Bunching onions (Allium fistulosum) have a hard time deciding what group they want to belong to. In some books they are listed (correctly) as biennials, producing flowers in their second year. Sometimes they will set flowers after a short cold period, allowing them to set seed in a single growing season (as annuals do). And in some books, they are labeled as perennials, due to their continual dividing habit.
Bunching onions grow quickly from seed sown in early in the spring (March in mild-winter areas, April in colder areas).
They will grow throughout the year, and overwinter in the garden throughout the United States. Mulching them will protect them both from overly hot summers and from cold winters. It will also help keep weeds down.
Flowers from bunching onions are pollinated by insects, and can cross-pollinate with flowers of common onions. For this reason, make sure that only one type of bunching onion or common onion is flowering in your garden at one time. Or you could use bagging & hand-pollination, or alternate day caging to prevent cross-pollination.
When the flowers begin to turn brown, bend the stalks and place the flowers in a brown paper bag before snipping the stems. Allow the flowers to finish drying on a tarp in a well-ventilated area. Many of the seeds will fall out. Threshing will remove the rest. Winnowing with a fan on the lowest setting is effective.