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Silybum, Genus, marianum, Species,

Genus & species

milk thistle

Asteraceae

Mediterranean Europe, northern Africa, western Asia

Attracts Pollinators Attracts Butterflies Deer Resistant Drought Tolerant

SunFull Sun

WaterLow · drought-tolerant

HardinessZones 5–10

Overview

Silybum marianum is a stout, erect annual or biennial in the daisy family (Asteraceae) reaching 24-72 inches (60-180 cm) tall with a 24-48 inch (60-120 cm) spread. Stems are erect, stout, branching, smooth to slightly hairy. Leaves are the primary identification feature: glossy, dark green with conspicuous white marbling (variegation) along the veins, deeply pinnately lobed, spiny-margined with stout yellowish spines up to 0.4 inch (10 mm) long. Basal rosette leaves are 12-24 inches (30-60 cm) long. The white vein marbling pattern is rare among common thistles and is immediately diagnostic. Flower heads are solitary, 1.5-2.5 inches (4-6 cm) in diameter, with purple to pink disc florets and no ray florets. The involucre has broad, spine-tipped phyllaries that spread outward and downward. Fruit is an achene 0.2-0.3 inch (5-7 mm) long with a white pappus. A single plant produces 5,000-20,000 seeds. Seeds contain silymarin, a flavonoid complex used in herbal medicine for liver support. Listed as a Class A noxious weed in Washington State (limited distribution, high priority for eradication).

Native Range

Silybum marianum is native to Mediterranean Europe, northern Africa, and western Asia, occurring in roadsides, waste ground, pastures, and disturbed sites from sea level to approximately 5,000 feet (1,500 m). Widely naturalized in California and at scattered locations in the Pacific Northwest, the species is listed as Class A in Washington State due to its limited distribution and the priority for eradication.

Suggested Uses

Used in noxious weed identification courses for the Class A (eradication priority) category and the rare white-marbled leaf pattern. The thistle comparison set covers four genera: Cirsium (no wing, no marbling), Onopordum (spiny-winged woolly), Carduus (no wing, nodding), and Silybum (white-marbled). Seeds are the commercial source of silymarin (milk thistle extract), used as a liver support supplement and studied in hepatoprotective pharmacology. The white vein marbling is studied in leaf pigmentation genetics. S. marianum is a regulated Class A noxious weed in Washington State and is not planted horticulturally.

How to Identify

Immediately identified by the glossy, dark green leaves with conspicuous white marbling (variegation) along the veins — no other common thistle has this leaf pattern. Separated from Onopordum acanthium (Scotch thistle) by the white-marbled leaves (versus uniformly gray-woolly), the smooth green stems (versus spiny-winged woolly stems), and the large broad-based spiny phyllaries spreading outward. Separated from Cirsium thistles by the white vein marbling and the broad, spreading, spine-tipped involucral bracts.

Appearance

Size & Dimensions

Height2' - 6'

Width/Spread2' - 4'

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~4 weeks

Flowers: May–August

Spring Summer

Flowers appear May through August. Large solitary heads bloom over 3-4 weeks. Pollinated by bees and butterflies. Seeds mature 4-6 weeks after flowering. In the Pacific Northwest, the limited distribution means new sightings are reported to county noxious weed boards.

Detailed Descriptions

Flower Description

Purple to pink solitary heads 1.5-2.5 inches (4-6 cm) with broad spine-tipped phyllaries spreading outward

Foliage Description

Glossy dark green with conspicuous white marbling along the veins; spiny-margined; large, deeply lobed

Growing Conditions

Sun Requirements

SunFull Sun

Requires 6-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.5 - 8.0 (Acidic to alkaline)

3579

Soil Types

loam sand silt clay

Drainagewell drained

Water & Climate

Water Needs

Low

Frost Tolerance

hardy

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

S. marianum is listed as a Class A noxious weed in Washington State, where the management goal is eradication wherever the species is found. Sightings are reported to the county noxious weed board. Rosettes are dug before bolting; the taproot is extracted to a depth of 6-8 inches (15-20 cm). Bolted plants are cut at ground level 2-4 inches (5-10 cm) below the soil surface before flowering. The spiny leaves and phyllaries are sharp enough to puncture standard garden gloves; leather gloves and arm protection are used during removal. Seed heads are bagged and disposed of in landfill, since composting does not kill viable seed. The Class A listing reflects that the species is not yet established in Washington and the state goal is complete eradication.

Pruning

Pruning is not applicable. Plants are dug (rosettes) or severed below ground (bolted plants) and all material is disposed of. Seed heads are bagged. New sightings are reported to the county noxious weed board.