Onion in Zone 10A — Southern California

Growing onions in Southern California gives you access to fresh, flavorful bulbs year-round, plus the satisfaction of harvesting something you planted nearly a year ago. Our mild winters and long growing season mean you can plant onions when most of the country is dormant, then harvest massive bulbs by summer when store-bought options are expensive and shipped from far away.

Your biggest challenge here is timing the planting right for our unique climate patterns — you'll be starting seeds during our dry fall months and transplanting during our brief winter wet season. With 349 growing days per year, you have plenty of flexibility to get the timing right and grow the sweetest, most storage-worthy onions possible.

Starting onion seeds indoors makes sense if you want the largest possible bulbs or specific varieties not available as sets. Start seeds mid-October through early November, about 10 weeks before your planned transplant date in January. This gives you maximum growing time before our summer heat kicks in.

Set up seed trays in a warm spot (65-70°F) with good light — a south-facing window works, or use grow lights for 12-14 hours daily. Bottom watering works best for onion seeds since they're prone to damping off if kept too wet on top. The seeds are slow to germinate, taking 10-14 days even in ideal conditions.

With our very early spring character, indoor-started seedlings give you a jump on the season that direct-sown seeds can't match. You'll transplant pencil-thick seedlings in January, giving them months of cool weather to develop strong root systems before summer heat arrives.

Transplant your indoor-started onion seedlings from early to late January, when soil is workable and night temperatures stay above freezing. This timing takes advantage of our mild winter growing conditions while avoiding the worst of summer heat later. Start hardening off seedlings a week before transplanting by gradually exposing them to outdoor conditions for longer periods each day.

Space transplants 4-6 inches apart in rows, planting them at the same depth they were growing in their containers. January transplanting in Southern California means dealing with occasional rain and cool morning temperatures, so choose a stretch of settled weather when possible. The soil should be well-draining since our winter rains can create soggy conditions.

Your transplanted onions will establish slowly through winter, then put on rapid growth as temperatures warm in spring. This long, cool establishment period is exactly what onions need to develop the large, storage-quality bulbs that make the wait worthwhile.

Direct sowing onion seeds works well in Southern California from mid-December through early September, though most gardeners prefer the December through February window for best results. Wait until soil temperature reaches at least 50°F — usually by late December in most areas. The benefit of direct sowing is simplicity, but you'll get smaller bulbs than with transplanted starts.

Prepare your beds by working compost into well-draining soil, since onions hate soggy feet during our winter rains. Sow seeds ¼ inch deep and space them 4-6 inches apart, or sow more densely and thin later. Keep the soil consistently moist through germination, which takes longer in cool winter soil.

Many Southern California gardeners prefer planting onion sets (small bulbs) instead of seeds for direct sowing. Sets planted in early spring give you the easiest path to harvest, though variety selection is limited compared to seeds. Plant sets pointed end up, just barely covered with soil.

Onions need consistent moisture during their long growing season, especially during bulb formation in late spring and early summer. In Southern California's semi-arid climate, this means supplemental watering throughout most of the growing period since our 10-20 inches of annual rainfall comes mainly in winter. Check soil moisture with the finger test — if it's dry 2 inches down, it's time to water.

During winter establishment (January through March), natural rainfall often provides enough moisture, but watch for dry spells longer than 10 days. As spring arrives and temperatures climb, increase watering to about 1 inch per week. Summer heat inland can push this to 1.5 inches weekly, especially during hot spells when temperatures hit the 90s.

Water at the base of plants rather than overhead to prevent foliar diseases and reduce water loss to evaporation in our low-to-moderate humidity. Drip irrigation or soaker hoses work perfectly for onions' shallow root systems. Signs of underwatering include stunted growth and small bulbs, while overwatering shows up as yellowing leaves and soft, rotting bulbs.

Stop watering completely when onion tops begin falling over in mid-summer — this signals the start of bulb curing. A 2-inch layer of mulch helps retain moisture during active growth and keeps soil temperatures moderate during summer heat, but pull it back from bulbs as harvest approaches.

Your first onion harvest typically arrives from late March through early January, depending on when you planted and which varieties you chose. Long-day varieties suited to Southern California usually mature in 100 days from transplant, while short-day types can be ready sooner. The key indicator is when tops begin falling over and turning yellow-brown naturally.

Harvest on a dry day when soil isn't muddy from recent irrigation. Gently lift bulbs with a garden fork rather than pulling, which can damage the necks and reduce storage life. Brush off soil but don't wash the bulbs — wet onions are prone to rot during curing.

Cure harvested onions in a warm, dry, well-ventilated area for 2-3 weeks until the outer skins become papery and necks are completely dry. Our low humidity makes Southern California ideal for curing — a covered porch or garage works perfectly. Properly cured onions store for months in a cool, dry place.

Since first frost doesn't typically arrive until late December, you have an extended harvest window for any late-planted bulbs. Green onions can be harvested anytime by pulling alternate plants, giving the remaining bulbs more space to size up.

Onion Thrips These tiny, slender insects create silvery streaks and stippling on onion leaves, eventually causing tips to turn brown and papery. Heavy infestations weaken plants and reduce bulb size. Thrips thrive in our hot, dry summer conditions and can multiply rapidly during heat waves.

Hot summer heat inland creates perfect breeding conditions for thrips, while Santa Ana winds can blow in new populations from surrounding areas. Use reflective mulch early in the season to confuse adult thrips, and consider beneficial insects like minute pirate bugs. Avoid overhead watering, which thrips hate, and remove plant debris where they overwinter.

Pink Root This fungal disease turns onion roots pink, then red, and finally black as they rot away. Affected plants show stunted growth, yellowing leaves, and small bulbs. The fungus persists in soil for years and spreads through contaminated tools or water.

Pink root thrives in warm, wet soils — exactly what we get during winter rains followed by spring warming. Improve drainage by adding compost and planting in raised beds. Rotate onions to different beds each year and avoid working in wet soil. Choose resistant varieties when possible.

Bolting Onions send up flower stalks prematurely instead of forming bulbs, triggered by temperature swings, drought stress, or planting at the wrong time. Once bolted, the leaves become bitter and bulbs stop developing properly.

Our variable winter weather and potential for early heat spells create ideal bolting conditions if timing is off. Plant at the right time for long-day varieties, maintain consistent soil moisture, and provide afternoon shade during unexpected warm spells. Choose bolt-resistant varieties and avoid planting too early in fall.

Southern California Specific Challenges: Our combination of intense summer heat, periodic drought conditions, and Santa Ana winds creates stress that makes onions more susceptible to all these problems. Water restrictions during critical growing periods can trigger bolting, while fire season smoke can stress plants. Focus on building healthy soil, consistent moisture management, and choosing varieties bred for hot climates.

Onions make excellent companions for carrots, beets, lettuce, and tomatoes in Southern California gardens. Plant them alongside carrots to help repel carrot flies while the carrots' deep roots break up soil for shallow onion roots. Lettuce and beets appreciate the pest-deterrent properties of onions, while all these cool-season crops share similar water needs during our mild winter growing period.

Avoid planting onions near beans and peas, as onions can inhibit their growth through natural compounds released from their roots. This is especially important in our intensive year-round growing climate where plants are closely spaced and root zones overlap significantly.