Onion is a cool-weather vegetable that does well when planted two weeks before the last frost date in areas with short growing seasons. Long-day onions do well in areas where sunlight lasts 15 to 16 hours per day. Plant short-day onions in the early fall for southern areas with long growing seasons. Short-day onions mature with only 12 hours of daily sunlight. Northern areas have long days for growing and the southern areas grow gardens in the winter with shorter days.
Till the garden soil to loosen the dirt and break up any clumps formed over winter. Remove rocks and other foreign debris from the garden bed. Add manure or compost and mix well with the soil. Rake the soil smooth to create a level bed for the onion bulbs.
Form a trench with the garden hoe 1 to 2 inches deep. The length of the trench is determined by how many onions you want to grow. Onions grown for storage need at least 4 inches of space to grow to maturity. Green onions only need 2 inches of space between bulbs.
Set the onion bulbs in the trench, roots down, with the tops pointing up. Push the onion sets firmly into the soil when planting so the roots make good contact with the soil.
Cover the onion bulbs with soil, leaving no part of the bulb exposed. Water the onion bulbs so the soil is moist but not saturated. Water twice a week until the bulbs are established. After that, normal spring rainfall should provide enough moisture for the onions.
Weed the onion bed on a regular basis to keep the weeds from leaching the nutrients from the soil. Water at least once a week during the summer months for large, healthy bulbs to develop.
Harvest the onions in the fall after the stalks have turned brown and died back. Pull the onions from the ground and remove the dirt. Allow the onions to harden off by drying for two or three days in a shaded area. Cut the dead stalks off 1 inch from the top of the onion bulb. Store in a cool, dry place until ready for use.