Growing onions in your home garden provides fresh, firm bulbs for cooking and tender green onions for stir-fries and salads. They come in several varieties, from small round cooking onions to large slicing onions used in sandwiches. Onion varieties are classified as either "long day" or "short day," depending upon how early in the growing season they begin to form bulbs. Bulb size is determined in part by the variety you grow, but you can enhanced your onions' size with proper growing techniques.
Select onion seeds that match your preferences in onions. Onions are available in white, yellow and red varieties. Each variety has its own characteristics, size, shape and flavor.
Choose onion seeds of the appropriate day length for your region. Long day onions perform best in northern climates, while short day onions do best in southern growing zones.
Plant onion seeds inside eight to 12 weeks before setting them out in the spring. Onions started from seed and transplanted to the garden produce larger bulbs than onions started from onion sets.
Sow individual seeds 1/4 to 1/2 inch deep in trays or cells filled with moist seed starter. Keep the soil evenly moist and place the trays in a sunny location. Keep tops trimmed to 4 inches once seedlings emerge.
Prepare the soil in a sunny location that receives at least six hours of direct sunlight a day. Areas that receive full sun for the entire day enhance onion growth. Till to a depth of 8 inches. Add a 3-inch layer of compost or well-rotted manure and work in well with the existing soil.
Apply high phosphorus fertilizer (10-20-10) at a rate of 1/2 cup per 10 feet of row. Mix in well with existing soil.
Plant seedlings 1 inch deep spaced 4 inches apart in early spring as soon as the soil can be worked. Water to moisten the soil.
Side dress with high nitrogen fertilizer when seedlings are three to four weeks old. Repeat every two to three weeks until the neck of the onion begins to soften, about a month before harvest.
Water once a week or whenever the soil dries. Keeping soil moist promotes rapid growth and produces sweet onions. Onions grown in dry soil develop a strong flavor.