Photo: Ron Vanderhoff

Osteospermum calendulaceum Risk Assessment

Synonyms: Oligocarpus calendulaceus, Osteospermum parviflora, Calendula parviflora

Osteospermum calendulaceum -- California

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Created by: Ron Vanderhoff
Created on: Thursday, Nov 17th, 2022
Evaluation Summary
Osteospermum calendulaceum (syn. Oligocarpus calendulaceus, Osteospermum parviflora, Calendula parviflora) is an annual to short lived perennial plant in the small tribe Calenduleae of the Asteraceae family. The plant was assigned to the genus Oligocarpus in the 1830's, but continues to generally be referred to as Osteospermum calendulaceum by most authors. The plant is native to the South African Cape Provinces, Free State, KwaZulu-Natal, Lesotho, Northern Provinces, and Eswatini. It has been introduced into Hawaii, and Australia, where it is found in South Australia and Western Australia. It was first detected in the continental United States in the Laguna Beach area on 27 April 2022 over about 1.5 acres, comprising an estimated two thousand plants. A novel plant in California, its local ecology and environmental impacts are largely uncertain. It is under aggressive and ongoing management at its California occurrence location in Laguna Beach. The introduction of the species is concerning, due to its nearly year-round flowering and affinity for drylands habitat. The species was given an "A" pest rating by CDFA in Sept. 2022. It has not been declared invasive in Australia, where it is well-established, or in Hawaii, where it has a very limited distribution. With a PRE score of 16, the results of this evaluation suggest an invasive risk in California. The species scored low (a NO) on three of the four questions about impacts on native plants and animals, as well as on three of seven questions about reproductive strategies, and all three questions about dispersal. However, the species scored high (a YES) on questions about its displacement of native plants and questions about see production and germination. Although naturalized in other regions of the world with a match to California's climate, the lack of an Invasive status in these other regions contrast rather strongly with our observations in California. This assessment matrix, and all others, rely heavily on invasiveness in other areas. In Laguna Beach, we may indeed be seeing it for the first time as invasive. The plants behavior in Laguna Beach is concerning, including high density, high germination, likely year-round flowering and fruiting, and success in dryland conditions. Additional research of the Laguna Beach, California plants could be conducted to determine if these plants may comprise a phenotypic and genetic variation that facilitates local invasiveness. Certainly, its unusual behavior here in California was considered in this assessment. (See additional discussion in the Evaluation Notes.) This PRE has scored the species as "High". Due to limited published literature about the plants' natural history and the limited baseline of experience with the species in California, the confidence score of this PRE is low. More study is needed as well as future observations to see how this species behaves in California.
General Evaluation Information
Date of Evaluation: 
November 24, 2023
Evaluation Time (hrs): 
12 Hours
Evaluation Status: 
Completed
Plant Information
Plant: 
Osteospermum calendulaceum
Plant Material: 
If the plant is a cultivar, and if the cultivar's behavior differs from its parent's (behavior), explain how: 
The plant is not a cultivar, with none known to have been developed.
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-Osteospermum_calendulaceum-California-20230817.pdf1.1 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: 
Very High
Answer / Justification: 
The species is naturalized in Southern Australia in the states of New South Wales, South Australia, and Western Australia (since at least 1948) and the island of Hawaii (since 1983). A single 2001 Croatia collection is not considered naturalized. (GBIF, 08-13-23) The species is now also naturalized in a small portion of Laguna Beach, CA., the first record for the continental United States. (Calflora, 11-22-23)
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: 
The species is naturalized in Western and Southern Australia (since at least 1948) with approximately 12 of the 45 occurrences having a California climate match (GBIF, 08-13-23)(iNaturalist, 11-22-23).
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: 
Low
Answer / Justification: 
It is documented as naturalized (Flora of Australia, 11-24-2023) and "Alien" (Florabase, 08-15-2023) in Australia, but not invasive. On the island of Hawaii it is also documented as naturalized, with no references indicating invasive (Flora of the Hawaiian Islands, 08-15-2023). Its occurrence on the Hawaiian Islands may be relictual (see the Comment section at the conclusion of this assessment). In California, where a recent occurrence was detected and is being managed, we must rely on personal observation and documentation, not peer reviewed. However, these observations are consistent across multiple observers. Ron Vanderhoff (CNPS Orange County), Christopher McDonald (UCCE) and Chris Gort (Laguna Cyn. Foundation) all have extensive field experience with the species at the site. Other observers, including biologists from Orange County Parks would likely have similar remarks (McDonald, C., 2024 & Vanderhoff, R, 2022). My comment on an 08-31-2023 site visit states "This plant at this site is exhibiting invasive behavior. Its growth (when not managed) is as an essentially solid vegetative, spreading mass, crowding out other annuals and shading the soil below. Its seed production and germination also imply invasive qualities." (Calflora 08-31-2023). On 11-27-2022 I delineated a representative small plot, 15 cm x 15 cm and counted 62 seedlings (Calflora 11-27-2022). This density, if allowed to grow to maturity, would likely create a dense mat and implies a high likelihood of invasive behavior at this site. Comments contained in other Calflora observations by Christopher Macdonald and Chris Gort also mention dense mats. Even without certain invasiveness documented elsewhere (Australia and Hawaii), given the behavior of the plant in Laguna Beach, CA I am marking this question as YES. But without published, peer reviewed documentation am leaving it at a LOW confidence.
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: 
It is documented as naturalized only, not invasive anywhere in the world (Flora of Australia, 11-24-2023) (Florabase, 08-15-2023) (Flora of the Hawaiian Islands, 08-15-2023). Its occurrence on the Hawaiian Islands is not considered invasive, and may be relictual (see the Comment section at the conclusion of this assessment) which is also not a California climate match. In California, where a recent occurrence was detected and is being managed, we must rely on personal observation and documentation, not peer reviewed. However, these observations are consistent across multiple observers. Ron Vanderhoff (CNPS Orange County), Christopher McDonald (UCCE) and Chris Gort (Laguna Cyn. Foundation) all have extensive field experience with the species at the site. Other observers, including biologists from Orange County Parks would likely have similar remarks (McDonald, C., 2024 & Vanderhoff, R, 2022). My comment on an 08-31-2023 site visit states "This plant at this site is exhibiting invasive behavior. Its growth (when not managed) is as an essentially solid vegetative, spreading mass, crowding out other annuals and shading the soil below. The growth habit and density may also interfere with small animal movement. Its seed production and germination also imply invasive qualities." (Calflora 08-31-2023). On 11-27-2022 I delineated a representative small plot, 15 cm x 15 cm and counted 62 seedlings (Calflora 11-27-2022). This density, if allowed to grow to maturity, would likely create a dense mat and implies a high likelihood of invasive behavior at this site. Comments contained in other Calflora observations by Christopher Macdonald and Chris Gort also mention dense mats. Even without certain invasiveness documented elsewhere (Australia and Hawaii), given the behavior of the plant in Laguna Beach, CA I am marking this question as YES. But without published, peer reviewed documentation am leaving it at a 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: 
High
Answer / Justification: 
Chrysanthemoides monilifera is declared invasive in California by Cal-IPC (Cal-IPC, 08-15-2023). It is also given an "A" Rating by the CA Dept. of Food and Agriculture (Albrecht, CDFA, 11-23-2023). It is declared invasive in Australia, by Weeds Australia and others. (Weeds Australia, 09-10-23) (NSW Environment, 11-26-2023) Various authors now place this taxon in the genus Osteospermum, as Osteospermum moniliferum, a synonym of Chrysanthemoides monilifera (Kew POWO, 11-23-2023) (PlantsZAfrica, 09-10-2023) (IPNI, 09-10-23). O. moniliferum is declared invasive in both California and Australia and in a similar climate (Cal-IPC Inventory, 08-15-2023) (Weeds Australia, 09-10-23)(NSW Environment, 11-26-2023). Osteospermum ecklonis was assessed by Cal-IPC and given a Red Alert status, but was not added to the inventory (Cal-IPC Inventory, requires a click-through). Osteospermum fruticosum has been nominated for assessment consideration by Cal-IPC, but an assessment has not yet been performed (Cal-IPC, pers communication). CABI lists O. jucundum as naturalized, but not invasive, in parts of S. Africa, Europe, and Australia (CABI, 08-15-23). Although not declared invasive, a closely related species, O. monstrosum (syn. O. clandestinum) has been collected three times along the San Diego County coast (1959, 2012, 2019) and may be established (CCH2, 11-26-23). This species is also native to South Africa and also naturalized in Australia. With the exception of O. moniliferum (see above comment), no other certain Osteospermum species were found as declared invasive. However several related species in the Calenduleae tribe are documented as invasive. I am answering this as YES, because of the invasiveness of related species (in spite of the Osteospermum/Chrysanthemoides and other taxonomic issues).
Reference(s): 
6. Is the species (or cultivar or variety) found predominately in a climate matching the region of concern?
Yes or No: 
Yes
Points: 
2
Confidence Level: 
Very Low
Answer / Justification: 
Yes, but only very slightly. From GBIF (GBIF), in its native Southern African range it has a California climate match for 45 of the 98 records. Of the Australian occurrences it has a California climate match for 12 of the 45 occurrences. The 2 Hawaiian occurrences are not a match to the California climate (GBIF, 08-13-23). The sum of this is 57 of the 145 occurrences documented on GBIF match the CA climate - 39.3%. Of course the Orange County occurrence is a CA climate match, but is included as only ONE occurence in sum. However, iNaturalist (iNaturalist, 11-22-23) observations in South Africa indicate a larger proportion of California climate matches. Of the 112 observations, approximately 72 appear to match the GBIF CA climate match map. Taken together, this would sum as 130 of 257 observations with a California climate match: 50.6%. A YES answer, but barely.
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: 
Medium
Answer / Justification: 
Literature is very sparse on this plant, especially its environmental impacts. I can find nothing in the international literature of the species displacing, overtopping or dominating plant communities. The plant is primarily an annual and its habit is low and spreading (Flora of Australia, 11-24-2023). However, documentation of observations at the Laguna Beach, CA site DO indicate a broad, mat forming, mounding habit, dense cover, and dense foliage. An August 2023 observation states ". . . This large, dense patch was somehow missed during previous management. This is a nearly solid mass approx. 3m x 3m with a few outlier plants as well." Such a habit would infer displacement, overtopping or dominating of native plant communities (Calflora, 11-22-2023, esp. Calflora, 11-26-2023). Certainly, this plant is exhibiting invasive properties in Laguna Beach, California. It is overtopping other annuals and has a creeping, mat forming habit when left unmanaged in dense patches. This behavior has not been described in any other habitat it has invaded (Australia, Hawaii) or is native to. It is a new behavior observed only here in California (McDonald, C., 2024 & Vanderhoff, R, 2022). Based upon the plants behavior in Laguna Beach I am answering this as a Yes, but only medium confidence.
Reference(s): 
8. Is the plant noted as promoting fire and/or changing fire regimes?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
Literature is very sparse on this plant, but I can find no information of the plant promoting fire or changing fire regimes. The plant has a low and spreading growth habit, and remains in leaf and fairly hydrated throughout the year. Given the small stature of the plant, its primarily annual growth cycle, and its low and generally small biomass (Flora of Australia, 11-24-2023), this species would be unlikely to burn hot or produce much flame height. Additionally, due to its its co-occurrence with an abundance of other non-native, weedy annuals, it would be unlikely to alter fire regimes. However, there is some potential for drying plants to be an ignition source and assist in the spread the flames. Without documentation to the contrary, I am answering NO.
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: 
I can find very little documentation of this species health risk or impacts to grazing systems. Herbiguide (a non-technical Australian online resource) says this about the closely related Osteospermum clandestinum: "Toxicity: Not recorded as toxic." (Herbiguide, 08-17-2023). The CDFA Pest Rating Proposal for the species mentions "It is strong-scented and likely to be avoided by livestock". But no reference is given and a strong scent does not necessarily mean an impact to grazing systems (Albrecht, CDFA, 11-23-2023). A related Osteospermum in South Africa is referenced as forage for livestock. (Ntondini, et al) In Laguna Beach the plant has been goat grazed in a Fuel Management Zone, although the extent of the grazing is still uncertain (Calflora, 11-22-23)(Stinking Roger Outbreak, 08-20-2023).
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: 
Literature is very sparse on this plant, especially its environmental impacts. This is primarily a low growing, sprawling, herbaceous annual plant. (Flora of Australia, 11-24-2023)(Kew POWO, 11-23-2023). As such, it would be unlikely to produce impenetrable thickets, or slow most animals, livestock or humans. I can find no evidence to support a Yes answer. Documented observations at the Laguna Beach, CA site do not suggest any significant change in animal movement from the native vegetation in proximity. California ground squirrels are common at the site as are other small herbivores. (Calflora, 11-23-2023). With so little documentation I am assigning a LOW confidence.
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: 
No. This is essentially an annual species, therefore propagating by seed. All evidence suggests this is an obligate seeder (Flora of Australia) (Kew POWO, 11-23-2023). Flora of Australia, Kew Plants of the World and others all indicate an "Annual" growth cycle (Flora of Australia, 11-24-2023) (Kew POWO, 11-23-2023). Herbiguide provides additional information on a closely related species, Osteospermum clandestinum (syn's. O. monstrosum, Tripteris clandestina): "Reproduction: By seed. Vegetative Propagules: None. Population Dynamics and Dispersal: Spread by seed." (Herbiguide, 08-17-2023).
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: 
No. All evidence suggest this is an annual plant and an obligate seeder and does not reproduce vegetatively. (Flora of Australia, 11-24-2023) (Kew Plants of the World, 11-23-2023).
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: 
Yes, although germination experiments ex situ from the Laguna Beach colony have failed thus far. Germination was prolific at the Laguna Beach colony on 11-27-2022, with many thousands of seedlings. Very effective pre and post emergent herbicide management thereafter made further germination and growth difficult to assess. At this site was the statement "Seedling germination is very dense. One 15cm square area counted 62 seedlings. Some areas were denser than this, but over the entire colony the density will be considerably less. Given this density however, over an area of 100 sq meters (which is reasonable), this equals 280,000 plants." (Calflora, 11-26-2023). Another documented record at the same site states: "One medium/average size plant was removed and examined very carefully and all achenes present on the plant were carefully counted . . . a total achene count of 486 on this rather representative plant." (Calflora, 11-23-2023)
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
No
Points: 
0
Confidence Level: 
Medium
Answer / Justification: 
A field report from the Laguna Beach colony on 07-31-2022 says: "One medium/average size plant was removed and examined very carefully and all achenes present on the plant were carefully counted: 14 disk (short, wrinkled) achenes, 81 (long, smooth). However, because the disk achenes appear to drop earlier and average 5 per flower I can extrapolate a total achene count of 486 on this representative plant (81 + 405)." (Calflora, 11-23-2023). Another field report from the Laguna Beach colony on 08-24-2023 states: "One representative medium/average size plant . . . was removed and examined very carefully and all reasonably mature (presumed reproductive) achenes present on the plant were carefully counted: 264. In addition to these, approx. 40 heads had already dropped the fruit from the capitula; at an average of 5 achenes per (conservative) this would be another 200. +- 40 buds were also present on the plant; if each bud produces an average of 5 achenes this is another 200 fruit. In total, this plant at this time was carrying +- 664 achenes." (Calflora, 11-23-23) These two Laguna Beach examinations suggest 486 and 664 achenes per plant, less than the 1,000 required for a Yes answer. The CDFA 2022 Pest Rating Proposal states "Osteospermum calendulaceum reproduces by seed and can produce at least several hundred seed units on larger plants." (Albrecht, CDFA, 11-23-2023) Following the exact wording of this question I cannot quantify and meet the ">1000 seeds" each year requirement. Given that +486 seeds and 664 seeds were extrapolated from single representative plants on separate visits and that the flowering/fruiting period is at least seven continuous months (see question 17) a Yes answer may be possible, but I am remaining conservative without more supporting documentation.
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: 
There is also very little documentation of this topic. A 2022 Calflora observation following a moderate Fall rain event states "Seedling germination is very dense. One 15cm square area counted 62 seedlings. Some areas were denser than this, but over the entire colony the density will be considerably less. Given this density however, over an area of 100 sq meters (which is reasonable), this equals 280,000 plants." (Calflora, 11-22-2023) This infers high germination, but is a small baseline. The seed is sold commercially by at least one South African seed source. Their online information mentions no special germination requirements other than "Sow seeds in a well drained medium". (Lifestyle Seeds, 08-20-2023) Ex situ germination experiments were conducted at UC Irvine and by myself in Newport Beach. Surprisingly, both efforts were nearly complete germination failures. However, these efforts may not be conclusive and additional professional germination testing has been suggested. An old (1935) paper from South Africa (Mancini) South Africa suggests the similar Osteospermum muricatum does have "some" seeds germinating at up to 3 months, which could extrapolate to at least a 25% germination per year. Given the high germination documented at the Laguna Beach site, answering Yes. But without better documentation suggesting a Low confidence.
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: 
Being primarily an annual plant (Flora of Australia, 11-24-2023) there is no doubt that viable seed is produced each year. The CDFA 2022 Pest Rating Proposal indicates "Osteospermum calendulaceum reproduces by seed and can produce at least several hundred seed units on larger plants." (Albrecht, CDFA 11-23-2023)
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: 
High
Answer / Justification: 
Yes. Thorough observation of the Laguna Beach, CA colony indicates both flowering and fruiting in the months of Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Nov, and Dec. Sept (year-round) is likely as well, but is lacking site visits and management is aggressive and flowering/fruiting is difficult to verify, since plants are regularly removed. At least nine months of continuous flowers and fruit. (Calflora, 11-22-2023) Herbiguide (an Australian online resource) provides this information on the closely related Osteospermum clandestinum (syn. O. monstrosum): "Annual. Germinates autumn/winter. Flowers in May-October. Reproduction: By seed. Flowering times: August to October in SA, July to October in Perth, May to October in WA. Vegetative Propagules: None. Population Dynamics and Dispersal: Spread by seed. Weed of crops, pastures, roadsides, wet areas, granite outcrops, open bushland and wastelands. Toxicity: Not recorded as toxic." (Herbiguide, 08-17-2023) Flora of Australia says "Flowers and fruits September–April", or nine months (Flora of Australia, 11-24-2023). A review of iNaturalist records shows images of flowering plants every month of the year in South Africa (iNaturalist, 11-22-23). Although not counted, GBIF records from Australia also indicate flowers and fruit over a majority of months (GBIF, 08-13-2023).
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: 
Low
Answer / Justification: 
The CDFA 2022 Pest Rating Proposal states "The nonwinged forms of the seed units do not have clear adaptations for dispersal but may be spread from the area of the parent population by human or animal activity, by water, wind, or in soil. The more prominently winged seed units seen as one of the fruit variants outside of California would aid in local wind dispersal of the plant." The same document continues "Given their introduction into Australia, the Hawaiian Islands, and now coastal California, propagules of the plant are sometimes dispersed long distances via global trade or transport activities." (Albrecht ,CDFA, 11-23-2023) Dispersal by mammals, birds or domestic animals is not documented and appears to be speculative. The seed does not have any physical hooks, barbs or bristles to suggest attachment to animals. However, the achene is resinous and sticky and might adhere well to fur or hair. The seed, like most in the genus Osteospermum, is very hard and solid, making viable passage through birds a possibility - although there is no research or data to support this for this species. With such poor documentation I an answering this with a soft No.
Reference(s): 
19. Are the plant’s propagules frequently dispersed long distance (>100 m) by wind or water?
Yes or No: 
No
Points: 
0
Confidence Level: 
Low
Answer / Justification: 
The CDFA 2022 Pest Rating Proposal indicates "The nonwinged forms of the seed units do not have clear adaptations for dispersal but may be spread from the area of the parent population by human or animal activity, by water, wind, or in soil. The more prominently winged seed units seen as one of the fruit variants outside of California would aid in local wind dispersal of the plant." (Albrecht CDFA, 11-23-2023) Although perennial, an allied species, Osteospermum jacunda, with a similar range in South Africa, has a very similar achene shape, size and form. Brief dispersal details for that species are described: "The fruits (cypselae) of the ray florets are not adapted to any special kind of long distance dispersal, as they have no pappus, wings or glands. The cypselae that sometimes develop from the disc florets are light, winged and obviously adapted to wind dispersal." (CABI, 08-15-23) An even more closely allied annual species with similar seed morphology, but larger "wings" on the fruit is O. monstrosum (syn. O, clandestinum). About C. monstrosum, Plants of Africa states "Once mature, the fruits are pale brown and dry, and the wings serve to keep them tumbling in the wind for greater dispersal." (Plants of Africa, 11-26-2023). During my own germination experiment I did note that the fruit were buoyant, floating in water. However, the habitat of the species does not have a free water association, preferring drier hillsides, grasslands and upland scrublands (Plant of Africa, 11-26-2023). Given that the winged form of the seed appears to be a very minor percentage of the California plants (Albrecht, CDFA, 11-23-2023) and the speculation around this method of dispersal I am answering as No.
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: 
Medium
Answer / Justification: 
The seed is sold commercially by at least one South African seed source (Lifestyle Seeds, 08-20-2023). However, this was the only seed source I located and this does not at all seem to be a common horticultural or commercial species. The plant is documented as a weed of agriculture in Australia, implying the possibility of seed movement within agricultural shipments (Flora of Australia, 11-24-2023). Herbiguide (an Australian online resource) also states "Spread by seed. Weed of crops, pastures, roadsides, wet areas, granite outcrops, open bushland and wastelands." (Herbiguide, 08-17-2023). The CDFA CA Pest Rating Proposal for the species states "The nonwinged forms of the seed units do not have clear adaptations for dispersal but may be spread from the area of the parent population by human or animal activity, by water, wind, or in soil." (Albrecht, CDFA, 11-23-2023). The achene (fruit) is resinous and sticky and would adhere well to clothing and possibly equipment. Movement of the seed long distances (South Africa, Australia, Hawaiian Islands, and California) implies some form of assisted movement. The source of that intercontinental movement is unknown but not likely to be mammals, birds, wind or water, leaving anthropomorphic dispersal likely. I am marking this with a No answer, since most of the supporting evidence is inferred and not documented. Giving it only Medium confidence though, due to same.
Reference(s): 
Evaluation Notes

Information about this plant is sparse in the literature, with even less documentation about it ecology or possible invasiveness. There is documentation of the taxonomy, but very little about life history details, such as germination, dispersal, impacts to ecology, etc. The few references in the bibliography are therefore repeated across several of the questions and in most instances provide only small insights to the specific question/issue. Much of this lack of information and the resulting inferences accounts for the low confidence.

It is worth noting that the California plants do have morphologically unusual fruits, with only occasional and poorly developed wings. This is unlike the plants described from South Africa and Australia, with more well developed wings on all fruit. This somewhat unique seed morphology was noted by CDFA in their 2022 Pest Rating Proposal.

Perhaps most important is the unique behavior of the species in California. We are seeing this plant grow and spread here in ways that have not been described from the other two areas it has established (Australia and Hawaii). Dr. Christopher McDonald (UCCE) has had personal contacts with several botanists in Australia who have recorded this species and their description of the populations there (in both western Australia and southern Australia) are unlike what we are finding in California. In Australia the botanists have described solitary plants or a few patches of plants, and have not seen the dense patches we are seeing in California. They typically find a few plants growing long distances from known populations. There is one report in iNaturalist where a patch of plants is growing in a density that is still lower than in CA https://www.inaturalist.org/observations/154751850, the rest of the photographs are not dense infestations. 

In California, if left untreated we are seeing large amounts of seedlings germinating and dense mats of growth. Christopher has not been able to find anyone who has recently seen this plant in Hawaii (the original collector is deceased), which again points to its acting differently in California. Because this plants is behaving differently in California than in Australia and Hawaii, it makes relying on the few reports for this species problematic.

This species occurrence in Laguna Beach (if not extirpated) could be a laboratory to better understand its invasive status, at least in California's Mediterranean climate.

Total PRE Score

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

PRE Score: 
16
Number of questions answered: 
20
Screener Confidence (%): 
67.0
PRE Content Access and Privacy
Organization: 
California Invasive Plant Council (Cal-IPC)
2023-2024 Western IPM Project
Evaluation visibility: 
Public - accessible to all site users

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