispersed by wind (Smith 2002). The seed are able to disperse on the wind after fires and germinate well in these conditions (Starr et al. 2003).

(b) Long-distance (>100m) dispersal by human means
Answer: Rare

Comments: A number of C4 African grasses, most importantly, Hyparrhenia rufa …were brought in to support grazing in savanna regions and in cleared forests (Central and South America) (D'Antonio & Vitousek 1992). Outside of Brazil, it has perhaps reached its maximum development in the drier western side of Central America, especially in Guanacaste and in Nicaragua where it is recognized as the base of a substantial live-stock industry (Parsons 1972). Commonly cultivated throughout the tropics for cattle fodder (Starr et al. 2003). Spread is also by vehicles and machinery eg. Road graders (Smith 2002). It is dispersed by seed with uncommon ease (Parsons 1972). Seeds with long bristles are capable of catching on people and animals that walk past the plant (Starr et al. 2003).

References

Barnett, C, Ibarra, H, Noonan, K & Payne, C (2001) The effects of introduced exotic grasses, African Star Grass Hyparrhenia rufa and Anglinton Dichanthium annulatum on tropical dry forest regeneration, The Woodrow Wilson Foundation Leadership Program for Teachers, viewed 11/09/07, <http://www.woodrow.org/teachers/esi/2001/CostaRica/palo_verde1/grasses/>. Baruch, Z (1996) 'Ecophysiological aspects of the invasion by African grasses and their impact on biodiversity and function of Neotropical savannas', In: Biodiversity and Savanna Ecosystems Processes: A Global Perspective - Ecological Studies S O.T., E Medina & J Silva (eds), Springer-verlag, Berlin, pp. 79-93. Baruch, Z, Ludlow, MM & Davis, R (1985) 'Photosynthetic responses of native and introduced C4 Grasses from Venezuelan savannas', Oecologia, 67, 388-393. Boughey, AS, Mumo, PE, Meiklejohn, J, Strange, RM & Swift, MJ (1964) 'Antibiotic reactions between African savanna species', Nature, 203, 1302-3. Cameron, A, Miller, I, Harrison, P & Fritz, R (1984), A review of pasture plant introduction in the 600-1500 mm rainfall zone of the Northern Territory, Technical Bulletin No. 71, Northern Territory Department of Primary Production, Darwin. Clayton, WD, Harman, KT & Williamson, H (2006) Grass Base - The Online World Grass Flora The Board of Trustees, Royal Botanic Gardens Kew, viewed 12/09/2007, <http://www.kew.org/data/grassesdb/www/imp05516.htm>. Getzin, S (2002) Formation and Consequences of Fire-Induced Structures in Tropical and Sub-Tropical Savannas, Masters of Science Thesis, University of Potsdam, Potsdam. Global Invasive Species Programme (2003) South America Invaded: Grasses, viewed 10/09/2007, <http://www.gisp.org/casestudies/showcasestudy.asp?id=282&MyMenuItem=casestudies&worldmap=&country=>. More references available at: https://environment.nt.gov.au/media/docs/rangelands/weed-management-publications/wra-report-thatch-grass-updated-logo.pdf.