Onions are usually started from seeds or bulbs (sets), for 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 any time from early spring to early June. You can also buy started plants.
In mild-winter areas of the South and the Pacific Coast, plant onions in the fall for winter and early spring harvests.
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.
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 (1 to 2 inches apart for scallions).
Control weeds with frequent shallow cultivation.
Provide at least 1 inch of water each week.
Side-dress 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.
To develop scallions, leeks, or bunching onions with long, white stems, keep the soil hilled up around the stems.
If a plant sends up a flower stalk, pull the onion and use it.
See our article Summer's Bad Guys by Charlie Nardozzi for controls of common onion pests such as onion maggots.
Begin harvesting scallions when they're 6 to 8 inches tall.
Harvest bulb onions when about a quarter of the tops have fallen over and the bulbs have begun to develop papery skin. Gently knock over the rest of the tops with a hoe or by hand, bending, but not snapping them. Leave the onions in the ground for another few days. Then pull the bulbs and let them dry in a warm, airy place out of direct sun or rain for a week or two.
When storage onions are dry, hang them in mesh bags, old panty hose, or as braids in a cool, dry place.