Alliums

Like most bulbs, I'd leave them to die down naturally so that the goodness goes back into the bulb. Also a good feed is what I give mine to strengthen them after all their hard work flowering.

You can cut the flower stem off as this is of little use to the plant after flowering, or you can leave them bobbing away in the wind and you might get seedlings.

As Doris says leave the leaves to die back naturally.

Waste of time feeding them after the leaves have gone, feed them in later Winter when they begin to grow. Once they have fiinished flowering and sometimes before, the flower stems are no longer attached to the bulb.