The flowerheads of Allium 'Purple Sensation' are not as big (only 10cm/4") or densely packed as Alliums such as A. Globemaster or A. Ambassador, but this gives them an airy translucent quality. Make the most of it by planting them where they'll catch the setting sun and watch their floating purple bubbles glow. 

They will self seed but the resulting plants will be paler.