Eryngium giganteum
What a stunner this is. Also known as Sea Holly or, my favourite, Miss Wilmott's Ghost. So called, after the 19th-century gardener, Ellen Wilmott, who liked to secretly scatter seeds of the plant in other people's gardens. The following year the plant would appear; the ghost of Miss Wilmott. It is a biennial, but self-seeds freely & usually needs thinning out.