Title: Centaurea stoebe L.

Scientific Name: Centaurea stoebe L. subsp. micranthos (Gugler) Hayek

Common Name: spotted knapweed



Source & more images (via ALA)

Habitat: In the United States it occurs on well-drained, light-textured soils in areas of summer rainfall, in forest habitat as well as open grasslands and meadow. In central Washington it is often associated with irrigation where there is an abundance of moisture, deep soils and some degree of disturbance such as roadsides (Mauer et al, 1987).

Distribution:



Original source via CABI


Invasiveness Assessment

ESTABLISHMENT


1. Germination requirements? ‘..germinate whenever moisture and temperature are suitable, and both fall and spring seedling emergence is common (USDS 2004). Requires seasonal temperatures for germination.

2. Establishment requirements? ‘Canopy cover had no effect on emergence rate, with spotted knapweed germinating equally well over a range from 0-100% canopy cover’ (USFS 2004). Can establish without additional factors.

3. How much disturbance is required? ‘... usually found in disturbed areas, once a plant colony is established, it may invade adjacent areas that are relatively undisturbed’ Found on roadsides, foothill prairie habitats (CWMA n.d.). Most often found in disturbed ecosystems but has been found in relatively intact ecosystems also.

GROWTH / COMPETITIVE


4. Life form? Perennial forb (USDS 2004) – other

5. Allelopathic properties? ‘ .. displaces native plant species by exuding the phytotoxin (-) –catechin from its roots. .. inhibition of native species’ growth .. [causing] ultimately, death of root system’ (Bais et al 2003).

6. Tolerates herb pressure? In US, gall flies’ introduction has led to slight decrease in number of seeds that develop (Reebs 2002). ‘Unlike other grazers, sheep love knapweed, and .. they do a pretty good job of controlling the infestation’ (Alper 2004). Capable of flowering under moderate herbivory pressure.

7. Normal growth rate? ‘ smooth brome is capable of suppressing the growth of spotted knapweed .. vigorous grass slow knapweed invasion’ (USFS 2004). Maximum growth rate less than many species of same life-form.

8. Stress tolerance to frost, drought, w/logg, sal. etc? In Washington, thrives in irrigated land... produces large quantities of seed that can survive fire’ (USFS 2004). Drought tolerant, frost-tolerant. Not documented as to whether susceptible to salinity.

REPRODUCTION


9. Reproductive system? Reproduces almost entirely by seed. ‘Requires cross-pollination between flowers on different plants’ (USFS 2004).

10. Number of propagules produced? Depends on conditions and area but up to 25,260 seeds have been recorded on the one plant (USFS 2004).

11. Propagule longevity? ‘Thirty percent of seeds may be viable after eight years of burial’ (TNC 2005).

12. Reproductive period? ‘Form dense monocultures’ (CWMA n.d). ‘Often characterized by dense stands and virtually complete competitive exclusion of native species’ (Callaway 1999). ‘..can live at least 9 years’ (USFS 2004).

13. Time to reproductive maturity? ‘Fall and early spring germinating seedlings are capable of maturing into seed-producing adults in 1 yr’ (Sheley et al 1998).

DISPERSAL


14. Number of mechanisms? ‘Dispersal of achenes over long distances is facilitated by animals and birds…domestic sheep excrete viable seeds of spotted knapweed in their faeces for up to 7 days…can also be transported in rivers and other watercourses, and in crop seed and hay’ (USFS 2004).

15. How far do they disperse? ‘Dispersal of achenes over long distances is facilitated by animals and birds…domestic sheep excrete viable seeds of spotted knapweed in their faeces for up to 7 days…can also be transported in rivers and other watercourses, and in crop seed and hay’ (USFS 2004). Very likely that some propagules will disperse greater than 1 km.


Impact Assessment

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.

2. Reduce tourism? Weed has a purple flower and capable of forming monocultures (USFS 2004). Weed would have minor effect on aesthetics of site.

3. Injurious to people? ‘The sap of spotted knapweed can cause skin irritation in some people’ (CWMA n.d.).

4. Damage to cultural sites? No evidence to suggest weed has more than a negligible effect on aesthetics or structure of cultural sites.

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

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.

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.

8. Reduce biomass? Displaces other plants, especially grasses (MacDonald et al 2003). Likely that the biomass would increase.

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.

COMMUNITY HABITAT


10(a) Impact on composition of 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.

10(b) Impact on 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.

10(c) Impact on 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.

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.

12. Effect on threatened flora? No information available

FAUNA


13. Effect on threatened fauna? No information available

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.

15. Benefits fauna? Although both livestock and wildlife may eat it (USFS 2005), it provides very little support to desirable species.

16. Injurious to fauna? Not known to be injurious to fauna.

PEST ANIMAL


17. Food source to pests? No evidence that plant is food source to pest species (Alper 2004).

18. Provides harbor? Not known to provide harbour for pests.

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.

20. Impact quality? No evidence to suggest the weed would impact upon quality.

21. Affect land values? 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.

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.

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.

24. Disease host/vector? Not a known host / vector of disease.




Feedback

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Assessment ratings originally made by the Victorian Department of Primary Industries.
The entry of this assessment was made possible through the generous support of Stephen L. Young, West Central Research & Extension Center,University of Nebraska-Lincoln.






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Date: 21 February 2012; 12:14:34 PM AEDT

Author Name: David Low
Author ID: adminDavid