RMD-13-04: Consolidated Pest Risk Management Document for pest plants regulated by Canada
Appendix 9A: Pest Risk Assessment Summary for Nassella trichotoma (serrated tussock)

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Identity of Organism

Name: Nassella trichotoma (family Poaceae, subfamily Pooideae, tribe Stipeae) (USDA-NRCS 2009).

Synonyms: Stipa trichotoma (1829), Urachne trichotoma (1834), Agrostis trichotoma (1841), Urachne macrathera (1853), Piptochaetium trichotomum (1879), Piptatherum macrantherum (1894), Stipa macrathera (1901), nom. illeg., non Stipa macrathera (1896), Oryzopsis trichotoma (1910).

English common names: Serrated tussock, serrated tussock grass, nassella tussock, Yass tussock, Yass River tussock, tumbleweed.

French common names: Stipe à feuilles dentées

Description: Microstegium vimineum is an annual grass with a sprawling habit. It germinates in spring and grows slowly through the summer months, ultimately reaching heights of 0.6-1 m with the reclining stems growing up to 1 metre long. Slender stalks of tiny flowers are produced in August through to September or early October. In late fall they fade to pale greenish-yellow or turns purple in colour (Swearing 2004, Mehrhoff et al. 2003).

Nassella trichotoma is a tall (at least 60 cm), tussock-forming, perennial grass of the tribe Stipeae (family Poaceae), native to the pampas of South America and introduced into dry, warm-temperate regions in other parts of the world, primarily in the southern hemisphere. In Australia and New Zealand it is a serious, invasive weed in pasture lands and is considered one of their most serious weed problems. In pasture, it reduces the carrying capacity due to its low nutritive value and poor palatability to livestock.

Organism Status

Nassella trichotoma is not reported to occur in Canada, and no evidence was found that it iscultivated here (CFIA 2008, Scoggan 1979). Based on this information, it is considered to be absent from the PRA area.

Nassella trichotomahas not yet become established in North America:

Current Regulatory Status

Canada: Listed as Class 1 "Prohibited Noxious Weed", under the Weed Seeds Order (Seeds Act). All imported and domestic seed must be free of prohibited noxious weed seeds.

United States: Listed in the USDA-APHIS Footnote 1 Federal Noxious Weed List under the Noxious Weed Regulation (Federal Noxious Weed Act). No person may move a federal noxious weed into or through the United States, or interstate, unless this person obtains a permit for such movement (Noxious Weed Regulation).

European Plant Protection Organization (EPPO): Nassella trichotoma is not regulated in Europe, however, it was added to the EPPO Alert List because it presents a risk to the EPPO region. The objective of the EPPO Alert List is to provide early warning and eventually to propose candidates which may be subjected to a PRA (EPPO 2009).

Probability of Entry

The main pathway through which Nassella trichotoma is likely to be introduced into Canada is in seed lots of forage grasses for planting. However, the species does not seem to be a common weed in seed production fields in either its native range or where introduced and it has only rarely been intercepted in imported lots. Nassella trichotoma has been listed as a prohibited noxious weed seed on the Weed Seeds Order: this should greatly reduce the likelihood of it entering Canada as a seed contaminant. A second pathway is available through the sale of plants and seeds for planting in home gardens. See table 1, Summary of Pathways for Nassella trichotoma (serrated tussock).

Table 1: Summary of Pathways for Nassella trichotoma
Type of pathway Specific Pathways
Natural dispersalSeed: the wind distributes seeds over long distances.
Intentional Introduction
  • Seed of ornamental grass
  • Ornamental plants for planting
Unintentional Introduction
  • Contaminant of seed lots of forage grasses
  • Contaminant of hay and straw
  • Contaminant in raw wool
  • Spread by livestocka.

Probability of Establishment

The potential range of Nassella trichotoma in Canada would be very limited. Its range in South America and Australia suggests that it would be confined to Canadian Plant Hardiness Zone 8, possibly extending into zone 7. Even if zone 7 is included, the potential range would be limited to coastal British Columbia where there is a limited amount of suitable habitat (Figure 1).

Climate modeling, based on Nassella trichotoma's current worldwide range, limits the potential distribution even further to extreme Southwestern BC

Figure 1: Potential range of Nassella trichotoma (serrated tussock) in Canada

Figure 1. Description follows.
Description of Figure 1

This map shows the potential range of Nassella trichotoma (serrated tussock) in Canada, more specifically in British Columbia. Red is used to indicate the regions in which Nassella trichotoma could survive according to the Canadian Plant Hardiness Zones map, in this case NAPPFAST Hardiness Zone 8 and possibly zone 7. In Canada, the potential range of Nassella trichotoma is very limited to only coastal British Columbia. This image is narrowed to include the lower coastal regions of BC as well as Washington and Oregon which all have a scattered presence of red.

NAPPFAST zones 7-9

Probability of Spread

Natural spread potential is high. Nassella trichotoma spreads by seed, which may be produced in large quantities. The large panicles break off and blow in the wind, like tumbleweeds, distributing seeds over long distances in open habitats. Under suitable conditions, the plant can spread very rapidly. The seeds can persist for many years in the soil. Since Nassella trichotoma does not usually grow in cultivated fields, this reduces the likelihood of the seeds being moved around on machinery.

Potential Economic Consequences

Potential economic impact is not significant. The primary economic impact of Nassella trichotoma is a reduction in carrying capacity of range and pasture. As most of the ranching areas in Canada are outside of the potential range, the impact would be limited. In badly-infested areas in Australia land can become unusable for pasture, greatly reducing land values. A similar level of impact is unlikely to occur in Canada.

Potential Environmental and Social Consequences

Potential environmental impact is high. Nassella trichotoma has demonstrated an ability to out-compete native and introduced grasses on grazed lands where it has been introduced in Australia, New Zealand and South Africa. It has the potential to become established in native grasslands and lightly forested terrain, including the Garry oak meadows of Southwestern BC. While this area is very limited (see Figure 2), these meadows represent a critically endangered ecosystem within Canada, which provides habitat for about 54 species of vascular plants and three butterfly taxa that are listed as endangered or threatened in Canada.

There are no reports of toxicity to livestock or humans, but domestic animals have died from heavy feeding on Nassella trichotoma due to the indigestible material forming large balls in the stomach. Heavy infestations of conservation lands with Nassella trichotoma decrease their biological diversity, as well as their aesthetic value.

Figure 2: Distribution of Garry Oak Ecosystems in British Columbia

Figure 2. Description follows.
Description of Figure 2:

This map shows the distribution of Garry oak ecosystems in British Columbia through the use of the colour red. It is present, predominantly, on the coastal region of Southern Vancouver Island surrounding the Victoria and Nanaimo regions extending slightly upwards. The distribution of the Garry Oak ecosystem can be found on mainland BC as well which is signified by the two red circles in the vicinity of Hope. There is a black and white arrow on the mainland of the province pointing upwards to identify the direction of north.

Source: GOERT 2009


This assessment is based on the current understanding of the species' range and climatic tolerances. There are no records of the plant becoming established beyond the equivalent of a Canadian hardiness zone 8. Any evidence to show that it could survive in zone 6 or beyond would dramatically affect this assessment. Establishment potential and economic impact, in particular, would have to be re-assessed. If it was determined that the species was not a threat to endangered species in the Garry oak meadows of Vancouver Island, the environmental impact could be down-graded.


The overall risk associated with Nassella trichotoma is moderate. This indicates that specific phytosanitary measures are necessary. In 2005, the species was listed as a prohibited noxious weed seed. This should greatly reduce the likelihood of Nassella trichotoma being introduced into Canada. The importation and sale of plants for the public should also be regulated. The combination of these two measures would provide adequate phytosanitary protection to Canada against this species. It is recommended that Nassella trichotoma be considered for inclusion on the List of Pests Regulated by Canada.

Technical Issues for Consideration


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