Centaurea montana–known as perennial cornflower–bears thistle-like flowers in shades of purple, blue, white, or pink. Perennial cornflower is also called mountain knapweed, and mountain bluet. It is part of the Centaurea genus which also includes the annual cornflower called bachelor’s button, Centaurea cyanus, as well as Dusty Miller, Centaurea gymnocarpa.
Centaurea montana–perennial cornflower–is also called mountain bluet and mountain knapweed. It has wide, feathery, pale blue flowers about twice as wide as the bachelor’s button, the annual cornflower. Flowers bloom from spring to summer. They are cottage garden favorites; let them spread beside a wall or hedge. They are also a good choice for rock gardens and arid gardens.
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Get to know Centaurea — perennial cornflower
- Plant type: Perennial
- Growing Zones and range: Zones 4-8
- Height and width: 18 to 24 inches (45-61cm) tall and wide
- Foliage: Spreading plant with oval, tapering narrow leaves that are hairy beneath
- Flowers: Starburst-shaped deep blue flowers 2-inches (5cm) across
- Bloom time: Spring through mid-summer
- Uses: Meadow gardens, cottage gardens, rock gardens, mass in large beds
- Common name: Perennial cornflower, mountain bluet, mountain knapweed
- Botanical name: Centaurea montana
- Family: Asteraceae
- Origin: Europe and the Mediterranean
Where to plant Centaurea — perennial cornflower
- Plant Centaurea-perennial cornflowers in full sun.
- Grow Centaurea-perennial cornflower in average to sandy soil with excellent drainage.
When to plant Centaurea — perennial cornflower
- Start seed indoors 8 to 10 weeks before the last frost in spring. In Zones 9-11, start seed in autumn.
- Set Centaurea-perennial cornflower seedlings in the garden in the early spring of fall.
Planting and spacing Centaurea — perennial cornflower
- Indoors sow seed in flats or individual pots; sow seeds 1/8 inch deep; keep the temperature at 65° to 75°F (18°-24°C); seeds will germinate in about 7 days.
- Sow seed outdoors about 2 inches (5cm) apart and cover with 1/4 inch of soil. Keep the soil just moist.
- Set plants outdoors in their permanent place after hardening off seedlings for several days; transplant Centaurea-perennial cornflower to the garden after all danger of frost has passed.
- Space Centaurea-perennial cornflowers 18 to 24 inches apart.
How to water and feed Centaurea — perennial cornflower
- Keep the soil evenly moist.
- Fertilize cornflowers lightly with a slow-release fertilizer.
- Mulch around plants with chopped leaves or aged compost.
- Pinch off the top inch of growth to promote bushiness when plants are about 8 inches (20cm) tall.
- Pests and diseases are rarely a problem.
Centaurea — perennial cornflower care
- Mulch around Centaurea-perennial cornflowers with aged compost.
- Trim off spent blooms to prevent excessive reseeding and to promote new blooms.
- Stake tall plants to prevent them from flopping over.
- Divide Centaurea-perennial cornflowers every 3 to 4 years.
- Centaurea cornflowers are susceptible to slugs and snails.
Centaurea — perennial cornflower propagation
- Divide and reset crowded perennials in spring when new growth begins.
Other members of the centaurea genus
- Centaurea cyanus, bachelor’s button, cornflower, is a hardy annual with bright blue tufted round blossoms.
- C. dealbata, Persian cornflowers, is a perennial that grows to 2 feet tall and 3 feet wide.
- C. gymnocarpa, dusty miller, is a tender perennial grown for its divided, feltlike, gray-white leaves.
- C. moschata, Sweet sultan, is a half-hardy annual that needs warm weather; its flowers are large and silky.
Centaurea Montana — Mountain bluet
Centaurea Montana bears cobalt-blue flowers that are rounded with tagged-edged petals with a reddish-purple center. Overlapping scales beneath the flowers have a shingled appearance, enhancing the individual flowers. Blossoms first appear at the stem tips usually blooming from June into July, but will continue for months if deadhead. The lance-shaped foliage is covered with whish hairs when young but matures to bright green.
- Size: 1 ½ to 2 feet tall; spread to form large colonies.
- Light: Full sun.
- Soil and moisture: Average to sandy, infertile, well-drained soil. Moderate moisture, drought tolerant.
- Planting and propagation: Plant container-grown plants in spring, spacing 1 foot apart. Divide every three years in the fall.
- Special care: Deadhead regularly to extend blooming and prevent weedy self-seeding. Cut back stems to the ground when new growth appears at the base for late-summer and fall reblooming. May need staking to control floppiness. Spreads aggressively in fertile soil. Divide regularly to control.
- Pest and diseases: Root rot in heavy, moist soil.
- Climate: Zones 3-8; best where cool.
- Similar species and cultivars: ‘Alba’, white flowers; ‘Rosea.’ Pink flowers; ‘Violetta,’ violet flowers. C macrocephala (Armenian basket flower); golden yellow, thistlelike blossoms in early summer, 3 to 4 feet tall, Zones 3-7.
- Garden use: Best in informal or naturalistic gardens where neatness is not important. Blue flowers combine wonderfully with yellow, orange, and pink flowers.
Centaurea cineraria and chrysanthemum ptarmiciflorum — Dusty miller, silver lace
Leaves of Centaurea cineraria are bluntly lobed, while those of Chrysanthemum ptarmiciflorum are lacier. The silver-white leaves of both enhance the colorful flowers of other plants when tucked between or planted in a drift. The common name dusty miller refers to the silvery gray, felt-covered leaves.
- Size: 6 inches to 1 ½ feet tall and as wide.
- Light: Full sun.
- Soil and moisture: Average to rich, well-drained soil; moderate moisture.
- Planting and propagation: Sow uncovered indoors, at 75° to 80°F, 10 weeks before the last frost, or use nursery starts. In Zones 10-11, set out in fall. Space 1 to 1 ½ feet apart.
- Special care: Remove flower stems in bad stage; cut back to keep compact.
- Pest and diseases: Root rot is common in wet soil; aphids, downy mildew, aster yellow-virus, and rust are sometimes a problem.
- Climate: C. cineraria, half-hardy annual in Zones 2-4; perennial in Zones 5-11, but best replanted each year and treated as annual, Chrysanthemum ptarmiciflorum, annual in Zones 2-11. Both prefer warm weather.
- Cultivars and similar species: ‘Cirrus,’ less lobed than species; ‘Silverdust,’ 7 inches tall, more finely cut. Chrysanthemum ptarmiciflorum ‘Silver Lace,’ 7 inches tall, lacy.
- Garden use: Plant in groups in beds and borders to form bold contrast; use as edging or in window boxes and containers.
Centaurea cyanus — Bachelor’s button, cornflower
Bright blue cornflowers bloom midsummer in mixtures that combine blue with white, pink, and purple. The 1 ½ inch-wide flowers stand out against the gray-green stems and small, narrow leaves. Cornflowers often self-sow.
- Size: 1 to 3 feet tall; half as wide.
- Light: The best bloom occurs in full sun.
- Soil and moisture: Best in sandy loam with moderate moisture; tolerates poor soil and drought.
- Planting and propagation: Sow in the garden after the last frost, or indoors, at 60° to 76°F, four weeks earlier for summer bloom. Sow in the garden in late summer or fall for spring bloom. Space 6 inches to 1 ¼ feet apart.
- Special care: Deadhead after first flush of bloom to prolong bloom period.
- Pest and diseases: Aphids can sometimes be serious.
- Climate: Biennial grown as warm-season annual in Zones 2-8; cool-season annual in warmer parts of Zone 9 and in Zones 10-11.
- Cultivars: ‘Blue Boy,’ 2 to 3 feet tall, blue; ‘Jubilee Fem,’ 1-foot tall, deep blue, dense; ‘Polka Dot,’ 1 ¼ feet tall, white, blue, crimson.
- Garden use: Delicate plant for weaving with other flowers in borders, cottage gardens, and meadow planting; good cut flower.
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