allotment blue cornflower Cornflowers flower flowers horticulture border of field. border cornockle environment field meadow natural organic papaver-rhoeas petal plant poppy poppy field cornflower field red violett spring summer wildflower blue flowers garden flowers nature purple flowers batchelors button bloom garden bahelor button Centaurea cutout cyanus isolated cornfield green Asteraceae Centaurea cyanus cultivate England english Great Britain grow plants