Variegated Thistle

What is variegated thistle?

(Silybum marianum)

Variegated thistle flower, photo: M. Baker
Variegated thistle is a pasture and crop weed.

How to identify variegated thistle

  • Variegated thistle is a large biennial plant (living for two years) forming rosettes of a metre or more in diameter and a flowering stem up to 2 metres in height.
  • Rosette leaves are lobed and with spiny margins, and the upper surface is a bright, shining green with distinct white variegations. The flower heads are large and showy, bright purple in colour, and surrounded by long, stiff, spiny bracts.
  • Most seed germination takes place in early autumn but can also occur in spring and summer in irrigated areas. Variegated thistle normally over-winters as a rosette but in spring-sown crops it can establish as a seedling then flower and produce seed within the life of the crop.

Variegated thistle infestation, photo: M. BakerVariegated thistle leaves
Image top: Variegated thistle flower, photo: M. Baker.
Images above, left to right: Variegated thistle infestation, photo: M. Baker; Variegated thistle leaves, photo: M. Baker.

Variegated thistle in Tasmania

  • Variegated thistle occurs in most agricultural areas of Tasmania. Infestations are most common in the Midlands and in the south, lighter in the north-east and north-west, and very light in the far north-west.
  • Variegated thistle is found on roadsides and in waste areas, in pastures and occasionally in crops. Variegated thistle grows best in areas of high fertility such as sheep camps and stock-yards.
  • The rosettes are strongly competitive in pastures and crops causing severe yield losses. Dense infestations kill other vegetation, leaving large bare patches when the thistles die off, which are then vulnerable for re-infestation by weeds.
  • Some livestock (especially cattle) can suffer from nitrate poisoning from grazing variegated thistle. Grazing of variegated thistle can lead to the rapid onset of toxicity symptoms and death, or to chronic symptoms of ill-thrift depending on nitrate concentrations in the plant and the amount of material consumed. Variegated Thistles may be consumed in large quantities where little alternative feed is available or palatability is increased by cutting or by herbicide application.

Detailed management and control guidelines for variegated thistle can be found in the Variegated thistle Control Guide. Refer also to Herbicides for Variegated thistle Control. For further information see DPIPWE Weed Links and Resources.

See also:

Variegated Thistle Control Guide
Herbicides for Variegated Thistle Control
DPIPWE Weed Links and Resources

Other useful links:

Pest Genie

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