Onions grow best in loose sandy soil, or soil that has been lightened by adding lots of compost. They thrive in a raised bed, as the good drainage and rich soil suits them. Full sized onions need to be spaced 3-4 inches apart, but you can start out by planting them more densely. Just thin out the smaller plants as you want to eat them, leaving the others to reach full size. Onions need to be kept weeded. Since their roots are shallow they need to get about 1” of water each week.
Onions can be started as sets or as plants. Sets are miniature onions, produced last season in crowded conditions to keep their size small. Sets are sold by the pound in early spring at local garden centers. They will usually produce relatively good mature onions, but the size and keeping quality of your harvest will be much better if you plant seedlings rather than sets.
Onion seedlings may be purchased through the mail, or you can grow them yourself. Sow the seed early indoors, 10-12 weeks before the last frost date. The seed can be sown quite thickly in a small pot or seed flat (up to 50 seeds in a 4” x 6” flat). As the plants grow, use scissors to cut back the grassy growth and keep the plants at a stocky, 3” height. Tease the plants apart carefully when it’s time to plant them into the garden.
The development of an onion bulb is dependent on day length. Southern gardeners grow different varieties of onions (short day) than northern gardeners (long day). Buy plants or seeds locally, or choose carefully from the seeds catalogs. There are also a few day-neutral onions that will grow well just about everywhere.
Some onions store well, others are will not keep and are best for eating fresh. These characteristics are generally well specified in catalogs or on seed packets. The pungency of an onion depends on how much sulfur it picks up from the soil. Sweet onions like the Vidalia or Walla-Walla are less sweet when grown in New England where the soil contains more sulfur than it does in the South or far West. The Vidalia is also a short-day onion and will not form a big bulb when it’s grown north of the Mason-Dixon line.
Scallions or bunching onions, are just young onions that have not formed bulbs. You can sow them much closer together – as little as an inch apart.
Onions are mature and ready to harvest when their tops flop over. Let the whole plant dry slowly in a well-ventilated, dark place for several weeks. Once the stems have withered and the onions are completely dry, they can be moved indoors to a cool, dark location for storage. Some onion varieties may be stored for up to 8 months, so with careful planning (and enough garden space) you can be eating home-grown onions all year round.