Impact Assessment - White weeping broom (Retama raetam) in Victoria

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Assessment of plant invasiveness is done by evaluating biological and ecological characteristics such as germination requirements, growth rate, competitive ability, reproduction methods and dispersal mechanisms. Assessment of plant impacts, however, is determined by the extent to which a plant affects a land manager’s environmental, economic and social resources.

The relative importance of these resources varies depending upon the value people place on them and, as such, the assessment process is subjective. For example, a farmer is likely to place a higher emphasis on the impact of a plant on production (economic resource) than its impact on areas of natural vegetation occurring on the farm. Conversely, a Landcare or Friends group would value environmental or social resources more than economic resources.

Recognising that the value of resources vary between different land tenures, plant impact assessments allow a prioritisation of resources by land managers. Assessments can apply at a local, regional or state level, and the relative values of each resource identified may differ at each level.

The impact assessment method used in the Victorian Pest Plant Prioritisation Process uses three broad resource categories: social, environmental and agricultural, each with a number of related attributes. For example, social resources include such attributes as how the plant affects human access for recreation, or if it creates a health risk due to toxicity or by producing allergens.

Each resource attribute, or criterion, is assessed relative to a list of intensity ratings. Depending upon information found in relation to each criterion, a rating of Low, Medium Low, Medium High or High is assigned. Descriptions of the impact criteria and intensity ratings used in this process can be viewed here.

The following table provides information on the impact of White weeping broom.

A more detailed description of the methodology of the Victorian Weed Risk Assessment (WRA) method can be viewed below:

Victorian Weed Risk Assessment (WRA) method (PDF - 630 KB)
Victorian Weed Risk Assessment (WRA) method (DOC - 1 MB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

Common Name: White weeping broom
Scientific name: Retama raetam

Question	Comments	Rating	Confidence
Recreation
1. Restrict human access?	“May infest grazing land and prevent access to stock” (CRC Weed Management, 2003), making access by people difficult.	MH	M
2. Reduce tourism?	As an invader of “scrub and woodland on coastal, sandy soils” (Csurhes & Edwards, 1998), this xerophytic plant (see photo in CRC Weed Management, 2003) would not be obvious to the average visitor.	L	M
3. Injurious to people?	Although it is toxic, “the plant is not usually eaten, its bitter taste repels animals…There are no characteristic legions” (El Bahri et al. 1999). The taste is likely to deter people from eating the plant and it doesn’t appear to irritate the skin.	L	H
4. Damage to cultural sites?	Although not obvious to the average visitor (see Q. 2), for people more familiar with cultural sites, this 3 m high and 6 m wide plant (CRC Weed Management, 2003) may have a moderate visual effect.	ML	M
Abiotic
5. Impact flow?	Terrestrial species known to invade “scrub and woodland on coastal, sandy soils” Csurhes & Edwards, 1998).	L	MH
6. Impact water quality?	Terrestrial species known to invade “scrub and woodland on coastal, sandy soils” Csurhes & Edwards, 1998).	L	MH
7. Increase soil erosion?	This plant has a “shallow and deep root system” and its “cladodes…are effective in capturing and retaining soil.” Being “poisonous and unpalatable animals avoid walking or grazing on its cladodes” (El-Bana et al, 2003) would also assist reduces erosion locally. However, since the plant can “form a scrub layer that can out compete and shade out native plants,” if these plants are destroyed by fire (CRC Weed Management, 2003) large areas of bare soil may be exposed, increasing the risk of erosion.	ML	M
8. Reduce biomass?	Where R. raetem shades and out competes native vegetation (CRC Weed Management, 2003), direct replacement of biomass is likely to occur.	ML	M
9. Change fire regime?	“It can…increase the severity of bushfire if it dies off in large stands” (CRC Weed Management, 2003).	ML	M
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Shallow Sands Woodland (E), CMA= Glenelg-Hopkins, Bioreg.=Gippsland Plain, CLIMATE=VH. This 3 m tall x 6 m wide shrub “form[s] a scrub layer that can out compete and shade out native plants” and invades scrub and woodlands (CRC Weed Management, 2003). Major displacement of some dominant species within different layers.	MH	M
(b) medium value EVC	EVC=Woorinen Sands Mallee (D), CMA= Mallee, Bioreg.=Murray Mallee, CLIMATE=VH. This 3 m tall x 6 m wide shrub “form[s] a scrub layer that can out compete and shade out native plants” and invades scrub and woodlands (CRC Weed Management, 2003). Major displacement of some dominant species within different layers.	MH	M
(c) low value EVC	EVC=Loamy Sands Mallee (Lc), CMA= Mallee, Bioreg.=Murray Mallee, CLIMATE=VH. This 3 m tall x 6 m wide shrub “form[s] a scrub layer that can out compete and shade out native plants” and invades scrub and woodlands (CRC Weed Management, 2003). Major displacement of some dominant species within different layers.	MH	M
11. Impact on structure?	This 3 m tall x 6 m wide shrub “form[s] a scrub layer that can out compete and shade out native plants” and invades scrub and woodlands (CRC Weed Management, 2003). It is likely to have a major impact on the ground and understorey and shrub layers, but less of an impact on the tree layer.	MH	M
12. Effect on threatened flora?	No information found.	MH	L
Fauna
13. Effect on threatened fauna?	No information found.	MH	L
14. Effect on non-threatened fauna?	As an unpalatable shrub (El-bana et al, 2003) that invades and out competes native understorey and shrub species, its presence may reduce the food available to fauna.	MH	H
15. Benefits fauna?	As a large shrub, with branches drooping to the ground (see photo in CRC Weed Management, 2003), it may provide some shelter.	MH	M
16. Injurious to fauna?	Toxic and “not usually eaten...Palatable for the dromedary…Dromedaries and goats are the most often affected” (Bahri et al, 1999). It is possible that indigenous fauna, which may be poisoned by it, might eat this plant.	H	H
Pest Animal
17. Food source to pests?	Toxic and “not usually eaten...Palatable for the dromedary…Dromedaries and goats are the most often affected” (Bahri et al, 1999). May be eaten by feral goats, but as it is toxic (Bahri et al, 1999) it would provide minimal food.	L	H
18. Provides harbour?	As a large shrub, with branches drooping to the ground (see photo in CRC Weed Management, 2003), it may harbour foxes or rabbits as overnight cover.	MH	M
Agriculture
19. Impact yield?	“It has the potential to become a significant threat to Australia’s pastoral industry.” As an “aggressive invader” that “may infest grazing land and prevent access to stock” (CRC Weed Management, 2003), and being an unpalatable shrub (El-bana et al, 2003), it could reduce the carrying capacity of the land by more than 5%. “All domestic species are susceptible” to intoxication which can kill (Bahri et al, 1999).	MH	MH
20. Impact quality?	“Its toxins can taint milk, giving it an acrid taste and nasty smell and may poison nursing animals” (El Bahri et al, 1999). This could cause milk products to be rejected for sale.	H	H
21. Affect land value?	Land value is likely to be affected where severe infestations make grazing not viable (see Q. 19).	MH	MH
22. Change land use?	Control is expensive, since any treatments will need to continue for years until the seedbank is depleted (CRC Weed Management, 2003). Since the shrub is unpalatable (El-bana et al, 2003) and toxic (Bahri et al, 1999), land use may have to change from grazing to cropping.	MH	MH
23. Increase harvest costs?	“May infest grazing land and prevent access to stock” and control is expensive, since any treatments will need to continue for years until the seedbank is depleted (CRC Weed Management, 2003). Managing stock on infested lands and the control of infestations could both cause a major increase in harvest costs.	H	M
24. Disease host/vector?	No reference of this plant as a disease host/vector in CRC Weed Management (2003).	L	M

Impact Assessment Record - White weeping broom (PDF - 25KB)
Impact Assessment Record - White weeping broom (DOC - 69KB)
This table can also be viewed as a PDF document (printer friendly). To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

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