Green Onions in Zone 8B — Desert Southwest

Green onions are a must-grow in our Desert Southwest gardens. Their mild, fresh flavor adds a zing to everything from tacos to salads, and there’s nothing quite like the satisfaction of snipping them fresh from your own backyard. Plus, they're super versatile, thriving in containers or tucked into garden beds, making them perfect for our sometimes-challenging landscapes.

Sure, desert gardening comes with its own set of hurdles, like extreme heat and alkaline soil. But with our long, 279-day growing season and careful timing, you can enjoy a continuous harvest of green onions. Knowing when to plant and how to water is key to summer survival for these tasty alliums.

You can transplant green onion seedlings outdoors in late January through late February. This gets them established before the real heat hits. Before you put them in the ground, be sure to harden them off by gradually exposing them to outdoor conditions for about a week.

When transplanting, space them about 2-3 inches apart. Keep an eye on the weather, as we can still get some chilly nights and even the occasional freeze this time of year. Protect your young transplants with frost cloth if needed.

Direct sowing is the easiest way to grow green onions in our region, and you can do it from late January through mid September. Choose a spot with well-draining soil and plenty of sunshine, though some afternoon shade can be beneficial during the hottest months. Amend the soil with compost to improve drainage and fertility.

Sow seeds about ¼ inch deep and 2-3 inches apart. Keep the soil consistently moist until germination, which usually takes about a week. For a continuous harvest, sow a new batch of seeds every few weeks. You can even plant the root ends of store-bought green onions for a quick and easy start!

Green onions need consistent moisture to thrive, especially in our arid climate. They aren’t drought-tolerant and their shallow roots dry out quickly. During the cooler spring and fall months, aim for about 1 inch of water per week.

In the extreme heat of summer, you’ll likely need to increase that to 2-3 inches per week, especially during the monsoon season when humidity fluctuates. 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 to avoid wetting the foliage, which can encourage fungal diseases in our unpredictable monsoon season.

Signs of underwatering include yellowing or wilting leaves, while overwatering can lead to root rot. A layer of mulch around your green onions helps retain moisture and keeps the soil cooler.

You can expect your first harvest of green onions in late March through early December, about 60-70 days after planting. They're ready to harvest when the stalks are about pencil-thick. You can either pull the entire plant or cut the green onion about an inch above the soil line.

If you cut above the base, the plant will regrow, giving you multiple harvests. For a continuous supply, stagger your plantings every few weeks. As the first frost approaches in early December, harvest any remaining green onions to use fresh or preserve.

Here are some common problems you might encounter growing green onions in the Desert Southwest:

• Onion Maggots
• What it looks like: Wilting or yellowing plants, stunted growth, and small white maggots feeding on the roots and bulb.
• What causes it: Onion maggots are the larvae of a fly that lays its eggs near the base of onion plants. They thrive in moist soil.
• How to fix/prevent it: Use row covers to prevent the flies from laying eggs. Apply beneficial nematodes to the soil to control the larvae. Remove and destroy any infested plants.
• Thrips
• What it looks like: Silvery or white streaks on the leaves, distorted growth, and tiny, slender insects crawling on the plants.
• What causes it: Thrips are tiny insects that suck the sap from plants, causing damage. They are more prevalent during hot, dry weather.
• How to fix/prevent it: Spray plants with insecticidal soap or neem oil. Encourage beneficial insects like ladybugs and lacewings. Provide adequate water to reduce plant stress.
• 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: Oomycete pathogen that spreads in cool, humid conditions via wind-carried spores.
• How to fix/prevent it: Remove affected leaves immediately. Improve air circulation. Water at the base, never overhead. Copper-based fungicides for prevention.
• Pink Root
• What it looks like: Roots turn pink or reddish, and the plant may be stunted or wilted.
• What causes it: A soilborne fungus that infects the roots, especially in warm, alkaline soils.
• How to fix/prevent it: Improve soil drainage. Rotate crops to avoid planting onions in the same location year after year. Amend the soil with organic matter. Use resistant varieties if available.

Desert Southwest Specific Challenges: The extreme heat and low humidity can stress green onions, making them more susceptible to pests and diseases. The monsoon season can bring periods of high humidity, which can promote fungal diseases like downy mildew. Alkaline soil can also be a challenge, so amend your soil with compost to improve its pH and drainage.

Green onions play well with others in the garden! Carrots make excellent companions because their scent deters onion flies. Lettuce benefits from the shade provided by green onions, which helps prevent bolting in our hot climate. Tomatoes and peppers also benefit from the pest-repelling properties of green onions. Beets are good companions as well because they don't compete for the same nutrients.

Avoid planting green onions near beans and peas. These legumes can inhibit the growth of onions.