You can cut them down or leave them, it,s up to you, if you leave them they will produce seed, which may help them spread eventually, the stems will detatch themselves from the bulbs, you just pick them up, Derek.

Don't curt the leaves, though. Leave the to die back naturaly.

Thats confusing - as Allium leaves die down before they fully flower and after flowering you are usually only left with the stem. I leave the seed heads on as I think they look as good as the flower did.

Hmm, well some do, Dorothy, but not all.