When To Plant Onions In California

If you’re a California gardener, knowing when to plant onions is the first step to a great harvest. The optimal timing for planting depends heavily on your specific region within the state, as California’s diverse climates create different growing windows.

Onions are a rewarding crop, but they need a long growing season to form nice bulbs. Planting at the right time ensures they get enough cool weather for leaf growth and enough warm weather for the bulbs to mature. Getting your timing wrong can lead to small bulbs or premature bolting, where the plant sends up a flower stalk instead of focusing on the bulb.

This simple rule covers most of the state: plant onion sets, seeds, or transplants in the fall or late winter. For a more precise schedule, we need to break California into its major climate zones. Your local frost dates are the most critical factor.

California Climate Zones & Planting Windows

California’s gardening regions fall into three main categories for onion planting. Use this as your starting guide.

Coastal and Southern California (Zones 9-11)

This includes areas like Los Angeles, San Diego, and the Bay Area. Mild winters and long growing seasons are perfect for onions.

Best Planting Time: October through January.
You can plant onion sets or transplants from fall into early winter. This gives them a head start for a spring harvest.
For seeds, start them in late summer to early fall for transplanting, or sow directly in fall.
The goal is to get them established during the cool months so they can bulb up as days lengthen in spring.

Central Valley and Inland Areas (Zones 8-9)

This region, including Sacramento and Fresno, has hotter summers and sharper frosts.

Best Planting Time: Late January through mid-March.
Plant as soon as the soil is workable in late winter. Fall planting is possible but riskier if winters are very cold.
You want to avoid having young seedlings hit by a hard freeze, but you also need to get them in the ground early enough to mature before the intense summer heat arrives.

Northern California and Mountain Regions (Zones 7-8)

Colder areas like Redding or the Sierra foothills have a shorter season.

Best Planting Time: March through April.
Wait until the threat of a hard spring frost has passed. Soil temperature should be at least 50°F.
Starting seeds indoors 8-10 weeks before your last spring frost date is highly recommended here to extend the season. Transplants can go out in early spring.

Understanding Onion Types: Day Length is Key

Choosing the right type of onion is just as important as timing. Onions are classified by the amount of daylight they need to form bulbs.

Short-Day Onions: Bulb when daylight lasts 10-12 hours. Ideal for Southern and Coastal California. Plant in fall for late spring harvest. Varieties: ‘Texas Early Grano’, ‘Red Burgundy’.
Intermediate-Day (Day-Neutral) Onions: Bulb with 12-14 hours of daylight. The most flexible type, great for most of California, especially the Central Valley. Plant in late winter. Varieties: ‘Candy’, ‘Stockton Red’.
Long-Day Onions: Need 14-16 hours of daylight to bulb. Best for Northern California with long summer days. Plant in spring. Varieties: ‘Walla Walla’, ‘Yellow Sweet Spanish’.

Picking the wrong type means your onions may never form good bulbs. Check seed packets carefully for this information.

Step-by-Step Planting Guide

Once you’ve got your timing and onion type sorted, follow these steps for success.

1. Preparing Your Soil

Onions need loose, well-draining soil with plenty of nutrients. They are heavy feeders.

Choose a spot with full sun (at least 6-8 hours).
Work the soil to a depth of about 8 inches, removing any rocks or clumps.
Mix in a generous amount of compost or well-rotted manure. Onions prefer a slightly acidic to neutral pH (6.0-7.0).
Avoid fresh manure, as it can introduce diseases and cause too much leafy growth.

2. Planting Seeds, Sets, or Transplants

You have three options for starting your crop. Each has it’s pros and cons.

Seeds: Most economical and offers the most variety. Start indoors 8-10 weeks before your outdoor planting date. Sow seeds ¼ inch deep. Transplant seedlings when they are about the thickness of a pencil.
Sets: These are small, dormant onion bulbs. They are the easiest and fastest. Plant them pointy-end up, just deep enough so the tip is level with the soil surface, about 4-6 inches apart.
Transplants: These are young seedlings sold in bunches. They give you a head start. Separate the plants gently and plant them about 1 inch deep, spacing 4-6 inches apart.

Rows should be spaced about 12-18 inches apart to allow for weeding and growth.

3. Watering and Feeding

Consistent care is crucial for developing bulbs.

Water regularly, keeping the soil evenly moist but never soggy. Onion roots are shallow.
Reduce watering as the bulbs mature and the tops begin to yellow and fall over—this helps them cure properly.
Feed with a balanced fertilizer or a side-dressing of compost every few weeks during the growing season until the bulbs begin to swell.

4. Weeding and Maintenance

Onions don’t compete well with weeds. Keep the area meticulously weeded, but be careful not to damage the shallow roots. A layer of straw or light mulch can help suppress weeds and retain moisture, which is especially helpful in warmer areas.

Common Problems and Solutions

Even with perfect timing, you might face a few issues.

Bolting: When an onion sends up a flower stalk. Caused by temperature fluctuations (a cold snap after warm weather) or planting the wrong type. The bulb stops growing and won’t store well. Harvest and use bolted onions first.
Small Bulbs: Usually from planting too late, using the wrong day-length type, or overcrowding. Ensure correct spacing and timing.
Pests: Onion thrips and maggots are common. Use floating row covers as a barrier and practice crop rotation—don’t plant onions in the same spot more than once every three years.
Diseases: Fungal issues like mildew can occur in damp conditions. Ensure good air circulation, avoid overhead watering, and rotate crops.

Harvesting and Storing Your Onions

Harvest time is the reward for your good timing.

Onions are ready when the tops turn yellow, brown, and flop over. Gently push over any stragglers to start the curing process.
On a dry day, carefully lift the bulbs with a garden fork. Let them lay on the soil surface for 1-2 days to dry, weather permitting.
Move them to a warm, dry, well-ventilated place out of direct sun (like a garage or covered porch) to cure for 2-4 weeks until the necks are tight and the outer skins are papery.
Once cured, trim the roots and cut the tops back to about 1 inch. Store in a cool, dry, dark place in mesh bags or a single layer. Check periodically for soft onions and use them right away.

FAQ: Planting Onions in California

Can I plant onions in spring in Southern California?

You can, but it’s not ideal. Spring-planted onions in mild-winter areas will have a very short growth period before summer heat arrives, often resulting in small bulbs. Fall planting is strongly preferred.

What is the best month to plant onions in Northern California?

For most of Northern California, aim for March. In the coldest mountain areas, wait until April. Always go by soil temperature and frost dates, not just the calendar.

How late is too late to plant onions?

If you plant after the spring has warmed significantly, the onions won’t have enough time to develop substantial foliage before triggering bulb formation. This leads to undersized bulbs. In most inland areas, planting after April is usually to late for a good harvest.

Can I grow onions from grocery store scraps?

You can regrow the green tops from a scallion or onion bottom in water for fresh greens, but you won’t get a new full-sized bulb from it. For bulb onions, it’s best to start with seeds, sets, or transplants specifically meant for gardening.

Do onions need a lot of water?

They need consistent moisture, especially during bulb formation. However, they hate soggy, waterlogged soil, which causes rot. Deep, infrequent watering that soaks the root zone is better than daily light sprinkling.

By matching your planting time to your local climate and choosing the correct onion type, you’ll set yourself up for a succesful and bountiful harvest. Paying attention to these details makes all the difference between a handful of small onions and a season’s supply of beautiful, storage-worthy bulbs. Grab your calendar, check your zone, and get those onions in the ground at the right time.