How to Plant and Grow Centaurea

Prized for its blue petals, the Centaurea genus includes annuals and perennials. Bachelor’s button or cornflower is the most widely known centaurea but it’s not the only one worth planting. A charming perennial centaurea is mountain bluet (Centaurea montana).

These fast-growing, vivacious late-spring and early-summer bloomers have the casual growth habit of wildflowers. Being prolific nectar producers, they are great for attracting butterflies. With their erect, unbranched stems, the thistle-like blooms also make good cut flowers.

The genus also includes the highly invasive spotted knapweed (Centaurea stoebe), sometimes called panicled knapweed, a species native to eastern Europe that has escaped cultivation in North America.

Where to Plant Centaurea

Plant centaurea in full or partial sun in any average, slightly alkaline soil. 

Centaurea looks best when planted in groups or drifts, either in mixed color combinations or single color blocks. A good place for centaurea is in an informal or wildflower garden or a cut flower garden. For an attractive red and blue display, interplant it with red poppies.

Later in the summer, after the bloom, the foliage becomes somewhat unsightly. This can be disguised by filling the space around the plants with other annuals and or perennials.

How and When to Plant Centaurea

Both annual and perennial centaurea varieties are commonly started from seeds. 

Plant them in the spring, either directly in the garden when temperatures are consistently above 65 degrees F, or start the seeds indoors about one month before the average last frost in your area. If your winters are mild, you can already plant annual centaurea in midfall for early spring bloom. The plants will start to grow before the first fall frost, then die back over the winter and restart growth in the spring. 

Cover the seeds with a ½-inch layer of soil and always keep the growth medium evenly moist. Annual centaura germinate in as little as 7 to 10 days while perennial varieties take 15 to 30 days to germinate.

Transplant seedlings into the garden when they are 4 to 6 inches tall. Space annual centaurea 12 inches apart. Thin direct-sown seedlings as needed. This close spacing allows the plants to support each other.

Perennial centaurea need more space to spread, about 2 to 3 feet between plants. Taller varieties, including mountain bluet, may need staking for support.

To extend the short bloom period of annual centaurea, start more seeds about three weeks after the first planting, and a third planting if desired.

Centaurea Care Tips

All types of centaurea are easy-care garden ornamentals.

Light

Choose a location in full sun or partial shade. In hot climates, a location with afternoon shade is preferable.

Soil and Water

Average garden soil is all that centaurea needs to thrive. Both annual and perennial species tolerate a wide range of soils, from quick-draining sandy soil to heavy clay. While the plant grows in mildly acidic and neutral soil, a pH of 7.0 or slightly above is ideal.

Because it is drought-tolerant, centaurea rarely needs supplemental watering after it establishes a strong root system. Too much moisture weakens the plant's stem and causes it to become floppy.

Temperature and Humidity

Centaurea does best in a temperate climate. It struggles in areas with hot, humid summers.

Fertilizer

Like most wildflowers, the plant has low fertilizer needs. A one-time application of a balanced granular slow-release fertilizer in the spring or at transplanting time is usually sufficient to sustain the plant during its active growth period.

Pruning

Deadheading spent blooms spurs plants to produce new flowers. This also controls the self-seeding of annual varieties.

Whether annual or perennial, centaurea often takes on a bedraggled look in midsummer. When the foliage wilts and turns light green and yellow, rejuvenate the plants by cutting them back by one-third or half. If moderate-to-cool weather prevails, expect centaurea to send up fresh foliage and flower stalks.

Potting and Repotting Centaurea

Centaurea does well in containers. Make sure the container has drainage holes and fill it with a lightweight, well-draining potting mix. Although Centaurea is drought-resistant in the landscape, potted plants need more frequent watering.

Despite its winter hardiness, potted perennial centaurea needs winter protection because growing it in a pot exposes its roots to frost and freeze-and-thaw cycles. Leave the potted plants outdoors during the winter but sink the pot in the ground to insulate the roots or place the pot in a second, larger pot to create a planting silo. 

Annual centaurea does not require repotting. Perennial varieties should be repotted to a larger pot with fresh soil or divided when the roots fill the container.

Pests and Problems

Aphids are a common pest found on centaurea. A simple yet effective way of control is washing the plants with a strong spray of water from a garden hose.

Especially in wet weather, the plant is prone to rust and powdery mildew. Your first line of defense against these fungal diseases is ensuring good air circulation. Removing infected leaves and stems promptly helps contain the spread.

How to Propagate Centaurea

In addition to propagation from seed, which is described under How and When to Plant Centaurea above, perennial centaurea can also be divided every two to three years. Division is also a good way to control its rapid spread. In the spring, dig up the entire clump, divide it into smaller sections, and replant them at the same depth as the original plant.

Types of Centaurea

'Amethyst in Snow' Mountain Bluet

Centaurea montana 'Amethyst in Snow' is a cultivar that combines the best of the white and blue forms of the species. A central purple head is set off by pure-white ray flowers. Zone 3-9

White Mountain Bluet

Centaurea montana 'Alba', also known as white perennial cornflower, is similar to the species, but with white flowers instead of blue ones. Zone 3-9

Giant Knapweed

Centaurea macrocephala, also known as Armenian basket flower, is a tall species in the genus. It grows 4 to 5 feet tall with bright yellow, thistlelike flowers in midsummer. The species is native to the Caucasus, Iran, and Turkey. Zone 3-8

Singleflower Knapweed

Centaurea uniflora is an alpine species native to the mountains of central and southeast Europe. It bears solitary purplish-pink flowers on a mound of prickly green leaves 15 to 20 inches tall. Zone 4-8

Perennial Bachelor's Button

Centaurea pulcherrima forms a low mound of deeply toothed gray-green leaves, which in early summer send up shaggy pink cornflower blooms. It is native to the Caucasus Mountains and tolerates hot, dry conditions. Zone 4-9

American Basketflower

Centaurea americana is an annual species native to the southwestern United States. Its fragrant, feathery, mauve flowers measure 4 inches across. With its strong stems and huge flowers, it makes an excellent cut flower.

Centaurea Companion Plants

Poppy

Oriental poppies have huge, exploding flowers of brilliant reds, pinks, white, oranges, and plum, some with double flowers in summer. Most are blotched with black at the base and centered with a boss of black stamens. They give their all at bloom time and the foliage dies back and looks ragged, so plan to fill the newly available space with annuals, dahlias, baby's breath, or other later-blooming plants. Zone 2-8

Russian Sage

With its tall wispy wands of lavender or blue flowers and silvery foliage, Russian sage is an important player in summer and fall gardens. It shows off well against most flowers and provides an elegant look to flower borders. The aromatic leaves are oblong, deeply cut along the edges. Foot-long panicles of flowers bloom for many weeks. Excellent drainage and full sun are ideal, although very light shade is tolerated. Plant close to avoid staking since the tall plants tend to flop. Zone 4-9

Butterfly Weed

Brightly colored butterfly weed is a butterfly magnet, attracting many kinds of butterflies to its colorful blooms. Monarch butterfly larvae feed on its leaves but seldom harm this native plant. It is slow to emerge in the spring, so mark its location to avoid accidental digging before new growth starts. If you don't want it to spread, deadhead faded blooms before seedpods mature. It is sometimes called milkweed because it produces a milky sap when cut. Zone 3-11

Garden Plans for Centaurea

Easy Slope Garden Plan

The mix of easy annuals and tough perennials in this garden plan will beautify any slope. Shrubby Nanking cherries and cottage garden favorites such as Shasta daisies, black-eyed Susans, and lupines add height and long-lasting color to the design and don't require much maintenance. Lower-growing plants such as petunias, sedum, and candytuft fill in the spaces between them. The effect is a lush, bloom-filled display that will look gorgeous from spring into fall.