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Invasiveness Assessment - Variegated thistle (Silybum marianum) 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 Variegated 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 - 1 MB)
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Common Name: Variegated thistle
Scientific name: Silybum marianum
| Question | Comments | Reference | Rating |
| Establishment | |||
| Germination requirements? | Seeds mainly germinate after autumn rains. | P & C (1992) | MH |
| Establishment requirements? | “Seedlings are quite shade tolerant”. | Champion (1995) | MH |
| How much disturbance is required? | Establishes as an environmental weed in minor disturbed ecosystems. E.g. dry sclerophyll forest and woodland and riparian vegetation. | Carr et al (1992) | MH |
| Growth/Competitive | |||
| Life form? | Erect annual or biennial herb. Other. | P & C (1992) | L |
| Allelopathic properties? | None described. | L | |
| Tolerates herb pressure? | Not readily eaten by stock. | P & C (1992) * | MH |
| Normal growth rate? | “Very competitive in pasture and when well established, eliminates most other plants”. | P & C (1992) | H |
| Stress tolerance to frost, drought, w/logg, sal. etc? | Tolerant of drought (occurs in arid areas), water logging (occurs in irrigation banks), frost (occurs in very cool areas). | P & C (1992) Robbins et al (1951) | MH |
| Reproduction | |||
| Reproductive system | Reproducing by seed. | P & C (1992) | ML |
| Number of propagules produced? | 50-200 seeds per head =125 30 flower heads per plant x 125 seeds per head = 3,750 seeds per plant. Champion (1995) states plants produce 6,350 seeds. | P & C (1992) | H |
| Propagule longevity? | “Some seeds may remain viable in the soil for at least 9 years “. | P & C (1992) | L |
| Reproductive period? | Annual or short lived perennial herb (NZ). Can form dense infestations. (See picture P & C 1992 p. 231). Annual or biennial (Australia). | Connor (1977) P & C (1992) | H |
| Time to reproductive maturity? | Annual/biennial < 1 year. | P & C (1992) | H |
| Dispersal | |||
| Number of mechanisms? | Seed, wind, contaminant of hay, water, mud agricultural produce, vehicles, machinery, grain, etc. Birds also stated by Anon., but not other references. | P & C (1992) | H |
| How far do they disperse? | “Because the seeds are heavy (20 mg), and plumes drop when mature, most seeds fall near the parent plant”. (Roche 1991). However, other mechanisms could transport many seeds further. | H | |
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