Onions are one of nature’s most versatile veggies and make a great addition to any garden. They are also packed with punches of flavor, antioxidants, and support digestive health. If you’re interested in learning more about how to grow onions then you’re in luck. Our friends at Grangetto’s Farm & Garden Supply know a thing or two about gardening in San Diego. Here, they share onion gardening advice to help get us started.

How to Start, Plant, and Care

Gardeners can start from seeds, purchase dormant “sets” or transplant. Prior to planting, it’s recommended (but optional) to soak sets or seeds in compost tea to encourage quick germination. 

Plant in the fall through early winter in a sunny location. Cool conditions and loose, rich, well-draining soil are ideal. Short-day varieties (12–14 hours of sunlight) grow best in Southern California.

Planting from sets is the easiest method. Space bulbs 4” to 5” apart, with the top barely visible. Till a garden bed with at least 2” of compost or planting mix. Place bulbs 1” deep with the roots down and space rows 9” apart. Cover with the newly amended soil and water until the top 3” to 4” of soil is moist. 

Keeping the upper area of your soil moist is key for onions. Watering with a drip system or drip tape will give you low-flowing irrigation. Turn on each day for short periods of time to maintain water content in the upper zone of the soil bed. Be sure to carefully remove surrounding weeds and fertilize regularly as well.


Young onions can be harvested just a few weeks after planting if you want to use them as “spring onions” or scallions. There is no perfect size, just pull when they are big enough to suit you. For full-sized bulbs, let onions grow and mature. They are ready to harvest when the bulbs are big and the tops begin to turn yellow and fall over. Manually bend any tops that are still standing, and carefully dig the bulbs out. Leave onions with tops attached outdoors (preferably protected from the sun) or in a well-ventilated area to cure for about two weeks. Tops, skin, and roots must be completely dry before storing.

These are just a few tricks of the trade but there’s always more to learn to perfect the art of gardening. There’s a detailed Guide to Growing Onions in San Diego and more expert growing advice on the Grangetto’s Farm & Garden website anytime you need it.

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