Your question made me wonder, as I had been gathering seeds from both ornamental allium and onions and leeks, so I looked it up in my Botanica's Organic Gardening. You don't say your zone, but onions can be grown from seed in warmer areas, and making 'sets' used for colder climates. It says in the first season, sow seed in spring to grow miniature onions (sets), then in the second growing plant the sets in spring.

I have noticed that some allium produce both little bulblets and seeds when they flower. Sorry I don't know more.

I'm in 7b. I was told to start my seeds now (I am growing an 80 day indeterminate and a 115 day indeterminate variety). When plants reach "set" size, pull, cure and store over winter. Then, plant out again next Spring. I'm going to try this method to see how it works. If not, I'll buy plants from dixondale farms and try again.

Hey -

I was googling on this a few days ago, and found this advice: Propagate by seed - To grow your own onion sets, sow seeds thickly in a block in midsummer. About 2 months after planting, roll down the tops, forcing the plants to form small bulbs. (Those about the size of a dime work best.) After tops dry, clip them off, leaving about ½ inch of stem. Cure and store in a cool, dry place as you would onions for eating. Plant your sets the following spring.

I am trying this method with two onion types. I planted seeds yesterday. I don't know what it means when it says "roll down the tops." ??

Here is a link that might be useful: link

Roll down the tops = bend the plants to the ground at the necks to stop the growth.