Impact Assessment - Spotted knapweed (Centaurea maculosa) 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 Spotted knapweed
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 - 1026 KB)
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: Spotted knapweed
Scientific name: Centaurea maculosa
| Question | Comments | Rating | Confidence |
| Recreation | |||
| 1. Restrict human access? | Weed grows up to 1.5 metres (USFS 2004). Forms monocultures but no evidence to suggest that human access would be restricted. | L | MH |
| 2. Reduce tourism? | Weed has a purple flower and capable of forming monocultures (USFS 2004). Weed would have minor effect on aesthetics of site. | ML | MH |
| 3. Injurious to people? | ‘The sap of spotted knapweed can cause skin irritation in some people’ (CWMA n.d.). | ML | M |
| 4. Damage to cultural sites? | No evidence to suggest weed has more than a negligible effect on aesthetics or structure of cultural sites. | L | MH |
| Abiotic | |||
| 5. Impact flow? | ‘Surface water runoff and stream sediment yield were 56 and 192% higher respectively for spotted knapweed dominated sites compared to bunchgrass-dominated sites’ (Sheley et al 1998). Minor impact on surface flow. | ML | H |
| 6. Impact water quality? | ‘Surface water runoff and stream sediment yield were 56 and 192% higher respectively for spotted knapweed dominated sites compared to bunchgrass-dominated sites ’ (Sheley et al 1998). Noticeable but minor effects. | ML | H |
| 7. Increase soil erosion? | ‘.. contributes to increased surface run-off and soil erosion’ (MacDonald et al 2003). ‘Surface water runoff and stream sediment yield were 56 and 192% higher respectively for spotted knapweed dominated sites compared to bunchgrass-dominated sites ’ (Sheley et al 1998). High probability of large scale soil movement with minor off-site implications. | MH | H |
| 8. Reduce biomass? | Displaces other plants, especially grasses (MacDonald et al 2003). Likely that the biomass would increase. | L | H |
| 9. Change fire regime? | ‘.. dense knapweed infestations can change the fire regime by changing the fuel characteristics and thus reducing the fire return interval at a given site’ (USFS 2004). Minor change in the frequency of fire risk. | ML | MH |
| Community Habitat | |||
| 10. Impact on composition (a) high value EVC | EVC= Plains grassy woodland (E); CMA=Corangamite; Bioreg=Victorian Volcanic Plain; CLIMATE potential=VH. ‘Reduces or displaces desirable plant species’(CWMA n.d.). Major displacement of some dominant species within a vegetation layer. | MH | M |
| (b) medium value EVC | EVC= Grassy dry forest (D); CMA=Goulburn Broken; Bioreg=Central Victorian Uplands; CLIMATE potential=VH. ‘Reduces or displaces desirable plant species’(CWMA n.d.). Major displacement of some dominant species within a vegetation layer. | MH | M |
| (c) low value EVC | EVC= Heathy dry forest (LC); CMA=Corangamite; Bioreg=Victorian Volcanic Plain; CLIMATE potential=VH. Reduces or displaces desirable plant species’(CWMA n.d.). Major displacement of some dominant species within a vegetation layer. | MH | M |
| 11. Impact on structure? | ‘Reduces or displaces desirable plant species’. Usually found in open, disturbed sites, replacing ground cover and capable of forming monospecific stands (CWMA n.d.). Likely to have a major effect on lower layer of the strata. | MH | M |
| 12. Effect on threatened flora? | No information available. | MH | L |
| Fauna | |||
| 13. Effect on threatened fauna? | No information available. | MH | L |
| 14. Effect on non-threatened fauna? | ‘Large-scale infestations can impede access to more desirable forage for livestock and wildlife’ (MacDonald et al 2003). ‘Large reductions in available forage and wildlife use have been reported on knapweed-infested range’ (USFS 2005). Likely to lead to a displacement of native animals, leading to reduction in number of animals. | MH | H |
| 15. Benefits fauna? | Although both livestock and wildlife may eat it (USFS 2005), it provides very little support to desirable species. | H | MH |
| 16. Injurious to fauna? | Not known to be injurious to fauna. | L | MH |
| Pest Animal | |||
| 17. Food source to pests? | No evidence that plant is food source to pest species (Alper 2004). | L | H |
| 18. Provides harbor? | Not known to provide harbour for pests. | L | MH |
| Agriculture | |||
| 19. Impact yield? | Reduces the productivity of desirable forage plants. ‘It invades pastureland and renders huge tracts commercially useless, because cattle, horses and most other animals turn up their noses at it’ (USFS 2005). ‘In Montana alone, it covers some 4.5 million acres and costs ranchers more than $40 million annually in herbicide and lost productivity’ (Alper 2004). Major impact on quantity. | MH | H |
| 20. Impact quality? | No evidence to suggest the weed would impact upon quality. | L | MH |
| 21. Affect land value? | In America it ‘invades pastureland and renders huge tracts commercially useless, because cattle, horses and most other animals turn up their noses at it’. ‘Tough for native plants to grow back when spotted knapweed has been eliminated’ (Alper 2004). Potential to also invade pastureland in Australia. | MH | H |
| 22. Change land use? | In America ‘it invades pastureland and renders huge tracts commercially useless, because cattle, horses and most other animals turn up their noses at it’, although sheep may eat it. ‘Tough for native plants to grow back when spotted knapweed has been eliminated’ (Alper 2004). Potential to have same effect in Australia and lead to a major detrimental change and significant loss for agricultural usage. | MH | H |
| 23. Increase harvest costs? | ‘In Montana alone, it covers some 4.5 million acres and costs ranchers more than $40 million annually in herbicide and lost productivity’ (Alper 2004). Increase in time and labour. | M | H |
| 24. Disease host/vector? | Not a known host / vector of disease. | L | MH |
Impact Assessment Record - Spotted knapweed (PDF - 26 KB)
Impact Assessment Record - Spotted knapweed (DOC - 67 KB)
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