As the latitude increases toward the North Pole, the summer days become longer. Cool weather at the start of the growing season encourages heavy leaf growth, and this helps produce larger bulbs. Actually, onions are sensitive to the dark period of the night rather than to the length of the day. Bulbing is not determined by day length alone but also by the interaction of day length, temperature, and light intensity. The rate of bulbing is more rapid with high light intensity and increased temperature.
Seeds are harder to establish then transplants.
The time between planting seed and harvesting is too short to make large bulbs in several areas of the country. Generally, a transplant with four or five leaves can be planted at the same time as seed, so you get a head start on making large bulbs. An onion set is a small bulb, stored from the previous year's onion crop. A transplant is a plant between 10 and 20 weeks old that has not been through the bulbing process. An onion is a biannual, which means it has two lives.