ON GROWING ONIONS:
Onions fit into three categories: short-day, intermediate-day and long-day varieties. Gardeners in plant hardiness Zone 7 and south will succeed best with short-day onion varieties. Zones 5 & 6 will probably be able to grow these varieties, too, if onions are planted in late winter instead of in the fall. Intermediate-day varieties will work for most gardeners in Zones 5 & 6 while long-day varieties will succeed in Zone 6 and colder zones. There is obviously some overlap with varieties that require shorter days being adapted to climates where much longer days occur. The key factor here is whether these varieties have sufficient time to make enough growth before lengthening days signal the bulbing response, and of course, the days have to get long enough to signal the response.
Gardeners from Zone 6 north will do well with the red, white and yellow onion sets and 'Walla Walla Sweet' onion seeds while everyone can grow the 'Evergreen Long White Bunching' onions, 'Early Italian' and 'Extra Select' garlics and 'Dawn Giant' leeks. Shallot sets will work in most areas of the country, but in the lower South look for 'Louisiana Multiplying' shallots. These white clump-forming shallots are readily available at the local feed store in most areas of the South in the fall. Gardeners in Zones 8 and 9 should look for short-day bulbing onions like 'Grano', 'Granex' and the various hybrids, 'Texas SuperSweet' and 'Burgundy'. These onion varieties are best planted from seed in the fall or transplants can be set out in January. Onion and garlic chives also thrive in the lower half of the country during the fall and winter and in most areas during the summer. Onion chives will fizzle out in midsummer in the lower South, but often come back as temperatures cool off in the fall. Garlic chives are almost a weed.
Onions, like most vegetables, thrive in a rich organic soil. Prior to planting, work 2 to 4 inches of compost or well-rotted manure into the soil plus 1 to 2 pounds of a complete fertilizer per 100 square feet. Try to be careful about the fertilizers you use for onions. Onions tend to be more pungent when grown on soils with a high sulfur content. Choose a nitrate-based fertilizer over a sulfate-based fertilizer if you can make that determination. It's difficult to do this with blended fertilizers, but ammonium nitrate would be better to use than ammonium sulfate for side dressing. The fertilizer bag, even on complete fertilizers, 12-24-12 for example, may indicate the components used to formulate the fertilizer. Onions also have a limited root system so, while you don't want to get fertilizer on the plants when you side-dress, you don't want it in the middle of the row either.
Try to space onion seeds 1/2 inch apart and cover them with 1 inch of soil mix. Later thin onions to stand 4 to 6 inches apart. If you're planting onion sets or setting out plants start at this spacing. For scallions (green onions), leeks, garlic sets (cloves) or bunching onion transplants, space them 2 inches apart. Onions need lots of moisture as the bulb initiation process begins, but less when the bulbs are approaching full size. When the tops begin to fall over naturally, it's time to pull the onions up.
Bunching onions are harvested as scallions. Plant seeds in the spring and again in the fall.
ONION GROWING TIPS
The common practice is to break or crush the onion stems if there are signs of flower heads. When the stems are dry, dig the bulbs, which can be left on top of the ground to cure and dry for several days.
Setting out onion plants that are too large, planting too early or using the wrong varieties usually causes onions to bolt or form undersize bulbs. -Birdy