Polygala myrtifolia_Luigi Rignanese
Photo by Luigi Rignanese

Polygala myrtifolia Risk Assessment

Common names: myrtle-leaf milkwort

Polygala myrtifolia -- California

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Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
August 9, 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: 
According to GRIN, this species has naturalized in Australia, New Zealand and Europe. GBIF lists occurrences in those countries as well. The occurrence record in GBIF in Florida is at a botanic garden. Calflora has 2 occurrences listed in California in Santa Cruz County and one in Marin (photo appears to show the correct plant).
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: 
This species has naturalized occurrences in Marin and Santa Cruz Counties in Calflora. Occurrences in southwestern coastal Western Australia as well as New South Wales, Australia also match California's climate, as do occurrences in Europe in Spain, France, Sardinia and Turkey.
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: 
Answer / Justification: 
DiTomaso and Healy list it in the table for "rarely naturalized in California," however note "local ornamental escape...may be invasive in mild climate regions." In invasive.org, entry states: "Polygala,myrtifolia, Myrtle-leaf Milkwort,"Dispersal: Water,Ant. Life Form: Small to medium shrub. RISK: Very serious threat to one or vegetation formations (Victoria). Vegetation Formations Invaded: 1,2,3,4,5,8,12","Introduced deliberately from: S Af and Commercially Available, In Victoria: Widespread, medium to large populations",Environmental Weed Invasions In Victoria". In Weeds Australia: "Myrtle-leaf Milkwort (Polygala myrtifolia) is a highly invasive species, particularly in near-coastal shrublands, and capable of invading intact vegetation."
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: 
Medium
Answer / Justification: 
Species described as invasive in Western Australia as well as in Victoria, both areas that support a climate match. In invasive.org, entry states: "Polygala,myrtifolia, Myrtle-leaf Milkwort,"Dispersal: Water,Ant. Life Form: Small to medium shrub. RISK: Very serious threat to one or vegetation formations (Victoria). Vegetation Formations Invaded: 1,2,3,4,5,8,12","Introduced deliberately from: S Af and Commercially Available, In Victoria: Widespread, medium to large populations",Environmental Weed Invasions In Victoria". In Weeds Australia: "Myrtle-leaf Milkwort (Polygala myrtifolia) is a highly invasive species, particularly in near-coastal shrublands, and capable of invading intact vegetation." Florabase in Western Australia: "A serious bushland weed in Victoria and South Australia, particularly of coastal habitats."
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: 
Medium
Answer / Justification: 
Polygala virgata is noted to be an "environmental weed in Victoria, New South Wales, South Australia, Queensland and Western Australia."
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: 
Medium
Answer / Justification: 
The main distribution of the native range of the species, in the Western Cape of South Africa matches California's climate, as do occurrences in California, Western Australia and New South Wales, Australia (GBIF). As well, most of the occurrences in GBIF in Europe (except one in North Germany, one in Northern Spain and one in Sweden) match California's climate. The areas that do not match California's climate are fewer, being the North Island of New Zealand, some of the occurrences in Tasmania (Australia), as well as the sparse records from northern Europe mentioned before.
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: 
In Australia, noted to outcompete native vegetation in the Environment Australia description. "Myrtle-leaf Milkwort (Polygala myrtifolia) is a highly invasive species, particularly in near-coastal shrublands, and capable of invading intact vegetation....By creating a dense shrub stratum, it prevents or limits the establishment of other species."
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: 
Noted that adult plants are killed by fire, but seeds rapidly post-fire (stimulated by fire) but no documentation that it promotes fire or changes regimes listed here, although it is possible.
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: 
Medium
Answer / Justification: 
No source mentioned toxicity and one source mentioned many animal species as dispersal vectors.
Reference(s): 
10. Does the plant produce impenetrable thickets, blocking or slowing movement of animals, livestock, or humans?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
High
Answer / Justification: 
Noted as producing thickets, or a "dense shrub stratum" in Australia, and the shrub is 1.5-4m high.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
No
Points: 
0
Confidence Level: 
High
Answer / Justification: 
Indicated by several sources to reproduce only by seed. In fact, a cultivar that was developed was thought to be seed sterile (var grandifolia) but in fact sets seed (Queensland Invasive Plants).
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: 
Species reproduces by seed and is said not to sucker or reproduce vegetatively.
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: 
Produces viable seed as documented in several sources.
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Low
Answer / Justification: 
Could be inferred that this is a "yes" based on the following from the Victoria Risk assessment: " Two seeded capsules about 10 mm long (Roy et al, 1998). Seedling density may exceed 2,000 m2 (Weber, 2003)." According to Rowles and O'Dowd (2009), they are small (10mg), and able to be dispersed by ants.
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: 
Victoria risk assessment: "Seeds germinate in the open or under dense vegetation generally in May to June (Carter et al, 1990) but at any time of the year if sufficient moisture (Blood, 2001). However Arnold (1981) states that the seeds appear to be capable of germinating any time of the year. Seedlings that germinate in autumn tend to have a better survival rate. Germinates prolifically after disturbance such as fire (ESC, 2002) and following soil or canopy disturbance (Muyt, 2001). Seeds are able to germinate throughout the year, whilst there is a seasonal component to survival, this question only deals with germination requirements."
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: 
Victoria Risk Assessment: "Can flower and seed when less than 50 cm high (2 years old) (Muyt, 2001; Carter et al, 1990). Older plants are around 15 years old and are still in good condition at this age (van der Walt, 2003). Plant is capable of flowering for more than 10 years....Plants are able to set seed at the age of 2 (Muyt, 2001; Carter et al, 1990)."
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: 
Florabase lists fruit set or occasional fruit set for 8 months per year, and Victoria's risk assessment cites Roy et al 1998 saying that this species can flower all year.
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: 
Medium
Answer / Justification: 
Victoria's risk assessment indicates that it is likely due to dispersal by bird that the species could disperse >1km via this method.
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: 
Medium
Answer / Justification: 
Noted to disperse via water and occupies habitats near water. "The seeds of Myrtle-leaf Milkwort are not suited to long-distance dispersal by wind, water or animal vectors. The hairy seeds might allow for short-distance wind dispersal or for some short-to medium-distance dispersal by animals through entanglement in fur or feathers." Victoria weed assessment indicates that seed is dispersed by birds, ants, water and cites Muyt (2001).
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: 
Dispersal seems to be fairly well documented in this species, with sources consistently listing water, birds, soil dumping and ants. No source lists the vectors indicated in this question.
Reference(s): 
Evaluation Notes
Total PRE Score

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

PRE Score: 
20
Number of questions answered: 
20
Screener Confidence (%): 
66.0
PRE Content Access and Privacy
Evaluation visibility: 
Public - accessible to all site users

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