Varieties: : If you wish to grow large onions, choose an intermediate type such as Candy, Red Candy Apple and Super Star. Long-day onions will also produce large bulbs but must be kept growing during hot weather.
Sets or plants: Though onions can be grown from seed if started inside, we are too late to raise seed-grown plants this year. Therefore, we must grow them from sets or plants. Sets look like miniature, mature onions and are most often unnamed. They will produce small mature onions. Therefore, don’t use sets if the goal is large onions but rather plants of one of the three varieties mentioned above or of another intermediate-day type. However, sets are fine if you want to grow green onions (scallions) or don’t care about the size of mature onions. Any sets that are larger than a nickel in diameter will likely go to seed and should be used as scallions. Those that are smaller than a nickel should produce mature bulbs.
Fertilizing: Onions have shallow root systems and need good, even moisture and adequate fertilizer to develop large bulbs. Fertilize according to soil test and work the fertilizer into the soil before planting. If a soil test hasn’t been done, use a complete, balanced fertilizer such as a 10-10-10 at the rate of 1 pound per 100 square feet. Actually, any fertilizer with the three numbers being similar will work; just follow the directions on the bag to determine how much to use.
Onions respond well to sidedressing (fertilizing as the plants are growing) about 3 weeks after the plants have started to grow. Use a fertilizer composed primarily of nitrogen such as nitrate of soda (16-0-0). This fertilizer may be applied at the rate of 2 pounds (about 4 cups) per 100 feet of row. High nitrogen lawn fertilizers such as a 27-3-3, 30-3-4, 29-5-4 or something similar are also good choices, but the rate should be 1 pound (2 cups) per 100 feet of row. Do not use lawn fertilizers that contain weed killers or weed preventers.
Planting: Space plants 4 inches apart to provide adequate room for bulb expansion. Set plants 1 to 1.5 inches deep. Rows can be spaced 12 to 16 inches apart or whatever is convenient.
Care: Keep the onions weeded to reduce competition. Water once per week if no rain. Onions should be ready for harvest the first half of July or earlier. (Ward Upham)