The nature of the onion is to grow tops in cool weather and form bulbs in warm weather. Those varieties grown in the north require 14 to 16 hours of daylight. The reason that we plant some of our varieties from plants rather than from sets is because the bulb will not get large enough unless it is already half grown and will mature into a big onion by the time we get to late summer and cooler weather of fall. Good sized transplants planted early will make a larger bulb than the normal onion set. Onions should be planted as soon as the ground is warm enough and the soil can be worked.
Onions are heavy feeders.
One should work manure and fertilizer into the soil before planting. A pound of manure to each square foot of ground and 4 to 5 lbs. A constant supply of moisture is essential. It is especially important during the bulb enlargement. Both plants and sets make excellent green onions fresh from the garden.