Variegated Thistle

(Silybum marianum)

Progressive Control Pest

Variegated Thistle is a very conspicuous, robust, spiny annual or biennial plant, with stems up to 100 millimetres in diameter. Its leaves have white veins and blotches that give it a variegated look. Its stems are up to 2 metres high and dominated by solitary red-purple flowers. The plants are found in pasture and wasteland and will form dense stands. The seed is spread by machinery and animals, and remains viable for over 20 years.

Reasons for the Strategy

Variegated thistle with a close up of the red purple flower.

Variegated Thistle will suppress and often exclude desirable pasture. Its spines can cause injury to animals and can be toxic. Dense clumps obstruct stock movement and provide habitat for vermin. Plants are poisonous to cattle, and to a lesser extent sheep, due to the high nitrate content of plants growing on high nitrogen soil. Variegated Thistle has the potential to have a significant impact on pastoral and crop production.

Variegated Thistle is assessed at “4” on the infestation curve. There are sites throughout Tasman District, excluding the Upper Buller. Plant numbers have reduced during the time of the last Strategy. The low incidence of Variegated Thistle in the Tasman-Nelson region, extensive areas of suitable habitat, and the potential for it to cause significant adverse effects, mean the benefits of progressive control far outweigh the costs.


To reduce the distribution and density of Variegated Thistle in the Tasman-Nelson region during the term of the Strategy.

Alternative Measures

The alternative option of “do nothing” or relying on voluntary control will not achieve the objective of reducing the distribution and density of Variegated Thistle, and will result in significant additional costs to the community with respect to lost production, and the increased cost of control in the future. Requiring total control is not practical given the long period of seed viability.

Strategy Rule for Variegated Thistle

The occupier shall destroy all adult and juvenile forms of Variegated Thistle on land that they occupy. A breach of Strategy Rule 5.12.5 is an offence under Section 154(r) of the Biosecurity Act 1993.

Biosecurity Act Requirement

No person shall knowingly sell, propagate, breed, release, or commercially display Variegated Thistle, under Sections 52 and 53 of the Act.