Always check a cultivar's daylength requirement or recommended latitudes before you buy, because daylength affects how and when onions form bulbs. Short-day onions, such as 'Red Hamburger', will form bulbs as soon as days reach 10 to 12 hours long. They're suitable for southern latitudes only. Long-day types, like 'Sweet Sandwich' and 'Southport Red Globe', need 13 to 16 hours of summer daylight in order to form bulbs. They're the type to grow in more northern latitudes.
Onions like cool weather in the early part of their growth, so plant them in spring, except in mild-winter areas, where onions are grown as a fall or winter crop. Generally speaking, onions grow tops in cool weather and form bulbs when the weather warms.
Plant onion seeds 4 to 6 weeks before the last average frost—or even earlier indoors or in a cold frame. When indoor seedlings are 2 to 3 inches tall, harden them off by exposing them to above-freezing night temperatures.
Outdoors, sow seeds thickly in rows about 1/2 inch deep. You can try mixing in radish seeds both to mark the planted rows and as a trap crop to lure root maggots away from the onions. Thin seedlings to 1 inch apart, and thin again in four weeks to 6 inches apart. For transplants or sets, use a dibble to make planting holes 2 inches deep and 4 to 6 inches apart. Use the closer spacing if you plan to harvest some young plants as green onions. For sets, open a furrow 2 inches deep and place the sets stem (pointed) end up 4 to 6 inches apart, and then fill in the furrow. One pound of sets will plant about 50 feet of row.
The practices you use will depend on the specific crop you're growing. In general, onions grow best if you keep them well weeded. Use a sharp hoe to cut off intruders; pulling or digging weeds up can damage the onions' shallow roots. Once the soil has warmed, put down a mulch around and between the plants to discourage weeds and to hold moisture in the soil.
Dry conditions cause bulbs to split, so water when necessary to provide at least 1 inch of water each week; keep in mind that transplants require more water than sets do. Onions can't compete well with weeds, so it's important to direct water right to the onion roots. Two good watering methods for achieving this are shown below.