Onion seed starting is easy. Onions grow best in fertile, well-draining soil. This should also be worked with organic matter, such as compost. Onion seeds can be planted directly in the garden bed. However, when growing onion seed, some people prefer to start them indoors. This can be done about late October to mid November. The best time for planting onion seeds outdoors is in spring, as soon as the soil can be worked, usually around March or April. Place them about an inch deep in the soil and approximately half an inch or more apart. If planting rows, space them at least one and half to two feet apart.Once seedlings have sufficient leaf growth, thin them down to around 3-4 inches apart. Transplant onion seedlings that were started indoors about 4-6 weeks before the last expected frost or freeze date, provided the ground is not frozen. Onion plants have shallow roots and require frequent irrigation throughout the growing season. However, once the tops begin to lay over, usually by late summer, watering should be stopped. At this point, onions can be lifted.