Onion in Zone 7B — Desert Southwest

Onions are perfectly suited for our Desert Southwest climate, giving you two distinct growing seasons to work with. Unlike store-bought onions that travel thousands of miles, homegrown varieties develop intense, complex flavors that transform your cooking. You can grow everything from sweet storage onions to sharp green onions, and they store beautifully through our mild winters when properly cured.

Our extreme heat and arid conditions challenge many crops, but onions actually thrive with proper timing. The key is working with our 245-day growing season and avoiding summer planting when temperatures soar above 105°F. Plant early enough in spring and you'll harvest before the worst heat hits, or time a fall crop to mature through our extended autumn season.

Starting onion seeds indoors makes sense if you want the largest bulbs possible, though it's not the primary method most Desert Southwest gardeners choose. Start seeds in mid-December through early January, about 10-12 weeks before your planned transplant date. This timing lets you get transplants in the ground during our very-early spring window.

Set up seed trays in a warm spot (65-70°F) with good light once seedlings emerge. Bottom watering works best to prevent damping-off and keeps soil moisture consistent. Your seedlings will be ready to harden off by early March, perfect timing for our spring planting season.

Keep in mind that onion sets (small bulbs) are much easier for beginners and give reliable results in our climate. Save indoor seed starting for when you want specific varieties or the satisfaction of growing from seed to storage onion.

Transplant your indoor-started onion seedlings from early to late March, when nighttime temperatures stay consistently above freezing. This timing takes advantage of our very-early spring and gets plants established before extreme heat arrives. Harden off seedlings gradually over a week, starting with just morning sun and working up to full outdoor exposure.

Space transplants 4-6 inches apart in well-draining soil, planting them slightly deeper than they were in their containers. Our spring weather can swing dramatically, so keep row cover handy for unexpected cold snaps. The key is getting roots established during these mild weeks.

Remember that onion sets are actually the easiest option for most Desert Southwest gardeners. These small bulbs transplant easily, establish quickly, and don't require the indoor setup that seedlings need. You'll find them available at local garden centers right when you need to plant.

Direct sowing onions works well in the Desert Southwest, though you'll need to choose your timing carefully. Sow seeds from mid-February through late July, depending on whether you're aiming for a spring or fall harvest. Spring sowings should go in as soon as soil can be worked, while summer sowings need the protection of our monsoon moisture.

Prepare your soil by working in compost and ensuring good drainage - our caliche layer can create waterlogged conditions that onions hate. Plant seeds ¼ inch deep and space them 4-6 inches apart, or plant closer and thin as they grow. Soil temperature should be at least 50°F for good germination.

The easier route is planting onion sets directly in early spring. These small bulbs establish faster than seeds and give you a head start on bulb development before summer heat arrives. Plant sets with just the tip showing above soil level.

Onions need consistent moisture throughout their growing season, which requires careful attention in our arid Desert Southwest climate. With only 8-15 inches of annual rainfall and extreme summer heat, you'll be providing most of their water needs yourself. During active growth, onions need about 1 inch of water per week, delivered slowly and deeply.

Use the finger test to check soil moisture - stick your finger 2 inches down and water when it feels dry at that depth. Our low humidity means soil dries out faster than in other climates, so check daily during hot weather. Water at the base of plants rather than overhead to reduce evaporation and prevent fungal issues in our arid conditions.

Stop watering completely when onion tops begin to fall over and yellow - this signals the start of the curing process. Continued watering at this stage can cause bulbs to rot in storage. Time this carefully with our monsoon season, as excess moisture during curing ruins months of careful growing.

A thin layer of mulch helps retain moisture and keep soil temperatures down, but don't pile it thick against the bulbs. Our extreme heat means even drought-tolerant plants like onions struggle without adequate water, so consistency is key to developing good bulb size before harvest time arrives.

Your first onion harvest typically comes in late May through late November, depending on planting time and variety. For storage onions, harvest begins when tops start falling over and turning yellow naturally - this usually happens about 100 days after planting. Don't wait for all tops to fall; start harvesting when about half have gone down.

Carefully dig bulbs on a dry day, brushing off soil but leaving roots and tops attached. The papery outer skin should be forming and feel dry to the touch. If you harvest too early, the necks won't cure properly and bulbs won't store well through our mild winters.

Cure harvested onions in a warm, dry, well-ventilated area for 2-3 weeks before trimming tops and storing. Our low humidity actually helps with curing, unlike more humid regions where this process is challenging. Properly cured onions will keep for months in a cool, dry place.

As our first frost approaches in mid-November, harvest any remaining bulbs regardless of size. Green onions can be pulled anytime once they reach pencil thickness, and these actually handle our summer heat better than bulb onions if you provide afternoon shade.

Onion Thrips These tiny, slender insects create silvery streaks and stippling on onion leaves, eventually causing tips to turn brown and die back. In severe infestations, you'll see the tiny yellow or brown insects themselves when you disturb the plants. Our hot, dry conditions actually favor thrips development, making this a common Desert Southwest problem.

Thrips thrive in our low humidity and can build up quickly during spring growth. Use reflective mulch around plants to confuse the insects, and encourage beneficial predators like minute pirate bugs. Insecticidal soap applied in early morning works well, but avoid spraying during extreme heat as it can damage stressed plants.

Pink Root This fungal disease causes roots to turn pink, then purple, and finally black as they rot away. Plants become stunted and may die in severe cases. You'll notice it first when pulling green onions - the roots look distinctly colored instead of healthy white. Our alkaline soils and inconsistent watering create conditions that favor this disease.

Pink root develops when soil stays too wet or when plants are stressed by heat and inconsistent moisture. Improve drainage in clay soils and avoid overwatering, especially during our monsoon season. Choose resistant varieties and rotate onion plantings to different areas of your garden each year.

Bolting Onions send up tall flower stalks prematurely, shifting energy from bulb development to seed production. Once bolted, leaves become bitter and tough, and bulb development stops. This is triggered by heat stress, long days, or inconsistent watering - all challenges we face in the Desert Southwest.

Plant at the right time for your variety (short-day types for our region) and maintain consistent soil moisture throughout the growing season. Provide afternoon shade during extreme heat and choose bolt-resistant varieties bred for hot climates. If plants bolt, harvest immediately and use quickly rather than trying to store.

Desert Southwest Specific Challenges: Our extreme heat and arid conditions stress onions more than gardeners in milder climates experience. The combination of 105°F+ temperatures, low humidity, and intense sun means even properly watered onions can struggle. Plan for extra shade protection during peak summer and time plantings to avoid the worst heat for best results.

Onions make excellent companions for carrots and beets because their strong scent helps deter root maggots and other soil pests that target these crops. Plant them in the same bed and you'll often see improved growth in both. Lettuce benefits from onions' pest-deterrent properties and uses space efficiently between onion rows during our spring growing season.

Tomatoes and onions work well together because onions may help repel certain insects that bother tomatoes, though this partnership works best in fall plantings when heat stress is less of a factor. Avoid planting onions near beans or peas - these legumes can be stunted by compounds that onions release into the soil, and they have different watering needs that don't match well in our arid climate.