A very long-flowering new hybrid, Allium 'Round and Purple' will, under the right conditions, give you huge (up to 10"/25cm across) silver lilac spheres for some 10 weeks between late April and mid July. 

It's pretty tall too, towering over A. Globemaster, but not quite making it to the heights of Alliums Gladiator and altissimum Goliath.  

Propagate by dividing bulbs.