evening primrose 'Crown Imperial'

'Crown Imperial' is an upright, clump-forming herbaceous perennial 40-50cm high with broad basal leaves and narrow, glossy, bright green stem leaves, green stems topped by clusters of flowers so that the plant looks like a small crown imperial fritillary, and flowers opening from red buds into large, bowl-shaped, vibrant yellow flowers from midsummer; the flowers are sterile so it does not self-seed