HARDINESS: Zones 3 to 7
PREFERRED SOIL pH: Near neutral
PREFERRED SOIL TYPE: Average
PREFERRED LIGHT Sun to partial shade
ATTRIBUTES: Long-flowering, fringed blue or rose pink flowers; for beds, cutting
SEASON OF INTEREST: Summer through fall
FAVORITES: C. dealbata 'Steen-bergii', 'John Coutts' for pink blossoms; C. montana for blue flowers
QUIRKS: Cutting blossoms boosts flower production
GOOD NEIGHBORS: Artemisia, coreopsis, ornamental grasses, rudbeckia, yarrow
WHERE IT GROWS BEST: Sun
POTENTIAL PROBLEMS: Some species can be invasive in cool climates; wet soil in winter can lead to root rot
RENEWING PLANTS: Lives many years; dig and divide clumps
CRITTER RESISTANCE: Excellent
SOURCE: Bedding plants, division
DIMENSIONS: 18-28 in (45.7-71.1 cm) tall and plants are equally wide
Centaurea in the Landscape
They may have humble origins as European wildflowers, but centaureas do deserve a place in any flower or cutting garden. The bushy, somewhat spreading plants produce thistlelike blossoms for weeks in midsummer, often continuing into fall if flowers are cut regularly. The best ones for the garden produce rosy purple or deep blue flowers chat pair beautifully with yellow-flowering perennials, such as coreopsis, rudbeckia, and yarrow.
Centaureas other common name, knapweed, gives a clue to the one risk involved in growing it. Popular centaureas reseed with enthusiasm if the old flowers are not snipped off before they go to seed.This is a welcome task, however, because the flowers are such pretty additions to bouquets.
A Centaurea Sampler
You may never tire of Persian cornflower (Centaurea dealbata), whose fringed blossoms rise on 2 ft (0.6 m) stems above woolly, gray-green foliage. Cut them soon after the buds open, and they'll keep in a vase for a week or more. The cultivar 'Steen-bergii' features a soft white center in each blossom, which gives it a delicate look. 'John Coutts' has a tight fragrance in addition to 2-3 in (5.1-7.6 cm) wide rose pink flowers atop 18 in (45.7 cm) tall stems. Except for regular flower cutting or deadheading, no special care is needed to keep this centaurea in flower all summer. In nurseries or catalogs, 'John Coutts' is sometimes listed as C. hypoleuca.
A blue-flowering centaurea, mountain bluet (C. montana) is an energetic spreader in cool gardens and should be considered invasive. However, hot summers suppress its growth, so it is definitely worthy of garden space in warmer regions.
Early in spring, centaureas may be purchased in containers from garden centers or as bare-root specimens from mail-order catalogs. Plant them in well-worked garden soil about 2 weeks before your last frost date. Set them 24 in (61 cm) apart and spread a 3 in (7.6 cm) thick layer of organic mulch around each plant to suppress weeds. Once flowering begins, gather cut blossoms for bouquets every few days or deadhead plants weekly to keep the new flower buds coming.
Centaureas grow so vigorously that their crowns become crowded in two or three seasons. In early fall or early spring, dig up plants, cut away and dispose of the old crowns, and reset divisions at the same depth at which they previously grew.
Mountain bluet spreads by underground roots, which hide beneath the foliage in summer, then emerge in the spring. To control its spread in cool-climate gardens, pull out about half of the outer stems in late summer. In warm regions, dig and divide plants every other year in the fall to help them maintain their natural vigor.
These virtually pest-free plants are drought resistant, but it's best to site them in well-drained soil, because wet soil in winter leads to root rot.