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Centaurea spp.

knapweeds

Asteraceae

Mediterranean, Eurasia, and North Africa

Attracts Pollinators Attracts Butterflies Deer Resistant Drought Tolerant Container Friendly

SunFull Sun

WaterLow · drought-tolerant

HardinessZones 3–9

Overview

Centaurea spp. is a genus of roughly 350 to 500 species of annual, biennial, and perennial herbs in the family Asteraceae, collectively called knapweeds, star-thistles, and cornflowers. Plants range from 8 inches to 6 feet (20 to 180 cm) tall, forming upright, branched stems from a taproot or rhizome. Leaves are alternate, gray-green, and variable, entire to deeply pinnately lobed, often with a coating of short hairs. The flower heads are thistle-like but spineless in most species, 0.5 to 2.5 inches (1 to 6 cm) wide, composed of tubular disc florets and frequently surrounded by enlarged, ray-like marginal florets that flare outward. Blooms appear in blue, purple, pink, magenta, yellow, or white from late spring through autumn. A defining feature is the involucre of overlapping bracts, each tipped with a fringed, comb-like, or spine-like appendage whose form distinguishes the species. Native mainly to the Mediterranean, Eurasia, and North Africa, Centaurea spp. has spread worldwide. Several species, including C. stoebe and C. solstitialis, are listed as noxious weeds across multiple western United States and Canadian provinces, where they invade rangeland and displace native vegetation. Seeds are wind-borne on a short pappus, and many species self-sow freely.

Native Range

Centaurea spp. is native chiefly to the Mediterranean Basin, southwestern and central Asia, and North Africa, with a center of diversity in the eastern Mediterranean and Caucasus. A small number of species reach the high mountains of eastern Africa. Many species are now naturalized far beyond this range, including across North America and Australia.

Suggested Uses

Ornamental Centaurea spp. such as cornflower and perennial mountain bluet are planted in cottage gardens, cutting beds, meadows, and pollinator borders, spaced 12 to 18 inches (30 to 45 cm) apart. The cut flowers hold up in arrangements and the seeds feed finches. Weedy species are not planted and are managed where they invade pasture and roadside.

How to Identify

Look for upright, branched herbs with alternate gray-green leaves and solitary thistle-like flower heads that lack the sharp spines of true thistles in most species. Each head sits above an involucre of tightly overlapping bracts, and the shape of the dark appendage at each bract tip, whether fringed, comb-toothed, or spine-tipped, is the main feature separating the species. Disc florets are often ringed by larger sterile florets that mimic petals. Seeds bear a short, bristly pappus rather than the long parachute of many other daisies.

Appearance

Size & Dimensions

Height8" - 6'

Width/Spread8" - 2'

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~12 weeks

Spring Summer Fall

Centaurea spp. blooms from late spring into autumn, with most species flowering June through September and annual cornflower types starting in May. Individual heads measure 0.5 to 2.5 inches (1 to 6 cm) across in blue, purple, pink, yellow, or white. Perennial knapweeds rebloom over many weeks when old heads are cut, while annuals flower in a single extended flush. Pollen and nectar draw bees and butterflies through the season.

Detailed Descriptions

Flower Description

blue, purple, pink, magenta, yellow, or white

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 sand chalk clay

Drainagewell drained

Water & Climate

Water Needs

Low

Frost Tolerance

hardy

Time to Maturity

perennials in 1-2 years; annuals in one season

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Centaurea spp. grows in full sun and average to lean, well-drained soil across a wide pH range of 6.0 to 8.0, tolerating drought, poor fertility, and exposed sites once established. Rich soil and shade produce floppy, sparse-flowering growth. Perennial species are cold hardy to USDA zone 3 or 4, while annuals are grown from spring-sown seed and complete their cycle in one season. Plants need little water after establishment and no supplemental feeding. Powdery mildew and aphids occur in crowded, humid conditions. Because several species self-sow heavily and some are regulated as invasive, spent heads are often removed before seed sets.

Pruning

Deadheading spent flower heads extends bloom on perennial Centaurea spp. and limits self-seeding. Perennial clumps are cut back by half after the first flush to prompt fresh growth and a second round of bloom, then taken to the ground in late autumn or early spring. Annual types are pulled once flowering ends.

Pruning Schedule

summerfall