Verbena bonariensis_S58-02
Photo courtesy UC Davis Weeds of California

Verbena bonariensis Risk Assessment

Common names: tall vervain

Verbena bonariensis -- California

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Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
September 20, 2016
Evaluation Time (hrs): 
3 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: 
Native to South America. Widely naturalized in Europe, South Africa, North America, Australia, Japan, New Zealand, Papua New Guinea, and other regions.
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: 
Naturalized in 27 counties in California, the region of interest. Also naturalized in New South Wales and Victoria in Australia in areas that match California's climate. Other areas with a climate match: South Africa, a few of the US East coast occurrences, areas of Spain and France.
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: 
Low
Answer / Justification: 
It is on the "watch list" for the Washington Noxious Weed Control Board site (not noxious, but watch). In Georgia EPC it is listed on the list: "Exotic plant that is naturalized in Georgia but generally does not pose a problem in Georgia natural areas or a potentially invasive plant in need of additional information to determine its true status." Invasive plant atlas does not give any details beyond: "It has escaped cultivation and become naturalized in disturbed areas across the southeastern United States and in California. It is drought and heat tolerant and is very common along roadsides and other disturbed areas." Flora of Australia online indicates: "Presumably introduced as a garden ornamental, now commonly naturalised and spreading on the Island," but no indication as to where in Australia this is. In a vegetation classification study in South Africa, V. bonariensis formed a community with up to 60% cover with Datura ferox and Argemone subfusiformis (DuPreez and Venter 1990) indicating its ability to form dense communities which restrict natives.
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: 
Low
Answer / Justification: 
In Murray and Phillips 2008, in the context of Australian weeds, it is listed as a "non-invasive" exotic species, ("classified as invasive across Australia (category ‘5A’ species). We used this source of information in conjunction with PIER (2009) to identify the invasive species in our dataset."). According to Queensland Biosecurity site: "Purpletop (Verbena bonariensis) is regarded as an environmental weed in Victoria, New South Wales, Queensland, Tasmania and the ACT." Note this is not a "significant" environmental weed. In the Syndney area, V. bonariensis was found to be one of the most abundant exotic species which germinated after fire (Thomson and Leishman 2005) and in South Africa V. bonariensis formed a community with up to 60% cover with Datura ferox and Argemone subfusiformis (DuPreez and Venter 1990) indicating its ability to form dense communities which restrict natives. In Australia and South Africa there are areas which match California's climate so this question is marked Yes with low confidence.
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: 
Low
Answer / Justification: 
Australian biosecurity site: "The purpletops (Verbena incompta and Verbena bonariensis) are regarded as environmental weeds in Victoria, New South Wales, Queensland, Tasmania and the ACT." Note they are not "significant environmental weeds." In Invasive Plant Atlas, several species, including V. littoralis, V. brasiliensis, V. montevidensis and this species are "invasive," there was no information about any noxious status or environmental harm found for Verbena species in similar climate areas.
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: 
The areas that match California's climate: distribution in South Africa and Tanzania (see very small match area in that area, without a large scale zoom map), some areas in Spain and France, areas in California, areas in New South Wales and Victoria, Australia, and areas along the Chile/Argentinian border. However, in Europe over half of the occurrences extend well beyond the climate match, into northern France and Germany, as well as Scandanavia and the U.K. In Australia, over half of the occurrences stretch inland beyond the climate-match, into inland NSW and Queensland. In New Zealand, none of the occurrence points match California's climate (it should be noted that in none of the species I have assessed have they matched in New Zealand) as well as most of the occurrences in Japan, South America, and the Eastern USA.
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: 
No
Points: 
0
Confidence Level: 
Low
Answer / Justification: 
Very little information about competition and community ecology for this plant available. One paper discussing butterfly distribution (Pryke and Samways 2004) lists: "In the least disturbed sites, both inside and outside the estate, indigenous flowers and Verbena bonariensis dominated."
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
No
Points: 
0
Confidence Level: 
Low
Answer / Justification: 
Unlikely considering habit, herbaceous perennial with sparse stems.
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: 
Low
Answer / Justification: 
Palatable to cattle. No other information about toxicity or health risk found.
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: 
Herbaceous perennial, no note or evidence given of thickets or impedance of movement of animals.
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: 
Murray and Phillips 2009 listed vegetative spread for this species. Also stated indirectly in Qld Biosecurity site: "Purpletop (Verbena bonariensis) reproduces mainly by seed" (emphasis on "mainly).
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: 
Low
Answer / Justification: 
Other species of verbena spread via rhizome, but based on lack of evidence, answering "no" to this question.
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: 
Readily self-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: 
In the botanical description in Jepson, the plant produces 8-17 spikes per plant, which appear that they could each produce at least 125 (1-1.25mm) fruits within the "dense" cyme. As well, an informal count of flowers on the plant resulted in a count of between 200 and 420 flowers per cluster, and with >5 per plant, this results in >1000 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: 
Medium
Answer / Justification: 
Missouri Botanical Garden gives information regarding sowing that implies no unusual conditions necessary.
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: 
High
Answer / Justification: 
V. incompta documented as producing seed in the first year. In Thomson and Leishman (2005) V. bonariensis is listed as one of the top species growing from seed after a fire.
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: 
Queensland biosecurity notes flowering from early spring-fall (October-March in Australia). Much > 3 months. Calflora lists flowering in May and June. Invasive Plant Atlas (US) indicates flowering "all summer until the first frost. Missouri Botanical Garden bloom time "June-frost."
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: 
Low
Answer / Justification: 
Queensland biosecurity lists dispersal by animals (animals, "external"), as well as wind and water.
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: 
Low
Answer / Justification: 
Queensland biosecurity lists dispersal by wind and water, but does not indicate its frequency.
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: 
Low
Answer / Justification: 
Listed by Queensland Government as "may also be spread in contaminated agricultural produce" but that doesn't give an estimate of frequency.
Reference(s): 
Evaluation Notes

Note: Calflora/TJM2 lists "Verbena incompta" as a synonym for this species. Tropicos does not. Invasive.org lists V. incompta as "Brazilian vervain" and  different than V. bonariensis. Review of taxonomy in Verloove 2010 and Nesom 2011 indicates this is still being debated, but here we go with the synonymy in GRIN, bonariensis = incompta.

Cal-IPC PAF was done, but this evaluator was unable to locate it online (evaluated not listed)

Invasive.org for V. incompta (assuming this is V. brasiliensis): http://www.invasive.org/browse/subinfo.cfm?sub=6988

Invasive.org for V. bonariensis: http://www.invasiveplantatlas.org/subject.html?sub=10107

Flora of Australia online: http://www.anbg.gov.au/abrs/online-resources/flora/redirect.jsp

GRIN: https://npgsweb.ars-grin.gov/gringlobal/taxonomydetail.aspx?id=408277

GRIN listing "auct" or plants misapplied this name as being V. brasiliensis: https://npgsweb.ars-grin.gov/gringlobal/taxonomydetail.aspx?id=408616

Queensland Biosecurity Edition: http://keyserver.lucidcentral.org/weeds/data/media/Html/verbena_bonarien...

Washington watch site (no real info): http://www.nwcb.wa.gov/weeds/tall-verbena

Dave's Garden: http://davesgarden.com/guides/pf/go/141/#b

MOBOT: http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.as...

Clemson Verbena page: http://www.clemson.edu/extension/hgic/plants/landscape/flowers/hgic1175....

Reviewed by Tim Hyland and Denise Knapp.

Total PRE Score

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

PRE Score: 
18
Number of questions answered: 
20
Screener Confidence (%): 
56.0
PRE Content Access and Privacy
Evaluation visibility: 
Public - accessible to all site users

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