Onion in Zone 7B — Northern California

Onions are one of the most rewarding crops you can grow in Northern California's Mediterranean climate. Our long 245-day growing season means you can grow storage onions that develop impressive size and complex flavors impossible to find in stores. The sweet, mild onions you'll harvest here showcase what our warm days and cool nights can produce – whether you're growing pungent storage varieties or sweet salad onions for immediate use.

Our dry summers and variable microclimates do present some challenges, particularly with consistent watering during bulb formation. However, onions are surprisingly adaptable to our conditions once you understand their rhythm of spring growth and summer maturation. With proper timing, you'll be harvesting your own onions from late spring through late fall.

Starting onion seeds indoors isn't the most common approach, but it gives you the longest growing period for the largest bulbs. You'll want to start seeds mid-December through early January, about 10 weeks before you plan to transplant in early spring. This timing takes advantage of our early spring conditions and gives onions maximum time to size up before summer heat triggers bulb formation.

Set up seed trays in a warm spot (65-70°F) with good light once seedlings emerge. Bottom watering works particularly well for onion seedlings – they're prone to damping off if kept too wet on the surface. Your seedlings will be ready to transplant by early to late March when our soil has warmed and the risk of hard frost has passed.

The main advantage of starting from seed is variety selection and cost – you can grow storage onions, sweet onions, or specialty varieties not available as sets. However, most Northern California gardeners find onion sets easier and more reliable given our excellent growing conditions.

Transplant your indoor-started onion seedlings from early to late March, once soil temperatures consistently stay above 40°F. This timing works perfectly with our early spring pattern – you'll avoid the worst of winter wet soil while getting plants established before summer heat arrives.

Harden off seedlings gradually over a week, starting with a few hours of morning sun and working up to full outdoor exposure. Our March weather can swing between mild and surprisingly cool, so watch for temperature drops that might stress young transplants. Space plants 4-6 inches apart in rows, giving them room to develop full-sized bulbs.

Plant sets about 2 inches deep with just the tips showing above soil level. Sets are actually the easiest method for beginners – they're less finicky than seedlings and more reliable than direct seeding. You'll find good selection at local nurseries in February and March, perfectly timed for our planting window.

Direct sowing onions works well in Northern California, particularly if you're planting sets or want a more hands-off approach. You can direct sow seeds from mid-February through late July, though early spring plantings (February through April) give you the longest growing season for storage onions.

Prepare your soil by working in compost – onions prefer loose, well-draining soil that won't stay soggy during our wet winter months. Seeds need soil temperatures around 50°F to germinate reliably, which usually happens by late February in most Northern California microclimates. Plant seeds about ½ inch deep and space them 4-6 inches apart, or closer if you plan to thin for green onions.

The main challenge with direct seeding is our variable spring weather and the patience required – seeds can take 2-3 weeks to emerge, especially in cooler microclimates. Sets give more predictable results, but seeds offer better variety selection and are more economical for larger plantings.

Onions need consistent moisture during their spring and early summer growing phase, which aligns well with our natural rainfall pattern but requires attention as summer progresses. During active growth (March through June), keep soil consistently moist but not waterlogged – about 1 inch of water per week including rainfall.

Use the finger test: stick your finger 2 inches deep into the soil near your onion plants. If it's dry at that depth, it's time to water. Our low humidity and warm summer afternoons mean you'll likely need to water 2-3 times per week once our dry season begins in earnest around May or June.

Water at soil level rather than overhead to prevent fungal issues and make the most of every drop during potential water restrictions. A soaker hose or drip irrigation works perfectly for onion beds. Stop watering completely when the tops begin to fall over and yellow – this signals that bulbs are maturing and need to cure properly.

Mulch around plants with straw or shredded leaves to retain moisture and reduce watering needs. This is particularly valuable during our hot, dry summers when consistent soil moisture becomes more challenging to maintain.

Your first onions will be ready for harvest as early as late May if you planted early and are growing quick-maturing varieties, with the main harvest season running through late November. Storage onions typically need the full 100 days to develop proper bulb size, while green onions can be harvested much earlier.

The key harvest indicator is when the tops begin to fall over and turn yellow-brown – this signals that the onion has stopped active growth and is ready to cure. Don't wait for all the tops to fall over completely, as this can lead to splitting or rot in our sometimes unpredictable fall weather. Harvest on a dry day when possible.

Gently lift onions with a fork rather than pulling to avoid bruising. Brush off excess soil but don't wash them – you want the outer skins to dry and form a protective barrier. Lay them in a warm, dry, well-ventilated area for 2-3 weeks to cure properly.

As our first frost approaches in mid-November, harvest any remaining onions regardless of size. Small onions store just fine and are perfect for cooking, while any that haven't fully matured can be used fresh rather than stored.

Onion Thrips These tiny, slender insects create silvery streaks and stippling on onion leaves, eventually causing tips to turn brown and dry. You'll see the damage before you see the thrips themselves – they're barely visible and move quickly when disturbed. Our warm, dry summers create ideal conditions for thrip populations to explode.

Thrips are particularly problematic during hot spells and drought stress, both common in Northern California summers. Keep plants consistently watered and consider row covers during peak summer heat. Beneficial insects like minute pirate bugs help control thrips naturally, so avoid broad-spectrum pesticides that kill beneficial populations.

Pink Root This soil-borne fungal disease causes onion roots to turn pink, then purple, eventually becoming black and dying back. Plants become stunted and may fail to bulb properly. Pink root thrives in warm, moist soil conditions and can persist in soil for years.

The disease is more common in areas with poor drainage or where onions are grown repeatedly in the same spot. Rotate your onion crops to different areas of your garden each year, and improve soil drainage with compost and raised beds. Choose resistant varieties when available, and avoid overwatering during warm weather.

Bolting Onion plants send up flower stalks prematurely, shifting energy from bulb development to seed production. You'll see a thick, hollow stem emerging from the center of the plant, topped with a round flower head. Once this happens, the bulb stops growing and may become bitter or develop a tough core.

Bolting is typically triggered by temperature swings, long day length, or drought stress – all potential issues in our variable spring weather and hot summers. Plant at the right time for your microclimate, provide consistent moisture, and choose varieties suited to our day length. If bolting occurs, cut the flower stalk immediately and use those onions fresh rather than storing them.

Northern California Specific Challenges Our Mediterranean climate's combination of dry summers and variable microclimates means onion watering becomes critical from May onward. Fire season and potential water restrictions can make it challenging to maintain the consistent moisture onions need during bulb formation, while our intense sun in inland areas can stress plants and trigger early bolting.

Onions make excellent companions for carrots, beets, and lettuce in Northern California gardens. The classic onion-carrot pairing works particularly well here because both crops enjoy similar watering schedules during our spring growing season, and onions help deter carrot root fly. Plant them in alternating rows or intermixed for natural pest control.

Tomatoes and onions also pair beautifully in our Mediterranean climate – onions planted around tomato plants help deter aphids and other pests while making efficient use of garden space. Both crops handle our dry summers well with proper watering, and you can harvest green onions early in the season without disturbing your tomato plants. Avoid planting onions near beans or peas, as onions can inhibit the nitrogen-fixing bacteria that legumes depend on, reducing their productivity in our nitrogen-hungry soils.