Let it stay on the plant until it develops seeds, the flower stem will yellow and die back nturally when the seeds are mature.

I have an even sillier question - since the Alliums you buy are bulbs, how long will it take for a WS Allium seed to bloom?

I collected seed from my allium this year also. I am wondering the same things as gardengirl.

Chey

SAME HERE, JUST COLLECTED SEEDS FROM MY ALLIUM BUT NOW WONDER IF THEY'LL EVER AMOUNT TO ANYTHING (lol)

I'm in the same boat as you, guys. I just want to know if it's gonna bloom for me, how long do I have to wait (years?)and if I'm just wasting my time in planting a seed instead of buying a bulb...? :)

I believe that allium take 4 or 5 years to flower from seed...the bulbs multiply readily,...I am not too sure how prolific globemaster is as I just put it in. I will tell you though that I have been allowing my seeds to mature and then putting them in the ground..the following year I had hundreds of babies (fine green onion type of growth) I started with my first alliums about eight years ago and I have at least a hundred bulbs...some of these are from bulbs.
Some of the articles I have read say that you should cut the seed head as it takes away from the bulb, but I can attest to the bulb being fine.

Globemaster has never produced seed for me. Which is sad since i just cut one in half with my shovel today :( sniff.
Star of persia seed doesn't seem to germinate for me.
I have succeeded with two smaller types from seed. Both took 2 -3 years to bloom.

Purple sensation can be done also but i read the seed grown ones aren't as deeply purple.