If the leeks are winter hardy, corms should be planted in the fall directly after harvest.

After the seedpods are swollen and ripe and before they open, cut them keeping 6 or more inches of stem attached. They can be placed in a paper bag and the seeds should be released in the bag.

Alternately, if unsure whether the seedpods are ripe, they can be inserted into a narrow neck vase (with water in it) which is placed inside a large bowl (which is dry inside) in order to collect the seeds as they drop.

Different varieties of leeks have different colored flowers, white, purple, pink, green, etc. Not usually much difference besides the color.

Seedpods look like tiny fruit with tiny stems while bulbils look like a little tiny garlic cloves without tiny stems.

Also, if more than one variety of leek are planted close to each other, cross-pollination often occurs, so seeds may not grow true. That may not necessarily be a bad thing, though. In most plants, seeds that develop due to cross-pollination often grow better and are more adapted to the area where the cross took place.

Hope this helps.

-Nick