As soon as soil can be prepared plant seeds 1/4" deep. Thin to 6" apart at the most. They do not produce large bulbs and can be planted in small spaces. Be sure to keep hills of soil built up to bottom of stalks to encourage more white onion growth under the surface of the soil. More bang for your buck! 
SEED INFORMATION: 
Lifecycle: annual (vegetable) 
Time to Maturity: about 2 months 
Sun Requirment: sun 
Spacing: 6" apart 
Depth: 1/4 inch 
Packet Weight: 600mg 
Seeds/Oz: 9000 
Ideal Germination Temperature: 50-85F 
Sowing Method: Direct 
Heirloom Year: Pre 1900 Heirloom