Onion in Zone 10B — Desert Southwest

Onions are one of the most rewarding crops for Desert Southwest gardeners, offering incredible storage life and intense flavor that puts grocery store varieties to shame. Our mild winters and two growing seasons mean you can grow onions for nearly the entire year, harvesting sweet spring onions in late March or waiting for full-sized storage onions through summer. The dry air that challenges many crops actually helps onions cure perfectly for long-term storage.

Growing onions in our extreme heat and alkaline soils requires smart timing, but our 349-day growing season gives you plenty of flexibility to work around summer's brutal temperatures. Plant during our mild winter months, and you'll have established plants ready to bulk up before the intense heat arrives.

Starting onion seeds indoors makes sense if you want the largest possible bulbs or specific varieties unavailable as sets. Start seeds indoors from mid-October through early November, about 10 weeks before your transplant window. This timing lets you grow strong seedlings during our pleasant fall weather.

Set up seed trays in a warm spot (65-75°F) with good light—a south-facing window works well during our mild autumn. Keep soil consistently moist using bottom watering to prevent damping off. The seedlings will be pencil-thin but tough enough for our very early spring transplant timing.

Remember that onion sets are much easier for beginners, but starting from seed gives you access to storage varieties that perform well in our alkaline soils and dry conditions.

Transplant your onion seedlings from early to late January, taking advantage of our mild winter weather. These hardy plants can handle our occasional January frost, but give them a week of hardening off first—gradually increasing their outdoor exposure time.

Space transplants 4-6 inches apart in well-amended soil, planting them at the same depth they were growing in containers. Our January weather is generally cooperative for transplanting, though you might face a cold snap or unexpected warm spell during this window.

Plant them in your sunniest spot since winter sun is precious here, and they'll need all the energy they can get before our extreme heat arrives. The cool soil and mild temperatures help roots establish quickly.

Direct sowing works well in the Desert Southwest, especially with onion sets (small bulbs). Plant from mid-December through early September, though the earlier dates work better for full-sized bulbs. Our winter soil stays workable, making this method quite practical.

Plant sets or seeds 4-6 inches apart and about 1 inch deep in loose, well-draining soil. Seeds germinate slowly in cool weather but establish strong root systems. Sets give you a head start and are nearly foolproof for beginners.

The key is getting them in the ground while soil temperatures are moderate. Our alkaline soil benefits from added compost, and the timing lets roots develop before summer's arrival.

Onions need consistent moisture during their growing season, which aligns well with our winter-to-spring planting schedule. Water deeply once or twice per week, applying about 1 inch total. Check soil moisture with your finger—if it's dry 2 inches down, it's time to water.

During our mild winter months, natural rainfall and lower evaporation rates mean less frequent watering. As spring temperatures climb and our dry, hot conditions intensify, you'll need to increase frequency while maintaining that consistent moisture level. Water at the base to avoid wetting foliage in our arid climate.

Stop watering completely once the tops begin to fall over and yellow—this signals the start of the curing process. Our hot, dry summer air is perfect for this natural drying, but premature watering can cause rot or prevent proper storage curing.

Watch for signs of stress in our extreme heat: blue-green color indicates good moisture, while gray-green leaves suggest they need water. A 2-inch mulch layer helps retain moisture and keeps roots cool as temperatures soar past 105°F.

Your first onions will be ready from late March through early January, depending on when you planted and whether you're harvesting green onions or waiting for full bulbs. Spring-planted onions typically mature by late June, just as our extreme heat peaks.

Harvest when the tops naturally fall over and begin yellowing—this usually happens after about 100 days of growth. Don't wait too long after tops fall, as bulbs can start deteriorating in soil. Pull them gently, brush off soil, and lay them in a single layer in a warm, dry, ventilated area.

Our hot, dry summer air is ideal for curing onions. Let them cure for 2-3 weeks until the outer skins are papery and necks are completely dry. Properly cured onions store for months in our low-humidity climate.

As late December frost approaches, harvest any remaining onions even if tops haven't fallen completely. They'll continue curing after harvest, and you don't want to risk losing your crop to unexpected cold.

Onion Thrips These tiny, slender insects create silver-white streaks and stippling on leaves, eventually causing tips to brown and die back. You'll see the damage before you spot the thrips themselves—they're barely visible and hide in leaf joints. Our hot, dry conditions favor thrips, especially during late spring as temperatures climb.

Thrips thrive in dusty conditions and stressed plants, both common in our extreme heat. Prevent problems with consistent watering and removing weeds that harbor them. Beneficial insects help control thrips, but severe infestations may need targeted sprays applied in early morning or evening.

Pink Root This soilborne fungus turns onion roots pink to red, eventually causing them to rot away completely. Plants become stunted with yellowing leaves, and bulbs stay small. Pink root is more common in our alkaline soils and during warm, wet periods like monsoon season.

The fungus persists in soil for years and spreads through contaminated tools or transplants. Improve drainage, rotate crops away from onion family plants for several years, and choose resistant varieties when possible. Avoid planting in areas with standing water during monsoons.

Bolting Plants send up flower stalks prematurely, making bulbs small and bitter. You'll see a thick, hollow stem emerging from the center with a round flower head. Once bolting starts, the bulb stops growing and becomes tough.

Heat, drought stress, or planting too early can trigger bolting. In our climate, late plantings that mature during extreme heat are especially prone. Plant at proper times, maintain consistent soil moisture, and choose varieties bred for warm climates. Remove flower stalks immediately if they appear.

Desert Southwest Specific Challenges Our extreme heat accelerates many problems, while alkaline soils and low humidity create unique stresses. Caliche layers restrict root growth, making plants more vulnerable to heat and drought. Plan irrigation carefully during monsoon season, as sudden moisture changes can trigger disease in our typically dry conditions.

Onions make excellent companions for carrots and beets in our Desert Southwest gardens—their shallow roots don't compete, and onions naturally repel many soil pests that target root crops. Plant lettuce between onion rows during winter months for efficient space use, as both crops prefer our mild winter growing conditions.

Tomatoes benefit from onion companions too, especially since both handle our alkaline soils well and have similar watering needs during establishment. Avoid planting onions near beans or peas, as onions can inhibit the nitrogen-fixing bacteria these legumes depend on—particularly important in our nutrient-challenged desert soils where every bit of natural nitrogen helps.