This easy annual plant also answers to Cornflower, Bachelor’s Button, Bluebottle, Hurtsickle, Hardheads, Knapweed, and Star Thistle. We use the genus name to keep it nice and simple. It grows well in any ordinary, well drained garden soil. Seeds will flourish within the pH range of 5.5-7.0. Deadhead regularly to prolong blooming and prevent self-sowing. Planting several times over the spring will produce a longer bloom time throughout summer and early fall. Continue reading below for some tips on how to grow Centaurea from seed.
Latin
Centaurea cyanus
Family: Asteraceae
Difficulty
Very easy
Season & Zone
Exposure: Full sun
Timing
Direct sow starting just before last frost, and again every two weeks until end of spring. Centaurea can be direct sown in autumn as well. Starting indoors is not necessary, but can be accomplished by keeping seed trays in the dark at the optimal soil temperature of 15-21°C (60-70°F). Move under bright lights once germination occurs, and transplant out in late spring, after all risk of frost has passed.
Starting
Just cover the small seeds. Thin or space to 15-30cm (6-12″) apart.
Growing
We acknowledge that Centaurea cyanus sometimes appears on lists of invasive species. In our experience, for the back yard or balcony gardener, this is not an issue, and the plants are easily controlled. They are short rooted, and can be pulled up with little effort. Unlike some plants, its seeds simply drop locally if allowed to mature, and do not eject, and they do not spread by underground rhizome. We respectfully ask that gardeners who live in threatened or sensitive ecosystems take this into consideration when planting Centaurea, just like any other type of seed.