There are many cultigens of top onion, which are thought to be hybrids of Allium cepa, bulb onion, and Allium fistulosum, bunching onion, or their wild ancestors, A. vavilovii and altaicum. According to Dr. Peter Hanelt, editor of Mansfeld's Encyclopedia of Agricultural and Horticultural Crops, it is unlikely to have occurred in nature, because the ranges of the parents do not overlap. The color of bulbs and bulbils ranges from white to yellow, red, and purple. Deeper colors are associated with increased pungency. Most cultivars have a tough outer layer, which must be peeled to render bulbs and bulbils edible. One white cultivar (McCullar's) does not need peeling and is treasured by chefs in the Chicago area.

Bulbils may be planted deep, 6 inches or so, and will produce blanched stalks resembling parent bunching onion.

Plants can become invasive because the stalks heavy with bulbils will fall over and then bulbils will germinate and take root. This growth habit gives the name "walking onion."