Onion bulbs grow just below the soil line or on it and bear green top leaves. The varieties of onion bulbs are numerous and include Bermuda, Spanish and Portugal. Depending on their variety, the onion's skin color might be white, yellow or red. Onions prefer a soil pH of 5.5 to 7.0. For regions where frost is expected in winter, schedule the planting for the spring as soon as the soil can be worked. They can be planted in frost-free regions during the fall for a spring harvest. The onion bulbs used for planting are called sets.
Dig a hole using a hand trowel, making it deep enough that the bulb, or set, can be placed 2 inches into the soil.
Place the set in the hole and cover it with soil. Space the sets 2 inches apart in a row, and space the rows between 12 and 18 inches apart.
Thin the plants when they are about 6 inches tall so that the spacing between plants is about 4 inches. The plants you remove can be eaten like scallions or green onions.
Scatter fertilizer along the side of the row, using about 3 ounces for each 10-foot row. Do this twice, once when the plants are 6 inches tall and again when they are 12 inches tall.
Irrigate so that the soil stays moist. The plant has shallow roots and requires constant moisture.
Bend the stems of the leaves horizontally when they begin to yellow, and push aside some of the top layer of soil to expose the upper half of the bulb.
Lift the onions from the soil when the leaves turn brown. It will take about 3 ½ months from planting to harvest.