The effect of combinations of cultivars and planting dates during summer on onions harvested during early winter in northern Japan was investigated. For the mid-August planting, which encountered moderate daylengths and temperatures during plant and bulb growth with a low rate of small (< 50 mm in diameter) and immature bulbs for very early (daylength threshold for bulbing < 11.5 h) and early (12 h) maturing cultivars, marketable bulb yield was the highest. For onions planted in early August, the rate of small bulbs was high owing to relatively long daylengths and high temperatures for bulbing, which resulted in lower yields in the very early and early maturing cultivars. In onions planted in late August, among the planting dates with high immature bulb rates in all cultivars, marketable bulb yields were the lowest because of insufficient daylengths and temperatures for bulbing and maturing, particularly in the later maturing cultivars. For middle maturing cultivars (daylength threshold for bulbing is 13 h) in the all planting dates, marketable bulb yield was low because of high rates of bulb immaturity with insufficient daylength and temperature for bulbing. Commercially high marketable yield was obtained by planting onion sets in mid-August for early winter harvesting in some very early and early maturing cultivars.