Thistles belong to the Thistle Subfamily of the Aster Family, along with artichokes. Compare an artichoke and a thistle, and notice that they are very similar, except in size. Like other members of the Aster Family, these are composite flowers.

Milk Thistle (Silybum marianum) is an annual or biennial plant which belongs to the Asteraceae family. It is native to the Mediterranean region, but is now found throughout the world. It grows 30-200 cm tall, has red purple flowers.