Because the seeds germinate readily, you do not need to sow seed thickly. If you are sowing in spring, it s a good idea to make more than one sowing of the annuals because centaureas are not long-blooming plants. Sow two to three times at two week intervals to have flowers through summer. If you sow in fall, plan to resow at least once the following spring. Centaureas prefer slightly alkaline soil, but they are really not fussy.
Annual cornflowers perform best when they are slightly crowded.
Thin the annuals to stand anywhere from 6 to 12 inches apart, depending on the species or cultivar. Space perennials 2 to 3 feet apart. Moisten the mix and let it drain. Sow the seeds in rows in the flats. Spritz the mix with water to moisten.