Nassella trichotoma (serrated tussock grass)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Biology and Ecology
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Vectors
- Plant Trade
- Impact Summary
- Environmental Impact
- Impact: Biodiversity
- Social Impact
- Risk and Impact Factors
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Nassella trichotoma (Nees) Hackel ex Arech., 1896
Preferred Common Name
- serrated tussock grass
Other Scientific Names
- Agrostis trichotoma Nees ex Trin.
- Oryzopsis trichotoma (Nees) Druce
- Piptochaetium trichotomum (Nees) Griseb.
- Stipa trichotoma Nees, 1829
- Urachne trichotoma (Nees) Trin.
International Common Names
- English: nassella tussock; serrated tussock; Yass River tussock
- Spanish: paja amargo; paja voladora
Local Common Names
- Argentina: pasta puna
- Australia: tumbleweed
- South Africa: nassella - polgras; saagtand - polgras
- Uruguay: spartillo
- STDTR (Nassella trichotoma)
Summary of InvasivenessTop of page Serrated tussock grass (N. trichotoma) has become a serious invasive weed in warm temperate grasslands in Australia, New Zealand and South Africa where it causes significant losses in livestock production due to reductions of up to 97% in the carrying capacity of infested pastures. It is also becoming an important invasive weed of conservation areas, particularly native grasslands. Because of its enormous seed production, efficient dispersal mechanisms and long-lived seed banks, N. trichotoma is difficult to control and has enormous potential to invade and spread further within and between countries.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Monocotyledonae
- Order: Cyperales
- Family: Poaceae
- Genus: Nassella
- Species: Nassella trichotoma
Notes on Taxonomy and NomenclatureTop of page First described by Nees in 1829 as Stipa trichotoma with the type from Uruguay; redescribed by Hackel in 1896 and transferred to the genus Nassella. N. trichotoma has since been shifted between the two genera (Barkworth, 1990). Parodi (1947) excluded it from his revision of Nassella, whilst Corti (1951) erected a new section Nassellopsis within Stipa to accommodate serrated tussock, and this is still recognized by South American botanists (Zuloaga et al., 1994), whereas Australian botanists include it in Nassella (Vickery and Jacobs, 1980). However, it is often referred to the genus Stipa in South Africa (Wells, 1978; Viljoen, 1987), with Nassella as a synonym. It is variously quoted as N. trichotoma (Nees) Arech. (Campbell, 1982; Cronk and Fuller, 1995), N. trichotoma (Nees) Hackel (Auld and Medd, 1987), N. trichotoma Hackel ex Arech. (USDA-NRCS, 2004) and N. trichotoma (Nees) Arechav. (Mabberley, 1997). Walsh (1998) noted that the transferral of species formerly included in Stipa to other genera such as Nassella is controversial and that "Future research may result in the reassignation of some species to their old genera or to some currently unrecognized, new genera": a view expressed earlier by De Winter (1965). The current accepted revision of the genus Nassella by Barkworth (1990) comprises 60 South American species including N. trichotoma, formerly assigned to the genus Stipa, and is characterized within the tribe Stipeae by the strongly overlapping lemma or bract margins with the lemma apices fused into a crown.
DescriptionTop of page N. trichotoma is a perennial, drought-resistant, tussock-forming grass, up to 50 cm high and 25 cm wide at the base. The leaves and culms of mature plants droop and are often bleached; culms up to twice as long as leaves and much branched; leaves emerging from base to form a large tussock, narrow (0.5 mm diam.), hard, tightly rolled, finely serrated; leaf blade linear with pointed tip, 80-500 mm in length; leaf sheath up to 160 mm long, rounded and smooth but serrated at the collar or ligule at the blade/sheath junction; ligule firmly membranous, short (ca 1 mm long), rounded, white, non-hirsute. Inflorescence an open, much-branched panicle, 20-35 cm long, but may be up to 75 cm, with brittle filiform branches; soon detaching, leaving the plants free of inflorescences for most of the year. Florets (spikelets) small and inconspicuous, 5-6 mm long, formed towards end of branchlets, with two reddish-brown to purple bracts (glumes) enclosing the seeds (ca 50 per inflorescence), imparting a purple tinge to the inflorescences. Each 'seed' borne singly on a thin stalk, 1.5-2 mm long, with a single 2-3 cm long, unbranched, filiform, twisted, slightly bent awl at the apex (which remains attached at maturity), and a tuft of white silky hairs at the base. When 'seeds' are ripe, the heads droop conspicuously. Grain 1-1.2 mm long. For drawings of 'seed' and grain, see Reed (1977). Roots diffuse and fibrous, forming a deep root system, down to 20 cm or deeper, making plants difficult to uproot.
Plant TypeTop of page Grass / sedge
DistributionTop of page Although N. trichotoma has a native range covering the moist/warm temperate pampas grasslands of South America, mainly between latitudes 30° and 40°S, it appears to be of rare occurrence over most of this range and shows little variation (Wells, 1978). The latter author, based on personal observations, considered that the true centre of origin and diversity lies in the drier southern Pampas region of Argentina, specifically in the Sierra de Ventana (Wells, 1977). Subsequent surveys (Erb, 1988; Evans and Ellison, 1995; Briese and Evans, 1998) showed that N. trichotoma is an integral component of the climax vegetation covering the subhumid rocky, southern highlands of Buenos Aires Province, with scattered outlying populations in the drier, upland grassland habitats of north-western Argentina, as well as in Bolivia, Chile and Peru. It is now widely distributed as an exotic invader of both natural and managed grasslands through the Southern Hemisphere, particularly in those countries such as Australia, New Zealand and South Africa which have regions with a Mediterranean or warm temperate climate. Climate data analysis reveals that its potential distribution within these countries could be significantly greater; Wells (1978) and Wells and de Beer (1987) consider that 36% of the total area of South Africa (ca 44 million ha) is at risk.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|South Africa||Restricted distribution||Introduced||ca. 1900||Invasive||Wells and de Beer, 1987; EPPO, 2014|
|-Canary Islands||Present||Introduced||USDA-ARS, 2004|
|-North Carolina||Widespread||Introduced||1988||Invasive||Westbrooks and Cross, 1993; USDA-NRCS, 2004|
|Argentina||Restricted distribution||Native||Not invasive||Erb, 1988|
|Bolivia||Present||Native||Not invasive||Zuloaga et al., 1994|
|Brazil||Present||Native||Not invasive||Missouri Botanical Garden, 2003|
|-Rio Grande do Sul||Present||Native||USDA-ARS, 2004|
|Chile||Present||Native||Not invasive||McLaren et al., 1998|
|Uruguay||Restricted distribution||Native||Not invasive||Erb, 1988|
|France||Present||Introduced||1920s||Invasive||Campbell, 1982; Campbell and Vere, 1995; EPPO, 2014|
|Italy||Present||Introduced||1920s||Invasive||Campbell, 1982; Wells and de Beer, 1987; EPPO, 2014|
|UK||Present||Introduced||1920s||Invasive||Campbell, 1982; Campbell and Vere, 1995|
|Australia||Restricted distribution||Introduced||USDA-ARS, 2004; EPPO, 2014|
|-New South Wales||Widespread||Introduced||ca. 1900||Invasive||Parsons and Cuthbertson, 1992; Campbell and Vere, 1995; McLaren et al., 2006|
|-New South Wales||Present||Parsons and Cuthbertson, 1992; Campbell and Vere, 1995; McLaren et al., 2006|
|-South Australia||Present||Introduced||Invasive||Campbell, 1982; McLaren et al., 1998|
|-Tasmania||Present||Introduced||1920s||Invasive||Fricke, 1956; Parsons and Cuthbertson, 1992|
|-Victoria||Widespread||Introduced||1954||Invasive||Campbell, 1982; McLaren et al., 1998|
|New Zealand||Restricted distribution||Introduced||1860||Invasive||Healy, 1945; Campbell, 1982; EPPO, 2014|
History of Introduction and SpreadTop of page The history of introduction of N. trichotoma is well documented, although the actual taxonomic status of the new alien was often not firmly established until many years after its critical arrival. For example, in New Zealand it was claimed to have been introduced from South America in the 1860s, either accidentally as a contaminant of packing materials, wool, ballast or pasture seed, or even deliberately to stabilize sand-dunes (Healy, 1945). However, it was not formally identified in New Zealand until the 1930s (Allen, 1935). Wells (1978) reports in detail on the likely means, as well as entry points, of its introduction into South Africa, postulating that the seeds arrived in hay, exported from Argentina for horse fodder by the British Army during the Boer War (1899-1902), through the ports of East London and Port Elizabeth. Once again, it was not until much later (1952) that it was correctly identified (Wells, 1977). Its manner of introduction into Australia is less precise. Anecdotal evidence from landholders put it in the late 1890s to early 1900s, although it was not identified until 1935, when it was known as Yass River tussock (Cross, 1937; Campbell, 1982). Its subsequent history of spread would suggest that this is a 'sleeper' species whose populations have a prolonged lag phase before increasing rapidly and dramatically (McLaren et al., 1998). Thus, from a minor infestation at Yass River in 1935, N. trichotoma had spread over 680,000 ha of New South Wales by 1975 (Campbell, 1982), and 20 years later this had increased to more than 870,000 ha (McLaren et al., 1998). In Victoria, N. trichotoma was not detected until 1954, but by 1979 it had increased to ca 130,000 ha (McLaren et al., 1998). The latter authors estimate that 32 million ha of southern Australia are at risk. N. trichotoma was introduced into Tasmania, probably in pea seeds imported from New Zealand during the 1920s (Goninon, 1998), although it was not identified until the mid-1950s (Fricke, 1956). Its minor incursions into Europe have been linked with wool imports (Campbell, 1982).
Risk of IntroductionTop of page Further spread is highly likely due to contamination by seed of N. trichotoma of agricultural seed and wool, which could lead to long-distance or intercontinental dispersal, and by farm animals (externally or internally) and machinery for in-country spread. Its potential distribution in Australia is calculated at 32 million ha, currently it infests just over 1 million ha (McLaren et al., 1998).
N. trichotoma is a proclaimed noxious weed in South Africa, and subject to all provisions of the Weed Act of 1937 (Wells, 1978). It is a declared noxious weed in New South Wales (1938), Victoria (1954), South Australia and Tasmania (1959) (Campbell and Vere, 1995; McLaren et al., 1998). An action plan, funded by the State government, has been enforced in Tasmania to isolate and contain N. trichotoma (Goninon, 1998). Government Committees in New South Wales oversee the control of N. trichotoma, and special low interest loans are available to landholders to implement recommended control measures. N. trichotoma was designated as a Federal Noxious Weed in the USA in 1983 (Westbrooks and Cross, 1993). However, in a somewhat bizarre case, N. trichotoma was intercepted in eight shipments of tall fescue grass seed from Argentina by USA quarantine in 1988, but no action could be taken because legislation was not in place to prevent onward distribution. By the time a recall order was granted, Festuca seed totalling over 24,000 kg, heavily-infested with N. trichotoma, had already been sold in the states of Illinois, Kentucky, Missouri and both North and South Carolina (Westbrooks, 1991; Westbrooks and Cross, 1993).
HabitatTop of page In its native South American range, N. trichotoma occurs mainly in the damp pampas or climax grasslands of Argentina and Uruguay between latitudes 30° and 40°S (Wells, 1978; Erb, 1988); although it can be found in drier regions on the lower eastern slopes of the Andes and in the southern highlands of Buenos Aires Province (Evans and Ellison, 1995). In its exotic range, it is dominant in native and introduced pastures of subhumid, subtropical and warm-temperate regions between latitudes 30° and 45°S (Campbell, 1982; Campbell and Vere, 1995). More recent reports from Australia demonstrate that it can also colonize and invade dry coastal vegetation, grassy woodland, sclerophyll forest and rocky outcrop vegetation (McLaren et al., 1998).
Habitat ListTop of page
|Coastal areas||Present, no further details||Harmful (pest or invasive)|
|Disturbed areas||Present, no further details|
|Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Rail / roadsides||Present, no further details|
|Natural forests||Present, no further details||Harmful (pest or invasive)|
|Natural grasslands||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page N. trichotoma replaces both native and introduced (pasture) grasses, especially in the Tableland grasslands of Australia and the South African veld (Wells and de Beer, 1987; Campbell and Vere, 1995; Cronk and Fuller, 1995).
Host Plants and Other Plants AffectedTop of page
Biology and EcologyTop of page Genetics
The chromosome number is 2n = 38, which is shared by all Nassella species (Watson and Dallwitz, 1992). There are no reports of hybridization between the various New World stipoid grasses present in Australia (McLaren et al., 1998), although Erb (1988) suggested that the great variation he observed in N. trichotoma in the southern highlands of Argentina may have been a result of crossing with other indigenous Nassella species. Campbell (1998) reported the occurrence of possible ecotypes in Australia with some plants producing lime-green ('albino') seedheads, with viable seeds which also produced 'albino' seedheads lacking the distinctive purple hues due to the pigmented glumes.
Physiology and Phenology
Mature seeds of N. trichotoma germinate over a wide range of conditions, which can continue over an 800-day period (Healy, 1945), suggesting that differences in permeability of the testa controls the germination periodicity (Campbell, 1998). Recent studies in Australia have revealed that freshly-collected seed has a low germination rate and that most germination is from seed set in the previous years, showing a requirement for dormancy (Joubert and Small, 1982; Gardener and Sindel, 1998); although earlier, Campbell (1982) had inferred that the seeds do not have a dormancy mechanism. Removal of the testa has now been shown to increase germination rate, as will cutting-off the awn end of the seed and applying gibberellic acid and potassium nitrate (Campbell, 1998). This staggered dormancy probably serves to ensure germination and seedling establishment over a protracted period, in autumn, winter or spring, when conditions are more favourable compared to the drier summer. In Australia, seeds germinate mainly in the autumn and winter months (Parsons and Cuthbertson, 1992), but growth is initially slow compared to pasture grasses, for example (Campbell, 1998), and plants rarely flower in the first year. Vegetative growth is continued into the second year when flowering begins in late spring or early summer. On poor soil, however, flowering may be delayed until the third or fourth year due to insufficient tussock growth. The flower stems, seen as thicker-than-normal tillers, first appear at the base of the tussock and the panicles begin to emerge 2 weeks later; the green florets turning purple over the next week. By the fourth week, the anthers (3) and stigma (2) become visible. The panicles begin to elongate and by 10 weeks are fully elongated and the seeds ripe (Campbell, 1998). The panicles readily detach and are blown away, hence the common names: tumbleweed and paja voladora. Under favourable conditions, a plant can produce up to 2000 panicles in a season.
The tussock is long-lived, although the actual age span has not been determined and, whilst the centre may die down due to drought, low temperatures or after burning, the tussocks can regrow.
The majority (84%) of flowers of N. trichotoma in New Zealand have been reported to be cleistogamic (Connor, 1979) i.e. showing self-fertilization of closed florets. Cleistogamous florets have only one fertile anther and two vestigial ones; whilst the remainder (16%) had three fertile anthers with either self- or cross-fertilization of open florets, presumably by windborne pollen. Propagation is entirely by seeds which are produced in enormous numbers, up to 100,000 seeds per plant per year (Wells, 1978; Campbell, 1982), giving an estimated production of 900-3400 millions per ha (Healy, 1945). The readily-detached seed-bearing panicles can be blown long distances, calculated at >8 km/day in strong winds, retaining up to 50% of their seeds (Healy, 1945; Taylor, 1987). Seeds are also spread by the rumen of sheep (Cook, 1998). Because of the longevity of the seeds, which can survive for at least 20 years in soil (Wells, 1978; Taylor, 1987), the seed banks are immense, ranging from 1755 to 42,930 seeds/m² in New Zealand (Healy, 1945) but up to 75,000 seeds/m² in South Africa (Joubert, 1984).
According to Campbell (1982, 1998), N. trichotoma grows in a continental climate in South America, oceanic in New Zealand and temperate Mediterranean in Australia. The climate is typified by annual rainfall from 450 to 990 mm, mean summer temperature of 19-21°C. It is tolerant of mean winter minimum temperature of -5°C and can survive frost and ice cover for varying periods (Healy, 1945). Hot summer temperatures are the least favourable climatic factor because the optimum temperature for growth is between 10 and 15°C, which is why it thrives in the upland grasslands of Australia (Tablelands of New South Wales) and South Africa (Veld and mountain Fynbos).
Healy (1945) concluded that occurrence was not correlated with soil type or soil fertility, although Campbell (1998) considered that it grows best on light-textured, acid soils (pH 4.5) of low fertility, subject to regular moisture deficiencies. In South Africa, Wells and de Beer (1987) reported that N. trichotoma is rarely found in areas with an average annual rainfall below 500 mm and grows over a wide range of climatic conditions and soil types, being able to tolerate floods, drought, exposure to salt and repeated frost. In South America, Erb (1988) described its range as warm-temperate, with an annual mean temperature between 15 and 20°C, a minimum mean of 9°C and a maximum of 24°C; typically on loess-based or sandy-clay soils, characteristic of the hilly grasslands, rather than the flat pampas. Thus, in the humid pampas of Uruguay and north-central Argentina, Erb (1988) concluded that N. trichotoma could only compete in localized drier pockets, such as well-drained railway embankments and around roadside tree plantings drained of water by root competition. This was endorsed during subsequent surveys (Evans and Ellison, 1995; Briese and Evans, 1998).
In its natural range, N. trichotoma occurs within a complex of stipoid grasses (Evans and Ellison, 1995). It invades and is associated with similar native stipoid grasses (Austrostipa) in Australia and is also associated with Eucalyptus trees in both Australia and Argentina (Erb, 1988; Hocking, 1998; McLaren et al., 1998).
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-10|
|Mean annual temperature (ºC)||10||15|
|Mean maximum temperature of hottest month (ºC)||18||30|
|Mean minimum temperature of coldest month (ºC)||-5||5|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||3||6||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||500||990||mm; lower/upper limits|
Rainfall RegimeTop of page Bimodal
Soil TolerancesTop of page
Special soil tolerances
Natural enemiesTop of page
Notes on Natural EnemiesTop of page A survey for natural enemies with biological control potential was undertaken by Wells (1977) in South America, who reported that N. trichotoma was rarely aggressive or weedy in its native range, in part due to pressures from natural enemies, including insects and fungi. A follow-up survey was undertaken later by Erb (1988) who identified 10 weevil (Curculionidae) and two moth (Satyridae) species associated with N. trichotoma, but no fungi, and concluded that these insects offered the best prospects for biocontrol.
Wapshere (1990) observed that no natural enemies had, thus far, been recorded on N. trichotoma in Australia, and that no information, specifically on the occurrence of fungal pathogens, was available from its South American range. Subsequently, Evans and Ellison (1995) carried out a pathogen survey in Argentina and recorded nine fungal pathogens, including a rust and a smut (see List of Natural Enemies), associated with N. trichotoma (Briese and Evans, 1998). Hussaini et al. (1998) followed this-up with a survey in Victoria (Australia) and discovered two fungal pathogens (Deuteromycota): Zinzipegasa argentinensis, and a Fusarium sp. Further surveys revealed two additional pathogens in Australia causing leaf spot of N. trichotoma: Dinemasporium sp. and Ascochyta sp. (Hussaini et al., 2000).
Means of Movement and DispersalTop of page Natural dispersal
Propagation is entirely by seeds which are produced in enormous numbers on windblown panicles, which can be dispersed up to 16 km, according to Healy (1945), or >8 km per day (Taylor, 1987), and this has recently been revised upwards of 20 km per day (Jones and Vere, 1998).
Dispersal by grazing animals, both externally and internally, has been well documented (Wells, 1978; Campbell, 1998; Cook, 1998), with sheep passing large quantities of seed up to 4 days after ingestion (Campbell, 1962). New guidelines on transport of stock are now being considered in Australia, including quarantine, shearing before sale and restricting sale of unshorn stock in the flowering period (Cook, 1998). Seed is also spread by machinery and man (Campbell, 1998).
Linked to vector transmission.
There are numerous examples of intercontinental movement of seed as contaminants of trade goods, especially in: hay or fodder to South Africa (Wells, 1978); seed of Medicago sativa to New Zealand (Healy, 1945); pea seed to Tasmania (Goninon, 1998); Festuca seed in the USA (Westbrooks, 1991); and wool in Europe (Campbell, 1982).
Exotic Nassella species, particularly those with drooping tussocks, are sold in nurseries in Australia as ornamentals. N. tenuissima (Mexican feather grass) is causing concern because it is taxonomically close to N. trichotoma (Jacobs et al., 1998). The latter authors detail attempts to apply pressures to the nursery industry to prevent the importation and sale of such South American grasses. Potentially, N. trichotoma could be sold alongside, or confused with, N. tenuissima in plant nurseries.
Pathway VectorsTop of page
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Stems (above ground)/Shoots/Trunks/Branches||seeds|
|True seeds (inc. grain)||seeds|
|Plant parts not known to carry the pest in trade/transport|
|Fruits (inc. pods)|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page N. trichotoma causes more reductions in carrying-capacity than any other pasture weed in Australia (Parsons and Cuthbertson, 1992; Campbell, 1998). Similarly, in New Zealand, N. trichotoma can reduce the carry-capacity by up to 90%, leading to the abandonment of sheep farming (Healy, 1945). If forced to eat N. trichotoma, both sheep and cattle may die, due to the rumen becoming blocked by undigested leaves (Campbell, 1998). The annual benefits from control of N. trichotoma in New South Wales, Australia alone, have been put at over AU$40 million (Jones and Vere, 1998), mainly due to losses incurred by the wool and lamb industries.
Environmental ImpactTop of page In South Africa, it has been known for many years that N. trichotoma invades and competes with natural vegetation, especially in the upland grasslands (Veld) and Fynbos (Wells, 1978). Only recently, however, has this environmental impact been appreciated in Australia, and techniques are being developed for the selective removal of N. trichotoma from native grasses swards and their replacement with native grasses (Hocking, 1998). It is also invading and impacting on conservation areas, including dry coastal vegetation, grassy woodland, dry sclerophyll forest and rocky outcrop vegetation in New South Wales and Victoria (McLaren et al., 1998), and Goninon (1998) has identified the threat posed to sensitive native grasslands in Tasmania.
Impact: BiodiversityTop of page Anecdotal evidence quoted by Gardener and Sindel (1998) indicates that there is a drop in biodiversity in grasslands invaded by N. trichotoma because the litter from the tussocks excludes shade-tolerant native herbs, and also eliminates mosses and lichens which typically colonize bare soil or rocky ground. McLaren et al. (1998) listed a number of native Eucalyptus spp. as well as understorey shrubs, in grassy woodland and dry sclerophyll forest in New South Wales (Australia), which are being threatened by invasion of N. trichotoma; although infestation into high-grade conservation areas, especially those dominated by native tussock grasses, appears to be slow (Hocking, 1998).
Social ImpactTop of page In worst case scenarios in New South Wales (Australia), farmers cannot sell infested properties and have to seek alternative employment (Campbell, 1998). In an economic assessment, Jones et al. (2000) concluded that on low rainfall and low soil fertility sites, the socially optimal control option for N. trichotoma is to retire the land from agriculture and plant trees. Similar social impacts have been experienced in New Zealand (Healy, 1945). Gardener and Sindell (1998) considered that its impact on conservation lands resulted in a decrease in their aesthetic value.
Risk and Impact FactorsTop of page Invasiveness
- Proved invasive outside its native range
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts agriculture
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Competition - monopolizing resources
- Highly likely to be transported internationally accidentally
- Highly likely to be transported internationally deliberately
- Difficult to identify/detect as a commodity contaminant
- Difficult to identify/detect in the field
- Difficult/costly to control
UsesTop of page N. trichotoma has no known uses. It is inferior to unpalatable as a pasture grass, but possibly could be sold with other 'weeping' tussock grasses as a garden ornamental.
Similarities to Other Species/ConditionsTop of page Many other tussock grasses have serrated leaves and thus the name serrated tussock can be misleading unless combined with other characters. The defining feature of N. trichotoma, for field identification, is the presence of a short, white, hairless ligule with a rounded apex, which distinguishes it from similar tussock species which either have no ligule or possess a ring of hairs around the top of the ligule (Campbell and Vere, 1995; Evans and Ellison, 1995). In his key to South American stipoid grasses in Australia, Walsh (1998) separates N. trichotoma from similar tussocks, not only in Nassella but also in closely-related genera such as Piptochaetium, by the firmly affixed awns, in addition to ligule morphology. Another recent arrival in Australia, N. tenuissima (Mexican feather grass) has also been closely compared with N. trichotoma and the distinguishing features are listed by Jacobs et al. (1998).
Prevention and ControlTop of page Cultural Control
Grazing by cattle and sheep cannot control N. trichotoma; however, at low weed density (10% of an improved pasture), goats can help to restrict weed population density (Campbell, 1982). Nevertheless, most reports claim that N. trichotoma is unpalatable and that animals will not readily graze it (Wells and de Beer, 1987). N. trichotoma burns readily but is quick to recover, whilst associated species are killed (Healy, 1945). Thus, ideal conditions are created for massive weed infestation because of the large seedbank (Campbell, 1982; Wells and de Beer, 1987). In general, therefore, burning can increase rather than decrease weed populations.
Afforestation has been used in both Australia and New Zealand to control N. trichotoma, especially with Pinus radiata (Healy, 1945; Campbell, 1982). However, it can take up to 6 years to shade-out the weed and prevent flowering, and a further 4 years before the tussocks die (Parsons and Cuthbertson, 1992; Miller, 1998). The planting of hedges or living windbreaks to trap windblown panicles has been practised in both Australia and South Africa (Campbell, 1982; Wells and de Beer, 1987).
Chipping, or severing the root system, has successfully been practised in New Zealand, following-on from other control measures, but is mainly used to kill isolated plants (Healy, 1945; Denne, 1988).
Ploughing can also have an impact but only in combination with other techniques (Parsons and Cuthbertson, 1992).
Both glyphosate and flupropanate have proven to be effective against N. trichotoma in Australia and, despite its slow action, flupropanate has been favoured until recently because of its ease of use, low mammalian toxicity and selectivity, especially when applied in late spring (Campbell and Ridings, 1988). In addition, it was found that herbicide residues in the soil killed the next flush of N. trichotoma seedlings (Campbell, 1998). Glyphosate has recently been shown to be effective in preventing seedhead production, if applied in spring, 2-8 weeks prior to seedhead emergence (Campbell et al., 1998).
On non-arable land in Australia, aerial application of flupropanate, pasture seed and fertilizer has been used: control depends on replacing N. trichotoma with a strongly competitive pasture (Campbell, 1998). Spot-spraying with flupropanate has been favoured by landholders in New South Wales but this involves frequent inspection.
Long-term trials have continued with flupropanate for management of N. trichotoma and initial results suggest that boom or aerial application at 3- or 10-year intervals could economically control the weed over large areas (Campbell et al., 2002). Unfortunately, flupropanate has since been withdrawn from sale in Australia and glyphosate now remains the only herbicide option (Campbell and Nicol, 2001).
Grasses are difficult targets for classical biological control, particularly using arthropod natural enemies, because of the uniform, simplistic architecture which favours polyphagy (Evans, 1991). Moreover, N. trichotoma was considered to be too closely-related to native Australian Stipa spp. 'to allow the introduction of agents' (Wapshere, 1990). However, this changed when the Australian tussock species were found to be so phylogentically distinct that they were accommodated in the new genus, Austrostipa (Jacobs and Everett, 1996). Several fungal pathogens, including the rust Puccinia nassellae and the smut Ustilago hypodytes, are currently being assessed in Argentina as potential biocontrol agents of N. trichotoma in Australia (Briese and Evans, 1998; Briese et al., 2000).
None of the methods outlined above can be used in isolation, and the management focus has now changed from trying to kill millions of seedlings, per hectare, up to 3400 million (Healy, 1945), by blanket coverage with herbicide, to promoting pasture competition and the strategic use of herbicides. Earlier, Wells (1978) in South Africa, had detailed an integrated strategy based on prevention of seeding by removing and burning seed heads, killing mature tussocks by chipping, ploughing or spot-spraying and prevention of seed dispersal by planting windbreaks, keeping stock away at seeding time, maintaining a good plant cover, particularly on the veld, by not burning. Essentially, this strategy is no different from that currently being advocated in Australia (Campbell, 1998; Miller, 1998), where it has been concluded that a plan to limit seed re-invasion must be in place to support other control measures, such as using appropriate growing regimes, fertilizers and spot-spraying. Underpinning this integrated control approach, is the realisation that a competitive environment needs to be maintained to prevent invasion and that C4 native grass species can outcompete C3 species such as N. trichotoma, especially in low fertility soils, but that over-grazing is a major factor in reducing their competitiveness (Badgery et al., 2003).
ReferencesTop of page
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