written by Jim Kwasnik, Master Gardener℠ volunteer
Growing edible onions (Allium capa) in North Carolina is fairly easy to do with proper soil preparation and garden care. Onions need full sun, well-drained soil with a good amount of organic matter (well-rotten compost), and pH 6.5 -6.8. The area planted should be weed free and watered as necessary during dry spells.
There basically two types of onions: (1) branching or green onions and (2) bulb onions. Bulb onions are further divided into the sweet (short-storage) onions or the cooking (long day/longer-storage) onions. Bunching onions do well under most conditions but bulb onions are dependent on day-light hours for bulb formation. If bulb onions do not have the correct light requirements they will produce only greens or very small, if any, bulbs.
Bulb onions are divided into three growing types depending on the amount of daylight needed for bulb formation: (1) short-day, 10-12 hours; (2) intermediate-day, 12-14 hours; and (3) long-day, 14-16 hours. Long-day onions are not recommended for our area. Unfortunately, the onion sets sold in big box stores are usually long-day onions and will not form properly in our area.
It is best to prepare ground in fall as average planting dates in Onslow County for live onion plants is February 10 through March 10. Later planting will produce a smaller bulb given same plant conditions. Onion varieties that grow well in our area that are found as plants includes (short-day) Yellow Granex, Granex 33, TX Early Granex 1015Y and (intermediate-day) Super Star, Candy, and Red Candy Apple. Plant the onions no deeper that 1” as planting deeper will prohibit bulb formation. In our acidic soils, fertilize every 2-3 weeks with a ½ cup of calcium nitrate (15.5-0-0) per 10 foot of row until the bulb starts to form.
When the tops die back, remove the onions from the ground, cut off any roots and allow them to cure in the sun for 2-3 days. Place the greens over the bulb to prevent sunscald. If rainy conditions are present, then move your harvest to a ventilated indoor storage area or garage. Once the tops are dry, cut off the tops and place the bulbs in mesh bags or open boxes for longer storage. Store in a cool area. Storage life is generally 30-60 days for short-day onions and 60-90 day for intermediate day onions. Check your harvest every few weeks and remove any soft onions to prevent others from rotting.