Silybum marianum has common names including cardus marianus, milk thistle, blessed milkthistle, Marian thistle, Mary thistle, Saint Mary's thistle, Mediterranean milk thistle, variegated thistle and Scotch thistle (though not to be confused with Onopordum acanthium). This species is an annual or biennial plant of the Asteraceae family. This fairly typical thistle has red to purple flowers and shiny pale green leaves with white veins. Originally a native of Southern Europe through to Asia, it is now found throughout the world.
Milk thistles can grow to be 30 to 200 cm (12 to 79 in) tall, and have an overall conical shape. The approximate maximum base diameter is 160 cm (63 in). The stem is grooved and more or less cottony. The largest specimens have hollow stems.
The leaves are oblong to lanceolate. They are either lobate or pinnate, with spiny edges. They are hairless, shiny green, with milk-white veins.
The flower heads are 4 to 12 cm long and wide, of red-purple colour. They flower from June to August in the North or December to February in the Southern Hemisphere (summer through autumn).
The bracts are hairless, with triangular, spine-edged appendages, tipped with a stout yellow spine.
The achenes are black, with a simple long white pappus, surrounded by a yellow basal ring.
Milk thistle has also been known to be used as food. The roots can be eaten raw or boiled and buttered or par-boiled and roasted. The young shoots in spring can be cut down to the root and boiled and buttered. The spiny bracts on the flower head were eaten in the past like globe artichoke, and the stems (after peeling) can be soaked overnight to remove bitterness and then stewed. The leaves can be trimmed of prickles and boiled as a spinach substitute or they can also be added raw to salads.
Because of potassium nitrate content, the plant has been found to be toxic to cattle and sheep. When potassium nitrate is eaten by ruminants, the bacteria in the animal's stomach breaks the chemical down, producing nitrite ions. Nitrite ions then combine with hemoglobin to produce methaemoglobin, blocking the transport of oxygen. The result is a form of oxygen deprivation.[