Grevillea robusta_Zoya Akulova
Photo by Zoya Akulova

Grevillea robusta Risk Assessment

Common names: silkoak

Grevillea robusta -- California

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Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
April 21, 2016
Evaluation Time (hrs): 
2 Hours
Evaluation Status: 
Completed
Plant Information
Plant Material: 
If the plant is a cultivar, and if the cultivar's behavior differs from its parent's (behavior), explain how: 
Regional Information
Region Name: 
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.
Climate Matching Maps PDF: 
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: 
Very High
Answer / Justification: 
Naturalized in California. Present in 8 counties according to Calflora. Native of Australia but widely introduced and naturalized in tropical and subtropical regions of the world. There are many points in GBIF. This species is cultivated for forestry. "The trees scatter their winged seeds and have become undesirable weeds in some pastures and rangelands of Hawaii, requiring eradication."
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: 
Very High
Answer / Justification: 
Naturalized in California. Present in 8 counties according to Calflora. There are many points in GBIF, including areas of South Africa that are similar to California.
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: 
High
Answer / Justification: 
Invasive in Jamaica, Zimbabwe, South Africa and Hawaii. In South Africa, invades forest edges, savannah and riparian areas. Hawaii: "The trees scatter their winged seeds and have become undesirable weeds in some pastures and rangelands of Hawaii, requiring eradication."
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: 
In South Africa, invades forest edges, savannah and riparian areas. A regulated weed in South Africa and a problem in Western Cape, Eastern Cape, KwaZulu-Natal, Gauteng, Limpopo and Mpumalanga.
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: 
There are numerous species of Grevillea listed in Randall 2012, although most have just a couple of references to it being naturalized or a weed. Grevillea banksii is invasive and a regulated plant in South Africa.
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: 
Very High
Answer / Justification: 
Based on locations in GBIF and description in CABI, it occurs mostly in areas that are more tropical than California. Map: http://www.gbif.org/species/2891791
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: 
Can displace native plant communities in South Africa. Extensive naturalized stands in Hawaii.
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
"G. robusta is vulnerable to fire and hence is excluded from the fire-prone Eucalyptus forests and grasslands that occupy much of its natural range."
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: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
No mention of toxicity in references, other than possible allelopathy. Can invade savannahs in South Africa but I found no mention of impacts to grazing specifically.
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: 
A tree that grows 40-70 ft high with many branches. Described as invasive but I did not find data to support it creating impenetrable stands.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Can reproduce vegetatively through root suckering.
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: 
High
Answer / Justification: 
This is a tree so would be unlikely to have this form of reproduction.
Reference(s): 
13. Does the species (or cultivar or variety) commonly produce viable seed?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Producing many seeds.
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Medium
Answer / Justification: 
Produces "great quantities" of seeds but I do not have a specific number. There are about 40,000 viable seeds/kg, but again, not sure how that translates to seeds per plant.
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: 
High
Answer / Justification: 
Described as a prolific seeder. No mention of any special requirements in the references I found.
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: 
Medium
Answer / Justification: 
Described as an early and prolific seed producer so I am assuming yes.
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: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
"Flowering commences in October in lowland occurrences in the native range, and seed is mature by the end of December. At high elevations, flowering and seed production occur 4-6 weeks later (Harwood, 1992a)"
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: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
wind-dispersed
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: 
Very High
Answer / Justification: 
Winged seeds are dispersed by wind.
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: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
Winged seeds are dispersed by wind.
Reference(s): 
Evaluation Notes

HEAR, Hawaii: http://www.hear.org/pier/species/grevillea_robusta.htm

Forest Trees of Hawaii: http://www.ctahr.hawaii.edu/forestry/trees/CommonTreesHI/CFT_Grevillea_robusta.pdf

GBIF: http://www.gbif.org/species/2891791

CABI datasheet: http://www.cabi.org/isc/datasheet/25866

South Africa: http://www.invasives.org.za/video/item/242-australian-silky-oak-greville...

Reviewed by Ramona Robison.

 

Total PRE Score

  • < 13 : accept (low risk of invasiveness)
  • 13 - 15 : evaluate further
  • > 15 : reject (high risk of invasiveness)

PRE Score: 
16
Number of questions answered: 
20
Screener Confidence (%): 
83.0
PRE Content Access and Privacy
Evaluation visibility: 
Public - accessible to all site users

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