Sea Holly. Clump - forming, herbaceous perennial. Large steely - blue prickly flowers, which age to dark blue in summer, over silvery grey foliage, strong stems. After care. cut back dead flowers spikes to encourage repeat flowers throughout summer and remove overwintered withered foliage in spring. Hardy to -10 to -15. Full sun free draining soil. Flower late spring into summer. Height 75cm spread 50cm. Pot Size 2lt.