Onion seeds like rich, loamy, well-draining soil. Texas A&M University recommends planting onion seeds in a garden bed amended with super phosphorous (0-20-0) for best growing results. In climates where the ground is frozen and covered in snow all winter, plant onion seeds in the spring as soon as the soil is workable and the last frost has passed. In warm areas where the winters are cool and mild and summers are hot, plant onion seeds in late fall.
Dig a 3-inch deep trench in a sunny garden bed. Spread a thin layer of super phosphate along the length of the trench. Texas A&M University recommends using 1/2 a cup for every 10 feet of planting row.
Fill the trench with 2 1/2 inches of soil, covering the phosphate completely. Sprinkle the onion seeds along the length of the planting trench and cover them with a 1/2 inch of soil.
Water the seeds immediately after planting until the soil is damp 3 inches down. The young seedlings will be visible in seven to 10 days. Keep the seeds damp but never saturated with frequent, light watering.
Thin the seedlings to a space of one plant every 2 to 3 inches eight to 10 weeks after the seeds sprout. Use the thinned seedlings in the kitchen as green onions or scallions.
Keep the area free of weeds. Weed by hand to avoid disturbing the shallow roots of the young onion seedlings. Spread a 1- to 2-inch layer of mulch around the seedlings to help keep the weeds down and keep moisture close to the ground.