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Onopordum acanthium

Scotch thistle

Attracts Pollinators Attracts Butterflies Deer Resistant Drought Tolerant

Overview

Onopordum acanthium is a massive spiny biennial in the family Asteraceae reaching 3-8 feet (0.9-2.4 m) tall and 3-6 feet (0.9-1.8 m) wide, ranking with the largest thistles of North America by height and rosette diameter. The entire plant is covered densely in white to gray woolly tomentum, giving a silver-gray appearance visible from a distance. First-year plants form a basal rosette up to 3 feet (0.9 m) across, carrying deeply lobed spine-tipped leaves 6-18 inches (15-45 cm) long. Second-year stems are erect, stout, up to 2 inches (5 cm) in diameter at the base, with spiny wings running the full length of the stem; the wings are 0.3-0.8 inch (8-20 mm) wide, continuous, and armed with stout spines up to 0.4 inch (10 mm) long. Flower heads are solitary or in small clusters, 1.5-2.5 inches (4-6 cm) in diameter, globe-shaped, with purple to pink disc florets and no ray florets. Phyllaries (involucral bracts) are spine-tipped. A single plant produces 8,000-40,000 seeds, with wind dispersal by the pappus. Seeds remain viable in soil for 10 years or more. The species is listed as a Class B noxious weed in Washington State and forms dense impenetrable stands on rangeland, roadsides, and waste ground.

Native Range

Onopordum acanthium is native to Europe and western Asia, in roadsides, waste ground, pastures, and disturbed sites from sea level to approximately 6,000 feet (1,800 m). Introduced to North America as an ornamental and escaped cultivation. Widely naturalized in the western United States, particularly in semi-arid rangeland of the intermountain West and eastern Washington and Oregon.

Suggested Uses

Used in noxious-weed identification training for rangeland managers. The spiny-winged stem morphology is the primary diagnostic teaching feature separating Onopordum spp. from Cirsium spp. and Carduus spp.. Studied in rangeland weed ecology and in impenetrable-thicket formation by large thistles. The dense woolly tomentum carries the species into leaf-surface-adaptation curricula for drought tolerance.

How to Identify

Separated from Cirsium spp. thistles by the continuous spiny wings running down the stems (absent in Cirsium spp.) and by the dense white-woolly tomentum covering the entire plant. Separated from Carduus nutans (musk thistle) by the winged versus unwinged stems and by the upright versus nodding flower heads. The plant's large size (up to 8 feet / 2.4 m), the silver-gray woolly appearance, and the spiny-winged stems identify the species at a glance. First-year rosettes can exceed 3 feet (0.9 m) in diameter.

Appearance

Size & Dimensions

Height3' - 8'

Width/Spread3' - 6'

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~4 weeks

Summer

Globe-shaped flower heads 1.5-2.5 inches (4-6 cm) in diameter with purple to pink disc florets and no ray florets, borne June through August in the second year over 3-4 weeks. Bees, butterflies, and other insects pollinate the flowers. Seeds mature 3-4 weeks after flowering and disperse by wind via the pappus. In the Pacific Northwest peak bloom falls in July. Plants die after seed set (monocarpic biennial).

Detailed Descriptions

Flower Description

Purple to pink; globe-shaped flower heads 1.5-2.5 inches (4-6 cm) with no ray florets; June-August (second year)

Foliage Description

Gray-green to silver-white from dense woolly tomentum on both surfaces; deeply lobed with spine-tipped margins; deciduous

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 - 8.5(Neutral)

357912

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

Listed as a Class B noxious weed in Washington State. Management focuses on first-year rosette removal before bolting. Digging the fleshy taproot of the first-year rosette works but uses protective clothing because of the stout leaf spines. Second-year bolted plants are cut at ground level 2-4 inches (5-10 cm) below the soil surface to sever the root crown; cutting above ground allows resprouting. The spiny-winged stems require leather gloves, long sleeves, and eye protection during handling. Mowing at the early bolting stage prevents seed set but must sever below the lowest branch. The large seed production and long-lived seed bank mean sustained management over 10 years or more is necessary. Biological control agents have been released in some western states.

Pruning

Pruning does not apply in a weed-management context. First-year rosettes are dug below the root crown. Second-year plants are severed 2-4 inches (5-10 cm) below ground level before seed set. Protective clothing (leather gloves, arm protection, face shield) is used because of the stout spines on stems, wings, and leaves.