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You will be delighted if you plant these beautiful perennials since they are both architectural and gorgeous! The delphinium cultivar Black Knight stands out for having some of the darkest indigo-blue flowers with black eyes. They begin to bloom in the early summer and frequently bloom twice. For effect around the back of the border, plant delphiniums in large clusters. A garden full of delphiniums is a wonderful sight, and this perennial is truly the pinnacle of cottage gardens. Height: 1.7m (5.5'). Spread: 75cm (29).