I planted Allium sphaerocephalon in a sunny border on the allotment a few years ago and they flower profusely in July. They propagate themselves profusely too.

 The bulbs themselves increase rapidly in number...

... each bulb begats more bulbs in the form of offsets.

 Resulting in numerous bulbs that can be gathered from the soil for re-planting.

Moreover each flower head produces bulbils which drop and grow into yet more bulbs.