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Centaurea solstitialis

yellow star-thistle

Attracts Pollinators Attracts Butterflies Deer Resistant Drought Tolerant

Overview

Centaurea solstitialis, yellow star-thistle, is an annual or winter-annual herb in the daisy family, growing 1-3 feet (0.3-0.9 m) tall, sometimes taller, with rigid, branching, winged stems covered in cottony gray hairs. The plant begins as a basal rosette of deeply lobed leaves, then bolts into a much-branched, stiff form. Yellow flower heads about 0.5-1 inch (1.3-2.5 cm) across sit at the branch tips, ringed at the base by sharp, straw-colored spines 0.5-1 inch (1.3-2.5 cm) long that radiate in a star. Flowering runs through summer, and a single plant can set thousands of seeds. A deep taproot lets it draw moisture late into the dry season, outlasting many native annuals. Native to the Mediterranean, it is a designated noxious weed across the western United States and forms dense, spiny stands on rangeland, roadsides, and disturbed ground. The foliage causes a fatal neurological disorder in horses that graze it over time, and the spines deter livestock and limit access.

Native Range

Centaurea solstitialis is native to the Mediterranean region of southern Europe and western Asia. It has spread as an invasive weed across the western United States, especially California, and into Argentina, Chile, and Australia, colonizing grasslands, rangeland, and roadsides in regions with dry summers.

Suggested Uses

Centaurea solstitialis is managed as an invasive weed rather than grown as a garden plant. Where established, it gives summer nectar for honeybees but displaces native vegetation and injures grazing animals.

How to Identify

Recognize yellow star-thistle by its yellow flower heads ringed with stiff, sharp, straw-colored spines up to 1 inch (2.5 cm) long, set on winged, cottony gray stems. The basal rosette has deeply lobed leaves, while upper leaves are narrow and run down the stem as wings. The long radiating spines separate it from yellow knapweeds and other thistles.

Appearance

Size & Dimensions

Height1' - 3'

Width/Spread1' - 2'

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~14 weeks

Summer

Flowering runs from June through September, peaking in midsummer. Yellow heads open at the branch tips and set large amounts of seed. Bees work the flowers, and the species is a summer nectar source where it has naturalized.

Detailed Descriptions

Foliage Description

gray-green

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 Range6.0 - 8.0(Neutral)

357912

Soil Types

loam clay sand rocky

Drainagewell drained

Water & Climate

Water Needs

Very Low

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Grows in full sun on dry, disturbed, well-drained soils and tolerates poor, rocky, and compacted ground. It germinates with fall and winter rains, overwinters as a rosette, then bolts and flowers as soils dry. A deep taproot supports growth into summer drought without irrigation. The plant spreads only by seed, which moves in hay, soil, and on vehicles. As a noxious weed it is regulated in many western states, where sale and transport are restricted. Repeated mowing, grazing management, and competition from perennials reduce its spread.

Pruning

Pruning is not a garden practice for this weed. Cutting or mowing before the spiny heads set seed reduces the next season stand. Plants cut or pulled at flowering, before seed matures, limit further spread.