Onion sets can be planted from September through mid- February. If you're using seed to start bulb onions the way Knobel does, sow seeds by the end of this month so the bulbs will be fully developed in June and July. Otherwise, seed for green bunching onions can be sown through April. Onion bulbs can't grow and expand in hard, adobe soil. Knobel says composted chicken manure dug to the depth of a shovel works best.
Onions appreciate fertilizer and can take twice as much as most other vegetables.
Some gardeners like to include bone meal, which is high in phosphorus and stimulates early bulb formation and root growth. If you do nothing else to your onion bed after planting, keep after the weeds. Shallow-rooted onions can't compete with weeds. Broadcast, or sprinkle, the tiny seeds, then spread a light, half-inch layer of soil to cover the seeds. Keep the bed moist but not soggy until the seeds germinate, then lightly water every two days or so.