Planting onions that develop into good, large onion bulbs is a bit tricky. The most important thing to take into account when growing onions is to make sure that you have chosen a variety that is appropriate for how far north or south you are. Onions are very sensitive to day length, and if you plant the wrong type for your region, the onion will not make a bulb but bolt and go to seed instead.
have a “trigger point” when they switch from trying to grow lots of green tops
to developing the bulb onion at the base. The more green tops they put on in
the spring, the larger the bulb will eventually be.
Plant onions from sets or transplants about 4-6 weeks before the last spring frost, to give the plants enough time to develop as much top growth as they can before the days start to get long and the plants reach their day-length trigger point.
Planting onions can be done from seed, sets, or transplants. Each has its advantages and disadvantages.
Growing onions from seed gives you the largest choice of onion varieties for your region. If starting from seed you should start them in seed flats indoors about 10 weeks before your last spring frost and plant them out about 4-6 weeks before last frost. Plant seeds about 1/2 " deep and 4" apart.
Onion sets are baby onions that were started the previous year. These are a good choice for short-season areas because they are already about 1/2" in diameter, and give you a head-start on the season. Onion sets are more prone to bolting, however, because they are already in their second season of growth.
Onion transplants and like onion sets, only the baby onions were started earlier this year. They offer the same head-start advantage, but are less likely to bolt than onion sets.
Read more about growing onions:
How to Grow Onions
Onions need soil that is loose and evenly moist so that they can easily push it aside when they start to form bulbs. This is the main how-to article that describes the best soil conditions, how day length affects bulb growth, fertilizing correctly, and how proper watering can prevent disease.