"Ogres are like onions," declared Shrek in the movie of the same name.
We don't know much about ogres, but onions are sweethearts in the garden. They take up little space--so you surely have a spot or two where you can plant them--and demand very little attention from you. Choose varieties that keep well in storage, put them in conditions that meet their needs, and you are on the way to a happy ending: a bin full of onions you can use in soups, stews, sandwiches, and salads long after the rest of your garden crops are a fading memory.
Though there are hundreds of onion varieties, they all can be organized into three distinct groups. "Long-day" onions need more than 15 hours of light each day during the peak of their growing cycle to form bulbs. So they grow best in the north, where summer nights are short. "Intermediate-day" onions need about 14 hours of daylight daily to trigger bulbing--they're the right choice in the country's midsection. "Short-day" onions form bulbs with just 8 to 12 hours of daily sunlight, so gardeners in the south typically rely on them.
Now, how to start your onion crop? The easiest, and most common, way is by planting sets (little bulblets) about two weeks before the average last-frost date for your area. For best results, choose sets that are 1/2 to 3/4 inch in diameter. Also easy is planting starts from your favorite nursery or mail-order supplier directly into the garden. Just remember, plants with many large leaves form larger bulbs than those with very small leaves.
Advanced gardeners often start onions from seeds. You get more varieties to choose from, so you can seek out those that grow well in your region, keep long in storage, and have the flavor or color you desire. And when you start seeds, you are gardening quite a bit sooner than you might otherwise--indoors, at least eight weeks before your last-frost date.
To start seeds, direct-sow them into flats or pots of sterile growing medium, top with a clear plastic cover, and keep the seedbed moist. A seedling heat mat can help speed germination. Once the seeds have sprouted, place the flats in a warm, sunny window or under fluorescent lights. If the seedlings begin to look leggy--that is, tall and very thin--trim them back to about 3 inches tall. You can do this repeatedly, if necessary, until they're about pencil-thick. Cutting the seedlings back in this way ensures a well-developed root system--just what onions need to grow large leaves and then large bulbs.
All successful gardening, especially organic gardening, starts with attention to the soil. Onions grow underground, so getting the right soil conditions is especially critical. The soil's pH, fertility, and structure affect the size--and even flavor--of onions. They grow best in a slightly acidic soil (pH between 6.6 and 6.8) that's well-draining and loaded with organic matter. Soils that are very rocky, composed of dense clay, or very compacted can restrict bulb formation. To improve soil that isn't hospitable to onion bulbs, mix in compost, shredded leaves, well-rotted manure, and other organic matter down to 2 feet deep.
As the bulbs form, they need room to expand. When planting, you can place the sets or transplants an inch apart and then pull immature shoots to eat as green onions until the remaining plants are 4 inches apart. Or simply place sets or plants 4 inches apart at the outset. Either way, they go in about an inch deep. Water them in well, and then make sure that they get at least an inch of water weekly, with periods of thorough drying in between.