The potato onion is a variety of multiplier onion, similar to the shallot, but which produces larger bulbs. It is remarkably easy to grow, keeps better than almost any other variety of onion, and is ideal for the home gardner with restricted space. It was very popular in the past, but like many old varieties, it has been passed over in favor of types more suitable for mechanical harvesting and mass marketing.
Information available on the internet for this onion tends to be confusing and contradictory. It is planted from bulbs, seldom, if ever, from seed. Most sources say it should be planted in the fall, but this probably applies only to areas with moderate climates. It can be planted in the spring as early as the ground can be worked and producess well when so planted, up to at least the most northern limits of planting zone 5.
Sources differ about planting depth, some saying shallow planting is appropriate and others calling for deeper planting. This onion does tend to grow very close to the surface and a planting hole perhaps an inch deeper than the diameter of the bulb seems to work well. The onions vary in size from half an inch to three inches in diameter. Reports state that a large bulb will produce several smaller bulbs when planted, while small bulbs will grow into one large bulb. Personal experience does not support this assertion. Relatively small bulbs, 1/4 inch to 1/2 inch diameter, planted in the spring of 2007, all produced a number of bulbs ranging in size from 1 inch in diameter down to 1/4 inch. Bulbs were planted relatively late in the season, but no small bulb produced a single larger bulb. In 2008, bulbs were planted as early as the ground could be worked (Connecticut, zone 5) and harvest results will be posted later in the year.
Onions can be pulled for green onions or scallions in the spring. Onions left to grow to maturity should be dug and dried after the tops have died (usually in July/August)