In this red onion cross section, you can clearly see the round tubular leaves that develop inside of each other. There are two flowering stalks in the middle of this bulb. This onion has two flowering stalks because one of its axillary buds has developed enough to form a second bulblet enclosed in a single outer tunic . These bulbs could be separated to yield two onion bulbs, each of which could be planted to produce more bulbs in subsequent growing seasons. This is a good example of asexual propagation .
Onions start bulb formation when the day length is right.
Bulbing varies with the variety of onion. In northern latitudes, long-day onions grow best. Southern latitudes grow short-day types.