Thistle, common name for several plants of the family of composite flowers. The name thistle is applied in a restricted sense to plants in several genera, usually possessing spiny leaves and branches, and sharp, spiny bracts surrounding the flowers. Technical definition of the thistle relies on the characteristics of the style and anthers. The common, plumed, or bull thistle bears purple flowers; the Canada thistle bears small lilac or white flowers. The musk thistle is common in fields in the northeastern United States and bears purple flowers. The globe thistle, which bears whitish or blue flowers, is frequently cultivated in the United States, as is the milk thistle or lady's thistle, which bears purple flowers. The cotton or Scotch thistle has large heads of purple flowers. The red star thistle bears purplish flowers; the blessed thistle bears large heads of yellow flowers.
The name thistle is also applied to other spiny plants. The sow thistles, for example, also belong to the family of composite flowers, and the Russian thistle belongs to the goosefoot family.
Scientific classification: Thistles belong to the family Asteraceae (formerly Compositae). The common, plumed, or bull thistle is classified as Cirsium vulgare; the Canadian thistle as Cirsium arvense; the globe thistle as Echinops sphaerocephalus; and the milk thistle or lady's thistle as Silybum marianum. The cotton or Scotch thistle is classified as Onopordum acanthium, the red star thistle as Centaurea calcitrapa, and the blessed thistle as Cnicus benedictus.