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The size of a mature onion bulb is determined by the size of the tops. For each leaf, there will be a ‘ring’ of onion. The larger the leaf, the larger the ring and ultimately, the larger the bulb. To get those large bulbs, other factors come into play.
Onion varieties are described as ‘short-day’, ‘long-day’ and ‘intermediate-day’ onions. To better understand these terms, you should know that onion-bulb formation is triggered by day length. Although contrary to what we might assume, the number of hours of daylight per day in northern states in the summer is actually longer than the number of hours of daylight per day in the South.
Long-day onions need 13 to 16 hours of daylight per day to trigger bulb formation, while short-day onions only require about 12 hours. Therefore, if you planted short-day onions in the North, bulbing would commence early and your resulting onion bulbs would be small. And if long-day onions were planted in the south, you may end up with only tops. The 35th parallel of latitude is the approximate dividing line between long-day and short-day onion growing.
So in a nutshell, plant long-day varieties in the North and short-day onion varieties in the South. As for the intermediate-day varieties, you can safely plant them almost anywhere, and they are especially good for those areas that fall in the ‘in-between’ area of long and short day varieties.
Onions need full sun and soil that is loose, fertile and well-drained. The planting area should be as weed-free as possible. It’s helpful to know if your soil is alkaline (pH above 7.0) or acid (pH below 7.0). Onions prefer a slightly acid soil pH between 6.2 and 6.8. If your soil is too acid, mix in ground limestone. If it’s too alkaline, add peat moss. Raised beds are excellent options for growing onions.
Onions can be planted early, about 4 to 6 weeks before the last estimated spring freeze. The soil should be at least 40 degrees F. Cool weather promotes healthy foliage growth.
Onions need fertilizer right from the start. A fertilizer with a high middle number is best, such as 10-20-10. Dig a trench 4” deep and 4” wide. Sprinkle ½ cup of fertilizer per 10 feet of row and cover with 2” of soil. Plant your onions about 6 inches from either side of the trench – not in the trench – about 1” deep and no deeper. Deeper planting will inhibit their ability to bulb.
Space plants 3 to 4” apart for growing full-size onions or if you prefer, space 2” apart and harvest every other plant during the growing season for scallions. Space rows 18 to 24 inches apart. The spacing from one fertilizer trench to the next should be 36”. Water immediately after planting.
Keep onion plants watered. Onions have shallow roots, so need a steady supply of moisture. At the same time, don’t overwater. The leaves will become yellowish if overwatered. As bulbs are forming, the need for water increases. When the onion tops start falling over, stop watering and let the soil dry out before harvesting.
Fertilize every 2 to 3 weeks after planting, sprinkling fertilizer on top of the original fertilizer trench and watering after application. Stop fertilizing when the onions start to bulb.
Keep onions weeded. Use careful, shallow cultivation to avoid damaging roots, pulling by hand any weeds growing within an inch or so of the onions. Mulching with a light layer of straw or grass clippings will keep weeds down and hold in moisture, but keep the mulch a couple inches away from the onions. A layer of Planter’s Paper between rows also works well.
Ideally, onions will have about 13 leaves when the tops begin to dry down, a good sign that the bulbs will be large. When the tops turn yellow and fall over, it’s time to harvest your onions !