Onions need room to form bulbs, so plant Yellow Sweet Spanish onion seedlings up to 4" deep, four to six inches apart in a row, and space rows 12" apart. The earlier you plant the starts, the larger the bulbs can grow. Water seedlings regularly, keeping the soil evenly moist. Fertilize throughout the growing season, but as soon as the onions start pushing the soil away from their necks, discontinue fertilizer. When the tops begin to fall over and become brown, stop watering. The onions are ready to harvest at that point.

When digging up the onions, take care not to damage the flesh and let the bulbs cure on top of the ground if the weather is dry, or lay them out in a dry protected area for a few days. Let dry for several more days, then trim roots to 1/4" and the tops to one inch. Any damaged onions should be used immediately but others can usually be stored in a cool dry place for a few months.