Invasiveness Assessment - Variegated thistle (Silybum marianum) 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 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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