Storage onion seeds are planted to produce onions bred to stay fresh for months on end. Onions begin to form bulbs in response to temperature, but also the length of the day. In southern Canada and the northern U.S., choose “long-day” onions. Our summer days are much longer than our winter days. “Short-day” onions are better suited to growing in the south, where the length of summer days is less pronounced. These bulb-up too rapidly in the north, whereas long-day onions may not form bulbs at all in the south.