Buddleja davidii_Louis-M. Landry
Photo courtesy Louis M. Landry

Buddleja davidii Risk Assessment

Common names: butterfly bush

Buddleja davidii -- California

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Evaluation Summary
Summary: 
General Evaluation Information
Date of Evaluation: 
June 21, 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: 
Naturalized in the US, England, Europe, New Zealand. References cited below are from Belgium and Germany. In California, present in a number of counties along the coast.
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: 
It invades much of western Oregon and Washington, in habitats ranging from floodplains to mountain slopes, often in disturbed areas. In California, present in a number of counties along the coast. A note to the Cal-IPC watchlist said it is becoming a problem in riparian areas near Santa Cruz. Considered "alien/established" in areas of Europe that are similar to California, including France, Italy, and Spain. Also in areas of eastern Australia and New Zealand 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: 
Very High
Answer / Justification: 
Classified as a B-rated noxious weed by the Oregon Dept of Agriculture, which wants to eradicate it from the wild and has banned most cultivars from sale. It invades much of western Oregon and Washington, in habitats ranging from floodplains to mountain slopes, often in disturbed areas (OR Extension). In New Zealand it is estimated that the weed pest is displacing valued native species and costing the forestry industry $0.5 to 2.9 million annually in control and lost production (USFS). Also a problem in the United Kingdom (Tallent-Halsell 2009).
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: 
Classified as a B-rated noxious weed by the Oregon Dept of Agriculture, which wants to eradicate it from the wild and has banned most cultivars from sale. It invades much of western Oregon and Washington, in habitats ranging from floodplains to mountain slopes, often in disturbed areas (OR Extension). In New Zealand it is estimated that the weed pest is displacing valued native species and costing the forestry industry $0.5 to 2.9 million annually in control and lost production. These are areas similar to California's climate.
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: 
There are 23 other species of Buddleja in Randall 2012 although B. davidii has by far the most number of citations. B. asiatica is an invasive species in Hawaii, where it displaces native species, but that is not similar climate to California (CABI datasheets). B. globosa is in the DAISIE database of invasive species in Europe, present in the United Kingdom and Portugal, but without details on impacts. Randall also has B. globosa cited from invasive lists in Victoria, Australia. B. madagascariensis is listed on the Victoria invasive plants website as displacing native species. Medium confidence because I don't have a lot of details on all of these.
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: 
High
Answer / Justification: 
This species has a wide ecological range, so while it has invaded many areas that are similar to California, it is also in many areas that are not, such as South America, northern Europe, the eastern US, and southeastern Asia.
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: 
Very High
Answer / Justification: 
Forms dense stands in riparian areas, especially those with frequent flooding, as well as disturbed sites such as reforestation areas. It can invade a range of sites including mountain slopes. Can grow 0.5 to 2m in height each year. At one site, plants dumped on a garbage heap grew to 2m and flowered in their first year, then within 4 years the thicket grew to 4m. Roots extend 4m into the soil. The plants generally die after about 20 years but some can grow for 30 years. It can both inhibit and facilitate native plants, depending on the species. It is uncertain whether B. davidii alters the trajectory of plant sucdession.
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: 
There was no mention of this in references.
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. The usual areas where it invades are riparian or forest gaps, rather than grasslands or other grazing areas.
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: 
Medium
Answer / Justification: 
Based on the description of rapid growth and dense thickets in Q7, impenetrable thickets sound possible. B. davidii is a shrub with multiple trunks.
Reference(s): 
Reproductive Strategies
11. Does this species (or cultivar or variety) reproduce and spread vegetatively?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
"Butterfly bush exhibited a high vegetative regeneration capacity from all types of vegetative parts, be it shredded or not." This experiment used a professional green waste shredder to shred aboveground and belowground plant material.
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: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
"Do not leave the clippings on the ground, as they can easily take root and create a new plant...Whatever you do, don't dump the clippings along a roadside or along a creek or river, as these are preferred habitats for escaped butterfly bush." "Butterfly bush exhibited a high vegetative regeneration capacity from all types of vegetative parts, be it shredded or not." This experiment used a professional green waste shredder to shred aboveground and belowground plant material.
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, see details in question 14. Seeds are thread-like and long-winged and contained in a capsule.
Reference(s): 
14. Does this plant produce copious viable seeds each year (> 1000)?
Yes or No: 
Yes
Points: 
1
Confidence Level: 
Very High
Answer / Justification: 
Yes, but it depends on the cultivar. Oregon is allowing specific sterile cultivars to be sold. "Some cultivars have been found to produce much less seed than others. For example, a study at Longwood Gardens in southern Pennsylvania found large differences in the amount of viable seed produced by B. davidii varieties. For example, cultivars 'Summer Rose' and 'Orchid Beauty' produced 20 times fewer viable seeds than 'Potter's Purple' and 'Border Beauty'. The study also found that a single flower cluster of 'Potter's Purple' was found to produce more than 40,000 seeds." "The flower clusters can be so profuse that they cause the branches to arch even more" (USFS).
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: 
Plants retain seeds on the plant over the winter and they are released in early spring to summer. In New Zealand, dispersal can start in late autumn or early winter. Seeds are short-lived in the seed bank. This reference did not specifically address germination rate, but given the facts above it sounds like B. davidii does not have specific requirements and does not hold seeds in the seed bank for special conditions.
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: 
Flowering occurs when the species is two years old, occasionally in the first year.
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: 
Depending on the location, it flowers from late spring to mid-autumn or from early summer to late summer/early autumn. In the UK, there is a wide variation among locations and individuals in the time of first flowering. Seed formation and ripening occurs within three weeks of flowering. In California, it flowers from March to September. Therefore, seed production for more than three months seems very likely.
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 is the main source of dispersal.
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: 
Seeds are mostly dispersed by wind but can also be dispersed in water in riparian areas, especially during flooding.
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: 
Very High
Answer / Justification: 
In Europe, it became naturalized as it spread along railroad tracks where seeds were either carried on locomotives or blown by the low-pressure drag created by the trains. Cars have also been found to disperse seeds physically.
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 (%): 
89.0
PRE Content Access and Privacy
Evaluation visibility: 
Public - accessible to all site users

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