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Statewide IPM Program, University of California

Blessed Milkthistle  (Silybum marianum)

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Life stages of Milkthistle mature plant seed head and flower head seedling seeds immature plant

Blessed milkthistle is a common winter or summer annual or biennial broadleaf plant. In California, it is found on the North Coast, North Coast Ranges, Central Valley, San Francisco Bay region, South Coast Ranges, South Coast, and Channel Islands, to about 1600 feet (500 m). It often forms dense competitive stands. Plants range from extremely small to very tall, depending on soil moisture levels and soil fertility. Blessed milkthistle inhabits agricultural land and other disturbed areas. Under stressful conditions like drought, mowing, or herbicide treatment, leaves can accumulate nitrate levels toxic to cattle. In 1971 a seed-eating weevil was released in southern California as a biological control agent for blessed milkthistle, but control to date has not been successful and the weevil attacks several native thistles. Blessed milkthistle has been used as a medicinal herb for at least 2000 years.

Habitat

Pastures, agronomic crop fields, orchards, trail margins of chaparral communities and woodlands; roadsides and other disturbed, unmanaged sites.

Seedling

Seedlings have thick, succulent cotyledons (seed leaves) that are broadly egg shaped, hairless, and about 2/5 to 3/5 of an inch (1–1.5 cm) long. The first pair of leaves are football to oblong in shape, hairless, and mottled, with white around the veins. Seedling leaves are typically 2/5 to 4/5 of an inch (1–2 cm) long, with a prickly-toothed edge, and are alternate to one another along the stem.

Mature plant

Mature plants are stiff, erect, sparsely branched and have prickly, white-marked leaves. Stem branches are thick, hollow, ribbed, sparsely hairy, lack wings or spines and grow 2 to 6-1/2 feet (0.6–2 m) tall. Lower leaves are 6 to 28 inches (15–70 cm) long, with leaf size diminishing towards the top of the plant. Leaves are spiny and deeply lobed and the leaf base wraps around the stem. All leaves show a characteristic white mottling or marbling around veins that distinguishes milkthistle from other related weedy species such as prickly lettuce, annual sowthistle, Canada thistle, and bull thistle.

Flowers

Flowers bloom from April through July. Flower heads are pink to purple, nearly round, shiny and have long stalks. Beneath the flower head, large green scalelike structures, called bracts, bear spines up to 2 inches (5 cm) long.

Fruits

Fruits are one-seeded (achene), smooth, flat, shiny, mottled black and brown, about 1/4 to 1/3 of an inch (6–8 mm) long, and have a tuft of bristles. Eventually the bristles detach as a unit, leaving a yellowish ring on the achene at the point of detachment.

Reproduction

Reproduces by seed.

Related or similar plants

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