Commercial Onion Planting

Take a soil test to find out what amendments are needed to grow onions commercially. This is done by contacting your local county agricultural extension office and asking for assistance. Adding amendments without knowing what is needed for onion production leads to failure and extra expense that erodes your profit margin.

Choose the right onion seed to plant for your area. Onion varieties are divided between short and long day onions. Short-day onions, such as the 'contessa' and 'yellow granex' varieties are planted below the 36th parallel that crosses the United States on the northern border of Arizona, New Mexico, Texas and North Carolina. Long-day onion varieties, such as 'walla walla' and 'yellow sweet Spanish,' are planted north of that line. Short day onions produce bulbs when day length reaches 10 to 12 hours and long-day onions at 14 to 16 hours. This is important because temperatures below the 36th parallel are too warm by the time sufficient daytime hours are available for long-day onions to produce quality onions that can be sold commercially.

Plant onion seeds indoors or in a greenhouse in a well-drained container filled with potting soil. Plant the seed 1/4-inch deep and keep the soil moist until planting. The new onion plants will emerge in five to seven days. The temperature of the seed planting area should be between 65 and 85 degrees Fahrenheit. The seeds should be planted 12 weeks before it is time to plant outdoors, which is 12 weeks before the last average frost date for your area.

Prepare a well-drained planting location for onions. Remove all competing vegetation and loosen the soil to a depth of 12 inches using a tiller or other mechanical methods.

Add the soil amendments at the rate recommended from the results of the soil test, along with a 1-inch layer of well-rotted compost. Use the tiller or shovel to incorporate the amendments and compost into the top 6 inches of soil. Rake the area smooth.

Install a drip irrigation system if you are planting more than a 100-foot row of onions. Onions need regular moisture and production will suffer if they are allowed to dry out. Hand watering multiple rows of onions is time consuming and you will be soaking the tops and the bulbs, which increases the chance of fungal or mildew disease.

Plant the onion plants, or sets, outside 12 weeks before the last frost. They should be about the width of a pencil when they are planted. Plant the sets 1-inch deep (counting from the bottom of the plant) and 3 to 4 inches apart. Water immediately to settle the plants. New growth will be seen within 14 days.

Harvest the bulbs when the green tops begin to turn brown and fall over. Allow the bulbs to dry in a well-vented and dry area for 4 to 5 weeks before storage.