Invasiveness Assessment - Stemless thistle (Onopordum acaulon) in Victoria (Nox)
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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 Stemless thistle.
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: Stemless thistle
Scientific name: Onopordum acaulon
| Question | Comments | Reference | Rating |
| Establishment | |||
| Germination requirements? | Seeds germinate after autumn rains. | P & C (1992) | MH |
| Establishment requirements? | Only known to establish in ‘open’ habitats. (See pic. P & C 1992 p. 220). | P & C (1992) | ML |
| How much disturbance is required? | An environmental weed (of ‘serious’ risk) in ‘Mallee shrubland’ and ‘lowland grassland and grassy woodland’. Also established in crops and pastures. | Carr et al (1992) P & C (1992) | MH |
| Growth/Competitive | |||
| Life form? | Prostrate annual or biennial herb. Other. | P & C (1992) | L |
| Allelopathic properties? | None described. |  | L |
| Tolerates herb pressure? | Rarely eaten by stock. | P & C (1992) | MH |
| Normal growth rate? | Reduces the carrying capacity of pastures so assumed to be at least = other herbaceous species. Also described as a ‘rapid spreader which can quickly take over in pasture or crops and seriously reduce production’. (Blood nd). | APB (1987) | MH |
| Stress tolerance to frost, drought, w/logg, sal. etc? | Tolerant of drought (occurs in Africa, Algeria, Morocco and Tunisia). | GRIN (2000) | L |
| Reproduction | |||
| Reproductive system | Reproducing by seed. | P & C (1992) | ML |
| Number of propagules produced? | | P & C (1992) APB (1987) | MH |
| Propagule longevity? | 8 flower heads per plant x 150 seeds per head = 1,200 seeds per plant. (Estimate based on pic. and descriptions), | P & C (1992) | M |
| Reproductive period? | Reproductive period = 1 year for annual species or 1-2 years for biennial species. | P & C (1992) | ML |
| Time to reproductive maturity? | Annual (sometimes biennial). < 1 year. | P & C (1992) | H |
| Dispersal | |||
| Number of mechanisms? | Seeds – prevailing winds, contaminate agricultural produce, vehicles, animals, etc. and water. Also, birds (not mentioned in published material), and in hay, silage, chaff and commercial seed (Anon.). | P & C (1992) | MH |
| How far do they disperse? | “The seeds are well adapted for wind disperse with a large feathery pappus”. In Victoria the spread of the plant has clearly been with prevailing wind, therefore wind assumed capable of spreading seed > 200 m. Other mechanisms could also disperse seed > 200 m. | APB (1987) P & C (1992) | MH |
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