There are many commonly grown garden plants that belong to the Centaurea genus, these include but are not limited to cornflower, Bachelor's buttons, Basket flower, Knapweed, Ragged robin, and Sweet sultan.
The variety means that the genus contains plants that are hardy annuals, biennials and perennials.
This means that the flowering season is also very variable, though annuals tend to flower from late in spring to late in the summer, whereas perennial members of Centaurea tend to flower in the spring or summer.
They tend to have deeply coloured petals of varying colours and vary in height from 30 to 90 cm, they make ideal plants for growing in simulated wild gardens.
Centaurea montana by Alexandre Dulaunoy.
Centaurea macrocephala by Matt Lavin.
Common Names: Knapweed, Starthistle, Loggerheads, Bluets, Sweet Sultan, Basket Flower, Cornflower, Ragged robin, Bachelor's buttons.
Life Cycle: Hardy annual. Hardy biennial. Hardy perennial.
Height: 12 to 36 inches (30 to 90 cm).
Growing Region: Annuals: zones 1 to 10; perennials: Zones 3 to 10.
Flowers: Spring and/or summer.
Flower Details: Blue, red, pink, yellow, white. Pompom-like. Fine petals.
Foliage: Weedy. Elongated lobes, often spiny.
Sow Outdoors: Cover seeds. Use successive sowing for continued blooms. Before last frost or autumn. Spacing 6 to 12 inches (15 to 30 cm).
Sow Indoors: Germination time: one to five weeks. Darkness. Temperature 60 to 70°F (15 to 21°C). Seven or eight weeks in advance. Transplant annuals outdoors at the end of spring. Perennials and biennials can be planted out in either late spring or early autumn.
Requirements: Full sunlight. Good drainage. Soil pH 5.5 to 7. Ordinary soil. Feed in spring. Regular watering. Deadhead. Stake taller species. Cut back perennials following blooming. Divide perennials in spring in cool areas or autumn in warmer areas.
Miscellaneous: These plants are attractive to insects, therefore they can be grown as an alternative source of food for insects when growing crops.
If planning to grow cornflower-like species outdoors then the seeds should be sown in sunny areas of the garden that have soil with good drainage, ideally the pH should be from 5.5 to 7.
It is best to sow the Centaurea seeds successively from just before the last frost of spring, or in the autumn. If first growing indoors then they normally take about one week to one month to germinate and should be started off about 6 or 7 weeks before you plan to put them out into the garden, in the late spring.
Centaurea require regular watering and ideally should be fertilized in the spring. If you require more seeds then Centaurea annuals will self seed and you can divide perennial varieties in the spring.
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