Plant onion seed as soon as the soil can be worked in the spring. Onion seed germinates in a wide range of soil temperature, 18-29°C (65-85°F). Sow and cover onion seed with ½” of soil and keep moist.Onions cCan be started indoors 6-8 weeks prior to planting in the spring. Thin or transplant to 3-4” apart in rows 18-24” apart.To plant onion sets, simply press sets into the soil about 2” apart.Later thin to about 4-6” apart to allow bulbs to mature.ow and cover onion seed with ½” of soil and keep moist. Onions cCan be started indoors 6-8 weeks prior to planting in the spring. Thin or transplant to 3-4” apart in rows 18-24” apart. To plant onion sets, simply press sets into the soil about 2” apart. Later thin to about 4-6” apart to allow bulbs to mature.