Spanish onions, or yellow onions, add flavor to many dishes and are easy to cultivate and grow on your own. Onions are hardy plants, and so with a little preparation and care, you can develop a sustainable vegetable garden.
Purchase Spanish onion seeds, seedlings, or bulbs at your local gardening store. Seeds are the cheapest option, but they have the lengthiest growing period and must begin growth indoors. Seedlings expedite the growing period but are more prone to spoilage. Bulbs have the quickest growth but the quality is variable.
Before planting, make sure the soil has fertilizer. Onions require moist soil, and manure and compost help stimulate growth. In most climates, late March or April are the best times to plant onion seeds. Temperatures should be between 55 and 75 degrees Fahrenheit during the growing season. Plant seeds 1" deep, with 3 to 4" between seeds. Multiple rows should be planted, with 6 to 10" between the rows. During the growing season, be sure to weed and keep the soil moist.
To harvest the onion bulbs, bend the stems horizontally when the leaves turn yellow. The bulbs are ready to be harvested when you can easily tug them out of the soil. Let the bulbs dry on the ground for several days, turning them over every few days and keeping them covered from any rain or moisture.The onions will turn brown and then trim the tops one inch from the bulb. On a wire screen in warm shade, spread the onions out and allow the onions to cure for two months. Check for spoilage after two months. The smallest bulbs can be cut off and used in next season's plantings. The longer the bulbs are allowed to cure, the stronger the flavor of your Spanish onions.