Photo: Gary French (Australia)

Nassella trichotoma Risk Assessment

Synonyms: Stipa trichotoma, Piptochaetium trichotomum (Nees) Griseb.

Common names: serrated tussock, serrated tussock grass, Yass tussock, Yass River tussock

Nassella trichotoma -- California

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Created by: Lauren Quon
Created on: Thursday, Dec 7th, 2023
Evaluation Summary
Serrated tussock (Nassella trichotoma) is a globally invasive perennial grass native to South America, where it occurs in warm temperate grasslands (CABI 2019, Humphries et al 2020). It is a major pasture weed in Australia and South Africa where it is widespread and invades agricultural areas (Invasives SA 2021, Humphries et al 2020, South Australia 2015). Invasion into conserved lands with native vegetation cover seems to be slower compared to invasion of agricultural lands, so most removal efforts are focused in native grasslands and conservation areas (CABI 2019). In the US, serrated tussock is on the USDA invasive weed list, and occurs in the following states: Illinois, Kentucky, North Carolina, South Carolina, Arizona, Florida, Hawaii, and Texas (Flora of North America 2021, CABI 2019, USDA-NRCS 2004, Texas invasives). Serrated tussock has not yet been observed in California. Serrated tussock reproduces and mainly grows from seed, and one plant may produce over 100,000 (up to 140,000) seeds that may be distributed by animals, wind, or water (Humphries 2020, South Australia 2015). It forms tussocks up to 50 cm tall and 25 cm across at the base, with diffuse fibrous roots within the first 20 cm of soil (CABI 2019, South Australia 2015). It is a poor unpalatable pasture grass, but can be used as an ornamental grass (CABI 2019).
General Evaluation Information
Date of Evaluation: 
December 7, 2023
Evaluation Time (hrs): 
8 Hours
Evaluation Status: 
Completed
Plant Information
Plant: 
Nassella trichotoma
Plant Material: 
If the plant is a cultivar, and if the cultivar's behavior differs from its parent's (behavior), explain how: 
N. trichotoma is not a cultivar.
Regional Information
Region Name: 
California
Climate Matching Map
These maps were built using a toolkit created in collaboration between GreenInfo Network, PlantRight, Cal-IPC, and Agricultural Sustainability Institute at UC Davis.
PRE Climate Map (external link)
Climate Matching Maps PDF: 
AttachmentSize
PDF icon ClimateMatch-Nassella_trichotoma-California-20231207.pdf1.12 MB
Invasive History and Climate Matching
1. Has the species (or cultivar or variety, if applicable; applies to subsequent "species" questions) become naturalized where it is not native?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Serrated tussock is native to South America, but has become naturalized in Australia and South Africa (Invasives SA 2021, Government of South Australia 2015).
Reference(s): 
2. Is the species (or cultivar or variety) noted as being naturalized in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
High
Answer / Justification: 
Serrated tussock is naturalized in Australia and South Africa, which have similar climates to the US (Invasives SA 2021, Government of South Australia 2015). According to the Flora of North America, it is on the USDA's noxious weed list in the US.
Reference(s): 
3. Is the species (or cultivar or variety) noted as being invasive in the U.S. or world?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
Very High
Answer / Justification: 
Serrated tussock is invasive in the US, where it is considered invasive in Illinois, Kentucky, North Carolina, South Carolina, Arizona, Florida, Hawaii, and Texas (Flora of North America 2021, CABI 2019, USDA-NRCS 2004, Texas invasives).
Reference(s): 
4. Is the species (or cultivar or variety) noted as being invasive in the US or world in a similar climate?
Yes or No: 
Yes
Points: 
3
Confidence Level: 
Very High
Answer / Justification: 
Serrated tussock is on the USDA's noxious weed list. The Flora of North America identifies it's occurrences in Illinois, Kentucky, North Carolina and South Carolina. CABI and Texas invasives.org lists this species as invasive in the following states: Arizona, Florida, Hawaii, Texas, and North Carolina. It is also considered invasive in in Australia, New Zealand, and South Africa, areas of similar climates (also see climate matching map attached and linked).
Reference(s): 
5. Are other species of the same genus (or closely related genera) invasive in a similar climate?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Nassella tenuissima, Mexican feather grass has similar features and is considered an invasive garden plant by some horticultural media. It is naturalized in Arizona, as an ornamental grass (CABI 2019). It was introduced in Belgium, UK, Australia, and South Africa, where Australia and South Africa are areas of similar climates to California.
Reference(s): 
6. Is the species (or cultivar or variety) found predominately in a climate matching the region of concern?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
In its native range in South America, it occurs mostly where climate does not match California. Outside of its native range, it is found in Madagascar, Europe, South Africa, Australia, and other parts of North America, where climate often does not match California. Link to climate matching map: https://weedmap.cal-ipc.org/climatematch/?areaType=states&areaList=06&mapView=2%2C0.00000%2C0.00000&datalayer=COMBINED&datalayeropacity=60&gbif_taxonkey=9613389&gbif_search=nassella+trichotoma&gbif_speciesname=Nassella+trichotoma
Reference(s): 
Impact on Native Plants and Animals
7. Does this plant displace native plants and dominate (overtop or smother) the plant community in areas where it has established?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Many sources state that serrated tussock grass invades pastures and can cause livestock production loss in temperate grasslands due to it being left ungrazed (Humphries 2020, CABI 2019, Weeds of Australia 2016, Government of South Australia 2015). If left ungrazed, serrated tussock can continue to spread into open spaces and displace native grass and other species (Humphries 2020, CABI 2019). In native grasslands and coastal vegetation, serrated tussock displaces native grass and degrades fauna habitat (Government of South Australia 2015).
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Serrated tussock grass fields can promote fire/be a fire hazard, especially infestations in the wildland-urban interface, where houses or other infrastructure borders infested areas (Government of South Australia 2015). Serrated tussock grasses can establish in post-fire bare ground areas and grows vigorously after it is burned (Victorian Serrated Tussock Working Party 2023).
Reference(s): 
9. Is the plant a health risk to humans or animals/fish? Has the species been noted as impacting grazing systems?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Serrated tussock grass is high in fibre, but low in protein, which can cause indigestible balls in livestock rumen and eventually lead to loss in livestock (CABI 2019, Government of South Australia 2015). Serrated tussock grass, if left ungrazed, can reduce up to 97% of losses in livestock production (CABI 2019).
Reference(s): 
10. Does the plant produce impenetrable thickets, blocking or slowing movement of animals, livestock, or humans?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Serrated tussock grass may be evenly distributed throughout an area, and would be unlikely to block or slow movement of animals, livestock, or humans. Current literature implies that it does not form thickets, but rather spreads out over bare ground and disturbed pastures (Humphries 2020, CABI 2019). Serrated tussock forms tussocks up to 50 cm tall and 25 cm across at the base, but it's unclear whether plants form dense, impenetrable thickets (CABI 2019, South Australia 2015).
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
No
Points: 
0
Confidence Level: 
Very High
Answer / Justification: 
Sources indicate that serrated tussock reproduces and grows from seed (Humphries et al 2020, CABI 2019, Weeds of Australia 2016). Serrated tussock is not rhizomatous, so vegetative spread is also unlikely (Flora of North America 2021).
Reference(s): 
12. If naturally detached fragments from this plant are capable of producing new plants, is this a common method of reproduction for the plant?
Yes or No: 
No
Points: 
0
Confidence Level: 
Very High
Answer / Justification: 
Sources indicate that new plants come from seeds (Humphries et al 2020, Weeds of Australia 2016). Serrated tussock is not rhizomatous, so vegetative spread is also unlikely (Flora of North America 2021).
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Serrated tussock individuals may produce over 100,000 (up to 140,000) seeds that are mostly viable (Humphries 2020, CABI 2019, South Australia 2015). Seeds are viable, with germination at 50% after 5 days, but seed viability decreases over the span of 12 months, where less than 10% of seeds may be viable (Humphries and Florentine 2022).
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Each plant produces more than 100,000 seeds, and up to 140,000 seeds annually (Humphries et al 2020, Government of South Australia 2015). Seed viability is noted as high within the first 5 days, but decreases over time (Humphries and Florentine 2022).
Reference(s): 
15. Is there significant germination (>25%) of seeds the next growing season, with no requirement of an infrequent environmental condition for seeds to germinate (i.e. fire) or long dormancy period?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Germination generally occurs throughout the year, but can increase if there is soil disturbance or fire. Drought and salinity can decrease germination (Humphries et al 2020). Though most seeds germinate within 6-12 months, some seeds may be dormant, or have staggered dormancy periods to ensure germination over time (Humphries et al 2020, CABI 2019).
Reference(s): 
16. Does this plant produce viable seed within the first three years (for an herbaceous species) to five years (for a woody species) after germination?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Serrated tussock reproduces and mainly grows from seed, and one plant may produce over 100,000 (up to 140,000) seeds that may be distributed by animals, wind, or water (Humphries and Florentine 2022, Humphries et al 2020, Government of South Australia 2015). Seed banks are viable for up to 3 years (Humphries and Florentine 2022).
Reference(s): 
17. Does this plant continuously produce seed for >3 months each year or does seed production occur more than once a year?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Serrated tussock may produce seed for more than 3 months each year, with seed production occurring between October through April, after flowering from August through May (Victorian State Government 2020).
Reference(s): 
Dispersal
18. Are the plant’s propagules frequently dispersed long distance (>100 m) by mammals or birds or via domestic animals?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Seeds can be dispersed by livestock when they get stuck on livestock fur coats or mud hooves (Government of South Australia 2015).
Reference(s): 
19. Are the plant’s propagules frequently dispersed long distance (>100 m) by wind or water?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Serrated tussock seeds can be dispersed by wind or water, but it is unknown if they travel long distances (CABI 2019, Weeds of Australia 2016, Government of South Australia 2015). We can assume that seeds can be dispersed long distances due to size and structure, which may increase mobility.
Reference(s): 
20. Are the plant’s propagules frequently dispersed via contaminated seed (agriculture or wildflower packets), equipment, vehicles, boats or clothing/shoes?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Plant seeds can be dispersed by contaminated machinery and equipment, hay, footwear, and clothing (Government of South Australia 2015). Frequency of distribution is unknown, but seeds are likely dispersed frequently in areas where plants are numerous.
Reference(s): 
Evaluation Notes
Notes: 
Total PRE Score

  • < 13 : Low Potential Risk
  • 13 - 15 : Moderate Potential Risk
  • > 15 : High Potential Risk

PRE Score: 
20
Number of questions answered: 
20
Screener Confidence (%): 
81.0
PRE Content Access and Privacy
Organization: 
2023-2024 Western IPM Project
California Invasive Plant Council (Cal-IPC)
Evaluation visibility: 
Public - accessible to all site users

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