Onion seeds are notoriously difficult to get to produce good onions. It's easier for the home gardener to grow onions from sets or transplants, but seeds can be nurtured into onions with good timing and techniques. Onion seeds germinate at different rates in different soil temperatures, but are versatile and frost-hardy enough to plant very early. Onion seeds are minuscule, so it's often good to plant a few in each hole to be sure of good sprouting.
Plant at the right time. Onions can be one of the first things to go into the garden in the spring, in late March or early April after severe frost danger is past. Cool temperatures and spring rains will help bulbs develop.
Sow onion seeds at the right depth as this has a large impact on bulb formation later. Plant onion seeds 1 inch deep in rich, moist soil. The soil should also be well-draining to cut down on possible rotting after bulbs form.
Follow spacing guidelines for your onion variety as this makes a big difference in good onion growth. Most seeds will range in spacing, from 1 to 4 inches apart. Rows should be 16 to 24 inches apart if you are planning a large plot of onions.
Weed religiously early on. Onions don't compete well with weeds, and will give up their growth if outcompeted. They also have very shallow roots, so weeding should be done by hand rather than with a garden hoe to avoid chopping roots.
Water or irrigate frequently as onions grow. This will help them produce large bulbs. Keep the soil moist, which will help onions grow round instead of misshapen as can occur if the soil dries out and cracks.