Common Knapweed (Lesser Knapweed / Hardhead)
Centaurea nigra · Herbaceous Perennial (Note: This is a wildflower/herbaceous perennial, not a woody tree species)
Plant Family
Asteraceae (Daisy / Sunflower family)
Tree Category
Wildflower / Herbaceous Meadow Perennial
Variety / Cultivar
Wild type (Centaurea nigra ssp. nigra)
Hardiness Zones
USDA Hardiness Zones 5a through 9b; tolerant of freezing winters and warm temperate summers.
About This Tree
An upright, clump-forming herbaceous perennial growing 30 to 90 cm tall with stiff, grooved stems, hard blackish-brown flower bracts, and vibrant purple discoid flower heads resembling small thistles without spines.
Leaf Characteristics
Basal leaves are lanceolate to ovate and often shallowly lobed or entire; upper stem leaves are smaller, unlobed, linear-lanceolate, alternate, rough-textured with fine short hairs, and green.
Bark Characteristics
Does not form true bark; stems are green to reddish-brown, tough, fibrous, grooved, and slightly hairy.
Growing Information
Origin Region
Native to Europe, including the British Isles; naturalized in parts of North America
Growing Season
Spring through autumn; flowers from June to September; dies back to basal rosette in winter; best planted/seeded in autumn or early spring
Sunlight & Water Needs
Requires Full Sun to light partial shade; low to moderate water needs; highly drought-tolerant once established in well-drained soils.
Mature Size
Height: 0.3 to 0.9 meters (1 to 3 feet); Spread: 0.3 to 0.5 meters; moderate to fast growth rate; individual plants live 3 to 8+ years.
Special Characteristics
Highly resistant to pests and diseases; excellent drought tolerance; crucial late-summer nectar source; non-spiny despite thistle-like appearance.
Wildlife & Ecology
Exceptional ecological value; rich nectar source for bumblebees, solitary bees, butterflies (such as Meadow Brown and Painted Lady), and hoverflies; seeds are eaten by goldfinches and other seed-eating birds in autumn and winter.