Title: Silybum marianum (L.) Gaertn

Scientific Name:

Silybum marianum (L.) Gaertn

Common Name: variegated thistle



Source & more images (via ALA)

Habitat: Subhumid warm-temperate regions occurring on fertile soils of alluvial or volcanic origin (Parsons and Cuthbertson 1992). It is a weed of cultivated land, roadsides, neglected areas and grazing land (Parsons and Cuthbertson 1992). Variegated thistle invades dry coastal vegetation, lowland grassland and grassy woodland, dry sclerophyll forest and woodland and riparian vegetation (Carr et al 1992).

Distribution:



Original source via CABI


Invasiveness Assessment

ESTABLISHMENT


1. Germination requirements? Seeds mainly germinate after autumn rains.

2. Establishment requirements? “Seedlings are quite shade tolerant”.

3. How much disturbance is required? Establishes as an environmental weed in minor disturbed ecosystems. E.g. dry sclerophyll forest and woodland and riparian vegetation.

GROWTH / COMPETITIVE


4. Life form? Erect annual or biennial herb. Other.

5. Allelopathic properties? None described.

6. Tolerates herb pressure? Not readily eaten by stock.

7. Normal growth rate? “Very competitive in pasture and when well established, eliminates most other plants”.

8. 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).

REPRODUCTION


9. Reproductive system? Reproducing by seed.

10. 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.

11. Propagule longevity? “Some seeds may remain viable in the soil for at least 9 years “.

12. Reproductive period? Annual or short lived perennial herb (NZ). Can form dense infestations. (See picture P & C 1992 p. 231). Annual or biennial (Australia).

13. Time to reproductive maturity? Annual/biennial < 1 year.

DISPERSAL


14. 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.

15. 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.


Impact Assessment

RECREATION


1. Restrict human access? “An erect annual or biennial herb, commonly 90 to 180 cm high. Stem leaves spiny, stiff and often reflexed; flower heads surrounded by large stiff reflexed bracts ending in sharp spines. Variegated thistle is very competitive in pastures and, when well established, eliminates most other plants.” The size of the plant, its spiny nature and its possible population density would be highly annoying to humans.

2. Reduce tourism? In dense patches, some recreational activities may be affected.

3. Injurious to people? “The spines in the leaves, stems and flower bracts can injure the delicate parts of animals and provide sites for infection.” Potentially harmful to humans; spines present for most of the year.

4. Damage to cultural sites? Dense infestations may create a negative visual impact.

ABIOTIC


5. Impact flow? Terrestrial species.

6. Impact water quality? Terrestrial species.

7. Increase soil erosion? “Clumps of variegated thistle leave the soil bare at the end of summer.” Moderate probability of soil erosion.

8. Reduce biomass? “A weed of cultivated land, roadsides, neglected areas and, most importantly, grazing land.” Invader generally replaces biomass.

9. Change fire regime? “Clumps of variegated thistle leave the soil bare at the end of summer.” Little fuel remains to establish or support fire. No change to fire regime.

COMMUNITY HABITAT


10(a) Impact on composition of high value EVC? EVC=Plains grassland (E); CMA=Port Phillip; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential.
Is very competitive in open situations such as pasture, roadsides and neglected areas. When well established it eliminates most other vegetation. Major displacement of grasses/forbs.

10(b) Impact on medium value EVC? EVC=Lowland forest (D); CMA=Corangamite; Bioreg=Otway Plain; VH CLIMATE potential.
Impact similar to 10(a) above, though forest canopy may restrict population density.

10(c) Impact on low value EVC? EVC=Lowland forest (LC); CMA=Glenelg Hopkins; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential.
Impact similar to 10(b) above.

11. Impact on structure? EVC=Lowland forest (LC); CMA=Glenelg Hopkins; Bioreg=Victorian Volcanic Plain; VH CLIMATE potential.
Impact similar to 10(b) above.

12. Effect on threatened flora?


FAUNA


13. Effect on threatened fauna?


14. Effect on non-threatened fauna? “Variegated thistle is very competitive in pastures and, when well established, eliminates most other plants.” Potential to significantly decrease useful fodder for fauna species.

15. Benefits fauna? No known benefits.

16. Injurious to fauna? “The spines in the leaves, stems and flower bracts can injure the delicate parts of animals and provide sites for infection.” Similar threat to fauna species.

PEST ANIMAL


17. Food source to pests? Not known as a food source to pest animals.

18. Provides harbor? “Seedlings prefer disturbed soil on which to establish so sheep camps, rabbit warrens and cultivated fire breaks are suitable sites. Dense clumps of variegated thistle provide harbour for vermin animals, particularly rabbits.” Capacity to provide permanent harbor.

AGRICULTURE


19. Impact yield? “Variegated thistle is very competitive in pastures and, when well established, eliminates most other plants.” Level of impact not documented. Assume serious impact on carrying capacity.

20. Impact quality? Not a serious weed of cropping. “Contamination of hay is also important [as a means of seed dispersal] because plants are flowering when hay is cut.” Possible major impact on hay quality.

21. Affect land values? “In Victoria, an estimated 4.8 million hectares is infested.” Like Cirsium vulgare (spear thistle), which infests 9.7 million hectares in Victoria, Sylibum marianum, may be, “so well established in much of southern Australia that it is accepted as a permanent part of the vegetation.” Little impact on land value.

22. Change land use? Predominantly a weed of cultivated land on fertile soils, control can be effected by, “encouraging pasture species able to utilise the raised fertility and compete with the weed species.” Little change to land use.

23. Increase harvest costs? Not known to affect harvest costs.

24. Disease host/vector? None evident.




Feedback

Do you have additional information about this plant that will improve the quality of the assessment? If so, we would value your contribution.


Assessment ratings originally made by the Victorian Department of Primary Industries.
The entry of this assessment was made possible through the generous support of Boyut Foreign Trade Corp.






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Date: 1 December 2011; 2:18:32 PM AEDT

Author Name: David Low
Author ID: adminDavid