I cut mine last year so that I could use the heads in floral decoartions. I hung them upside down in a plastic bag on the pergola and forgot to put them in the shed - result lots of allium seedlings everywhere. If I had put them in a cool dry place to dry off properly I would have ended up with seed in the bottom of the bag and some lovely dried heads. I'm not sure whether the seeds will grow bulbs this year and I will get flowers next year or maybe they will flower this year.

I like to leave them on, along with everything in the herbacious border, the seeds will eventually fall and the seed head will still look nice along with, when they are touched in the winter by the frost they take on added interest, as for growing from seed i have never tried them but the old seed heads are nice in a dried flower display, julien.

Search "Alliums from Seed" and you will get a detailed posting of how to grow them from seed.

Be aware that the giant types take 4-7 years to bloom from seed!