Variegated thistle, blessed milk thistle or just milk thistle, is a declared pest in Western Australia (WA). 

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Variegated thistle is usually an annual plant but occasionally does not flower until the second year. A large plant may cover an area of over one metre in diameter. It reproduces only from seed.

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Leaves: Variegated thistle forms a broad rosette of shiny dark green leaves variegated by a network of white veins and patches. The leaves are deeply lobed with a long spine at the point of each lobe.

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Flowers: In spring a branched flowering stem, without prickles, grows up to two metres high. Flowering starts in October and continues until early summer. One large purple flower head is produced at the end of each branch. Each head is enclosed by numerous overlapping bracts, each bract having a large stiff spine at the tip and shorter spines along the edge.

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Seeds: The seeds are black or brown, about five millimitres long, somewhat flattened with a smooth seed coat. They are equipped with a parachute-like pappus of pale, barbed hairs about two centimetres long. Seed set takes place in summer. 

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Pasture weed, toxic to stock.

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It is well established in areas such as Donnybrook, Bridgetown and competes with useful pasture plants for light, moisture and nutrients. If left unchecked, it can form very large infestations, choking out valuable pasture species as well as weeds. In such dense infestations, it can form impenetrable stands and provide shelter for pest species such as rabbits. 

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Variegated thistle is poisonous under certain conditions. It can kill cattle and occasionally sheep, especially when hungry animals consume large quantities in the absence of alternative feed, although its menacing spines act as a deterrent. It becomes more palatable to stock and more toxic when it has wilted, for example, after cutting, ploughing or spraying. 

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Its needle like spines can cause injury to animals, including domestic dogs. Variegated thistle can be a serious contaminant of wool.

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