Invasiveness Assessment - Spotted knapweed (Centaurea maculosa) in Victoria

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Plant invasiveness is determined by evaluating a plant’s biological and ecological characteristics against criteria that encompass establishment requirements, growth rate and competitive ability, methods of reproduction, and dispersal mechanisms.

Each characteristic, or criterion, is assessed against a list of intensity ratings. Depending upon information found, a rating of Low, Medium Low, Medium High or High is assigned to that criterion. Where no data is available to answer a criterion, a rating of medium (M) is applied. A description of the invasiveness criteria and intensity ratings used in this process can be viewed here.

The following table provides information on the invasiveness 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)
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Common Name: Spotted knapweed
Scientific name: Centaurea maculosa

Question	Comments	Rating	Confidence
Establishment
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.	MH	MH
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.	H	MH
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.	MH	M
Growth/Competitive
Life form?	Perennial forb (USDS 2004) – other	L	H
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).	MH	H
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.	MH	H
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.	ML	MH
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.	MH	MH
Reproduction
Reproductive system	Reproduces almost entirely by seed. ‘Requires cross-pollination between flowers on different plants’ (USFS 2004).	L	MH
Number of propagules produced?	Depends on conditions and area but up to 25,260 seeds have been recorded on the one plant (USFS 2004).	H	MH
Propagule longevity?	‘Thirty percent of seeds may be viable after eight years of burial’ (TNC 2005).	ML	MH
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).	H	MH
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).	ML	H
Dispersal
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).	H	MH
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.	H	MH

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