Status: We have settled into a pattern of producing potato onion seed in even numbered years (so we will have some late in 2016). Onion bulbs are usually available from August until the end of February.
Potato onions (Allium cepa var. aggregatum) and walking onions (Allium x proliferum) are vegetatively propagated types, similar to shallots. A single potato onion bulb planted in fall or spring will produce a cluster of, typically, 4 to 8 onions. The bulb or bulbil of a walking onion will grow into a normal looking onion plant with a smallish bulb, but instead of flowering, it will produce many bulbils which can be planted directly. These perennial onions are hardy to about USDA zone 4 and can be left in the ground year round. You can dig potato onion bulbs any time of year (although they obviously vary in size, depending on the stage of growth).