Milk thistle (Silybum marianum) is a biennial or winter annual native to Europe and Asia that has been naturalized throughout the world, including much of North America. Milk thistle was introduced into California in 1854. Its habitat includes crop fields, pastures and disturbed or waste area, preferring areas of high fertility. Many states consider milk thistle a noxious weed.
Throughout history, this member of the Aster Family was used to treat liver diseases, to prevent and treat cancer and as an antidote to Aminita (death cap mushroom) poisoning. Even today herbalists continue to employ milk thistle. The scientific literature is contradictory concerning milk thistle’s medical efficacy.
Milk thistle is toxic to cattle, sheep and other ruminants. Milk thistle contains potassium nitrate, which upon breakdown by ruminant bacteria results in compounds that cause oxygen deprivation.
For non-ruminants all parts of milk thistle are edible. The roots can be boiled or roasted while the young shoots in the spring are tasty when boiled as a vegetable. The leaves of milk thistle make a spinach substitute, raw or cooked, once they are trimmed of prickles. Milk thistle seeds make a coffee substitute. The stems, if peeled and soaked in water to remove bitterness, then coked, also make a good vegetable.
The stout, erect milk thistle stem grows up to 7 feet in height and is ridged with sparse hairs.
Milk thistle leaves are shiny, green and marbled with white veins and spots. The leaf margins are coarsely dentate (toothed) with yellowish prickles. The basal leaves are large and clasp the stems with ear-like lobes on the base. The stem leaves are much smaller and infrequent.
The large heads of milk thistle are solitary, terminal and nodding. The spiny-margined and spiny-tipped bracts occur in many rows. Red-purple disk flowers form the head.
The milk thistle fruit is a brown or black achene (dry, one seeded) with a ring of white bristles at the apex.
There is a story that the white marbling on milk thistle leaves resulted from drops of milk shed when Mary nursed the Christ child. This story gave rise to the species name, marianum, as well as to the other common names Lady’s thistle and blessed milkthistle. Variegated thistle is another colloquial name. The genus name, Silybum, is the Greek name for a thistle that was used as food.
These milk thistle plants were photographed along the Loop Trail near the Sundial Bridge in Redding Ca (Shasta County) last May.