I cast some cornflower seed (Centaurea cyanus) in early August and I am now the proud owner of 20 young plants, each in a small pot.
The normal procedure is to transplant them to a garden border in October, where they will overwinter then grow and flower in June next year. However, and to my deep dismay, I can now see flower buds forming on several of these plants. The cornflower functions as an annual in Northern Europe. I therefore assume that if it flowers before winter it will die. I will then be deprived next spring of, to quote John Clare
the blue cornbottles crowding their splendid colours in large sheets over the land and troubling the cornfields with destroying beauty.
but substitute ‘20 sq. feet of garden’ for ‘cornfields’.
I could sow more seed in spring for a summer flowering and I will certainly delay any future autumn sowings until autumn. That said, and to look at the current situation, if I stop these plants from flowering now, will they live through the winter and trouble my garden with destroying beauty next June?
I don’t have a huge amount of experience with cornflowers/Bachelor’s button’s, but I’d say stopping them from flowering won’t make much of a difference whether they come back from the roots in the spring or not. Let them flower, go to seed and sprinkle the seeds in situ, or save the seeds for spring.
In a mild climate, late-sown “annuals” quite often rest and come back the next year. But if by Northern Europe you mean hard freezes, I really can’t say.