Onion in Zone 7A — Northern California

Onions are one of the most satisfying crops you can grow in Northern California's Zone 7A. Our Mediterranean climate gives you an incredibly long window for growing onions – from planting sets in early spring through harvesting storage onions well into fall. You'll taste the difference immediately: homegrown onions have a depth of flavor and sweetness that store-bought varieties simply can't match, whether you're growing sweet Walla Wallas for summer salads or sharp storage varieties for winter cooking.

While our dry summers and occasional water restrictions might seem challenging for onion growing, the timing actually works in your favor. With our 235-day growing season and mild winters, you can plant early and let bulbs develop slowly through spring's moderate temperatures. The key is getting them established before summer heat arrives and understanding when to stop watering as harvest approaches.

Starting onion seeds indoors isn't the most common approach in Northern California, but it makes sense if you want the largest possible bulbs or specific varieties not available as sets. Start seeds from mid-December through early January, about 10 weeks before you plan to transplant outdoors in early spring.

Set up seed trays in a warm spot (65-70°F) with good light – a sunny south window works, though you'll get more consistent results with grow lights. Onion seeds are tiny and need consistent moisture, so bottom watering works best to avoid disturbing them. Fill your drip tray with water and let the soil wick up moisture from below.

The main advantage of starting from seed is variety selection and potentially larger bulbs, since indoor-started plants get a head start. However, onion sets are much easier for beginners and still produce excellent results in our climate. If you do start from seed, expect thin, grass-like seedlings that look surprisingly fragile but are actually quite hardy once established.

Transplant your indoor-started onion seedlings outdoors from early to late March, once the soil is workable and nighttime temperatures consistently stay above the mid-20s. In Northern California, this timing usually works perfectly since our spring comes early and temperature swings are generally mild.

Harden off seedlings gradually over a week by placing them outside for increasingly longer periods. Start with a few hours of morning sun and work up to full days outdoors. Space transplants 4-6 inches apart in rows, planting them at the same depth they were growing indoors – don't bury the bulb area.

For most Northern California gardeners, onion sets (small dormant bulbs) are actually the easier path than transplanting seedlings. Sets are widely available at local nurseries in spring, establish quickly, and are much more forgiving of planting mistakes. Plant sets pointed-end up, just barely covered with soil, at the same 4-6 inch spacing.

Direct sowing onion seeds works well in Northern California, especially from mid-February through mid-July for succession plantings. The key advantage is avoiding transplant shock, and our mild spring weather gives seeds plenty of time to establish before summer heat arrives.

Prepare your bed with loose, well-draining soil – onions hate wet feet, which is rarely a problem in our dry-summer climate. Sow seeds about ¼ inch deep and 1 inch apart, then thin to 4-6 inches as they grow. Keep the soil consistently moist during germination, which takes 7-14 days depending on soil temperature.

Direct seeding gives you the most variety options since you're not limited to available sets, and it's cost-effective for large plantings. However, germination can be spotty, and the seedlings develop more slowly than sets. If you're new to onion growing, starting with sets will give you more reliable results while you learn the crop's needs.

Onions need consistent moisture throughout their growing season, which requires some planning in Northern California's dry-summer climate. During active growth from spring through mid-summer, provide about 1 inch of water per week, checking soil moisture with the finger test – stick your finger 2 inches into the soil near the plants.

Early in the season, when winter rains typically end around May, you'll transition to regular irrigation. Water at the base of plants rather than overhead to avoid encouraging fungal issues in our low-humidity climate. Morning watering works best, giving plants time to dry before cooler evening temperatures.

The critical timing change comes in late summer when onion tops start falling over naturally – this signals the beginning of the curing process. Stop watering completely once about half the tops have fallen. This allows the outer layers to dry and form the papery skin that makes onions storeable. Many new gardeners make the mistake of continuing to water right up to harvest.

During our typically dry August and September, this natural curing process works perfectly with Northern California's climate. If you're growing short-day onions for fresh eating, you can maintain some moisture longer, but storage onions absolutely need that dry-down period to develop properly.

Your first onions will be ready from late May through mid-November, depending on when you planted and which varieties you're growing. Look for the telltale sign: tops beginning to fall over and turn yellow-brown. This usually happens about 100 days after planting, though it varies by variety and growing conditions.

Harvest on a dry day if possible – perfect timing for Northern California since late summer and early fall are typically bone dry. Gently lift bulbs with a garden fork, being careful not to bruise them. Brush off excess soil but don't wash them yet. The papery outer skin should be forming, giving the bulbs their characteristic onion appearance.

Cure freshly harvested onions in a warm, dry, well-ventilated area for 2-3 weeks before storing. In Northern California, this is usually easy – spread them on screens or hang in braids in a garage, covered porch, or spare room. Proper curing is essential for long-term storage, allowing the neck to seal and outer layers to fully dry.

As your first frost approaches in early November, make sure to harvest any remaining onions. Even if the tops haven't completely fallen over, they'll continue curing after harvest. Properly cured onions can store for months in a cool, dry place – perfect for enjoying your homegrown flavor all winter long.

Onion thrips show up as tiny, silvery streaks on leaves and can cause significant damage to your crop. These microscopic pests scrape leaf surfaces, leaving behind characteristic silver-white feeding scars that eventually turn brown. Heavy infestations cause leaves to curl and bulb development to slow significantly.

Northern California's dry summers actually favor thrips, since they thrive in hot, dusty conditions. Combat them by increasing air circulation around plants and using reflective mulches like aluminum foil strips, which confuse thrips. Beneficial insects like minute pirate bugs help control populations naturally, so avoid broad-spectrum pesticides that kill the good bugs along with the bad.

Pink root appears as pink to reddish-brown discoloration of the root system, eventually progressing to dark purple or black. Affected plants show stunted growth, yellowing leaves, and poor bulb development. This soilborne fungal disease thrives in warm, moist conditions and can persist in soil for years.

Prevention is key since there's no cure once plants are infected. Rotate onions to different beds each year, improve drainage, and avoid overwatering. In Northern California's heavy clay soils, amending with compost and planting in raised beds helps create the well-draining conditions onions prefer.

Bolting happens when onions send up a flower stalk prematurely, making the bulbs small and bitter. You'll see a thick, hollow stem emerge from the center with a round flower head at the top. Once this happens, the plant stops putting energy into bulb development.

Hot weather, drought stress, or planting at the wrong time typically trigger bolting. In Northern California, late plantings that experience our summer heat while still young are especially prone. Prevent bolting by planting at the right time for your variety – long-day onions in early spring, short-day varieties slightly later. Keep soil consistently moist during hot spells and choose bolt-resistant varieties if this is a recurring problem.

Northern California Specific Challenges: Our Mediterranean climate's combination of dry summers and variable microclimates means onion growing success often depends on your exact location and water management. Coastal gardeners deal with different timing than inland areas, while those in fire-prone regions need to balance water restrictions with consistent irrigation needs.

Onions make excellent companions for many vegetables commonly grown in Northern California gardens. Carrots and onions are the classic pairing – onions repel carrot flies while carrots help break up soil for onion root development. Beets and lettuce also thrive alongside onions, benefiting from the pest-deterrent properties while not competing for the same soil nutrients or space.

Tomatoes and onions work particularly well in our long growing season, since you can plant onions early and harvest them just as tomatoes hit their peak production in summer. This succession planting maximizes garden space efficiently. However, avoid planting onions near beans and peas – these nitrogen-fixing legumes can actually inhibit onion bulb development, and the different watering needs don't match well in our dry-summer climate.