Onions are one of the first crops you can grow in spring. Before you get started it's good to know about the onion growth cycle. Then follow our steps for growing the best onions ever.
Onions are photosensitive, forming bulbs in response to day length. The larger the plant is before the bulb formation is triggered, the larger the onion bulb will be. Onions are roughly broken into two groups; short and long day varieties. The dividing line is 36 degrees latitude, roughly a line that runs from Washington, DC, through southern Missouri and Kansas to San Francisco.
Long-day onion varieties, such as 'White Bermuda', are best grown in spring north of this line. Long-day onions grow green leaves until the long days of early summer initiate bulb formation. They are harvested soon after. Short-day onions, such as 'Texas Grano 1015Y', are best planted in the fall in warm southern and western areas south of this line that have mild winters. Short-day onions form bulbs in response to short days of early spring. They are usually harvested in early summer. Day-neutral varieties, such as 'Candy', are grown in spring in either the North or South.
Onions are usually started from seeds or bulbs (sets). To produce the largest bulbs, sow seeds indoors in flats 2 to 3 months before your average last frost date. Or sow directly outdoors as soon as the soil can be worked in early spring. Plant sets anytime from early spring to early June. You can also buy started plants.
Choose a fertile, well-drained, weed-free area of your garden to plant onions, especially if you are planting from seed. In the fall before planting, work manure or compost into the soil, or fertilize right before planting, using a balanced fertilizer, such as 10-10-10, applied at a rate of 1 pound per 20 square feet. Sow seeds indoors in flats 1/4 inch deep, 4 seeds per inch, in narrow rows. Keep the onion tops clipped to 3 inches tall and plants growing under grow lights.
Transplant plants started from seed when danger of heavy frost is past. Set transplants 4 inches apart. To direct seed, sow seeds 1/4 to 1/2 inch deep, 1 to 3 seeds per inch. To plant sets, push each bulb into the soil almost to its full depth, 4 to 5 inches apart, pointed end up.
Thin direct-seeded onions to stand 4 inches apart for bulb onions. Control weeds with frequent shallow cultivation. Provide at least 1 inch of water each week. Sidedress with 1 pound of 10-10-10 or equivalent fertilizer per 20 to 25 feet of row when plants are 4 to 6 inches tall and the bulbs are just beginning to swell. Harvest when one-half of the onion tops naturally fall over.
Lighting for Germinating Seeds
Q. Some seeds in my seed-starting trays have germinated and some have not. Do I put the whole tray under the light source now or do I wait until all the seeds have sprouted?
A. A rule of thumb is to set seed flats under lights when one-third to one-half of the seeds have germinated. Generally, all the same kind of seeds will germinate within a day or two of each other, assuming they're planted at about the same depth. If you wait too long to place them under lights, the early ones will get leggy.
If you planted many different types of seeds in the same tray, some will probably take quite a bit longer than others to germinate. Some people cut the large trays into "six-packs" so they can plant different seeds in each six-pack. That way, they can put the germinated seeds under lights and keep the slower-germinating seeds in a warm place to hasten germination.