Green Onions in Zone 9A — Northern California

Green onions are a fantastic addition to any Northern California garden. Their mild, fresh flavor elevates everything from tacos to salads, and they're incredibly easy to grow in our Mediterranean climate. Plus, you can snip off what you need without harvesting the entire plant, which is perfect for the home cook.

While we face challenges like drought and occasional water restrictions, our long growing season—roughly 288 days—gives us plenty of time to successfully cultivate green onions. With the right timing and a little attention to watering, you can enjoy a continuous supply of these versatile veggies.

You can transplant green onion seedlings outdoors in late January through mid February here in Northern California. Before planting, be sure to harden off your seedlings by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the sun and wind.

Space the seedlings 2-3 inches apart in well-draining soil. Keep an eye on the weather, as we sometimes experience unexpected temperature swings during this time of year. A row cover can provide extra protection if a cold snap is forecast.

Direct sowing is the easiest way to grow green onions in our region, and you can do it from late January through late September. Choose a spot with full sun to partial shade and prepare the soil by loosening it and adding compost. Green onions aren't too picky, but good drainage is always important, especially with our winter rains.

Sow seeds about ½ inch deep and 2-3 inches apart. Keep the soil consistently moist until germination, which usually takes about a week. You can also plant the root ends of store-bought green onions! Just stick them in the ground, and they'll regrow.

Green onions need consistent moisture to produce tender, mild stalks, but they aren’t drought-tolerant, which can be a challenge in our dry Northern California summers. Their shallow roots mean they can dry out quickly, especially in containers.

During the cooler, wetter months (late fall through early spring), rainfall might be enough. But during our dry summers, you'll need to water deeply about 1-2 times per week, providing about 1 inch of water each time. Use the "finger test"—stick your finger about 2 inches into the soil. If it feels dry, it's time to water. Water at the base of the plants rather than overhead to prevent fungal diseases, which can still occur even with our low humidity.

Signs of underwatering include wilting or yellowing leaves. Overwatering can lead to root rot, indicated by mushy or discolored stalks. Mulching around your green onions with straw or wood chips helps retain moisture and keeps the soil cooler during the hotter months, which is especially helpful with our water restrictions.

You can expect your first harvest of green onions in late March through mid December, about 60 days after planting. They're ready to harvest when the stalks are about pencil-thick.

To harvest, you can either pull up the entire plant or cut the stalks about an inch above the base. If you cut them, they'll regrow, giving you continuous harvests. To encourage continued production, be sure to water and fertilize regularly after harvesting.

As our first frost approaches in early December, you can either harvest all remaining green onions or cover them with a thick layer of mulch to protect them from the cold. They can often survive a light frost and keep producing through the winter, especially in our milder microclimates.

Here are some common problems you might encounter with green onions in Northern California:

• Onion Maggots
• What it looks like: Wilting or yellowing leaves, stunted growth, and small white maggots feeding on the roots and bulb.
• What causes it: These pests thrive in moist soil and can be attracted to the smell of onions, especially during the spring planting season.
• How to fix/prevent it: Use row covers to prevent adult flies from laying eggs. Rotate crops each year. Introduce beneficial nematodes to the soil.
• Thrips
• What it looks like: Silvery or white streaks on the leaves, distorted growth, and tiny, slender insects visible on the plant.
• What causes it: Thrips are more prevalent during our dry summers, as they prefer hot, dry conditions.
• How to fix/prevent it: Encourage beneficial insects like ladybugs and lacewings. Spray plants with insecticidal soap or neem oil, especially during dry spells.
• Downy Mildew
• What it looks like: Yellow patches on leaf tops with fuzzy gray-purple growth on undersides. Leaves brown and die from the bottom up.
• What causes it: An oomycete pathogen that spreads in cool, humid conditions via wind-carried spores. Different species affect different plants.
• How to fix/prevent it: Remove affected leaves immediately. Improve air circulation. Water at the base, never overhead. Copper-based fungicides for prevention. Plant resistant varieties where available.
• Pink Root
• What it looks like: Roots turn pink or reddish, and the plant's growth is stunted.
• What causes it: A soilborne fungus that thrives in warm, moist soil.
• How to fix/prevent it: Improve soil drainage. Rotate crops to avoid buildup of the fungus. Solarize the soil during the hottest part of the summer to kill the fungus.

Northern California Specific Challenges: Our moderate-to-hot summers, low humidity, and dry-summer rainfall present unique challenges. Hot weather can stress plants, making them more susceptible to pests and diseases. Water restrictions require careful watering practices to conserve water while keeping plants healthy. Monitor your plants closely and adjust your care as needed to adapt to our variable microclimates.

Companion planting can really boost your green onion crop here in Northern California. Carrots are great companions because their scent deters onion flies. Lettuce benefits from the protection green onions offer against aphids. Tomatoes and peppers thrive alongside green onions, as the onions help repel certain pests that can damage these crops. Beets also make good companions, as they don't compete for the same nutrients in the soil.

Avoid planting green onions near beans and peas. These legumes can inhibit the growth of onions, as they prefer different soil conditions and can compete for resources.