Green Onions in Zone 5B — Mountain West

Green onions are a must-grow for any Mountain gardener. Their fresh, mild flavor adds zest to everything from summer salads to grilled meats, and they’re incredibly versatile in the kitchen. Plus, there's nothing quite like the satisfaction of snipping a few stalks from your own garden, especially knowing those cool nights even in summer contribute to their exceptional taste.

Our high altitude, intense sun, and short season can present challenges, but green onions are surprisingly adaptable. With a little attention to timing within our 149-day growing season, you can enjoy a continuous harvest all summer long.

If you're starting green onions indoors, aim to transplant them outdoors from early April through early May. Before planting, be sure to harden them off. This means gradually exposing your seedlings to the outdoors for about a week, increasing their time outside each day, so they can handle our intense sun.

Plant your seedlings 2-3 inches apart in well-drained soil. Keep an eye on the weather forecast, as late frosts can still occur during this time. Be ready to cover your young plants if temperatures dip unexpectedly.

Direct sowing is the easiest way to grow green onions in our region. Sow seeds directly into the garden from early April through mid July. They’re pretty forgiving!

Choose a spot with full sun to partial shade and prepare the soil by loosening it and adding compost. Sow seeds about ¼ inch deep and 2-3 inches apart. Keep the soil consistently moist until germination, which usually takes about a week or two. For a continuous harvest, sow seeds every few weeks. And don’t forget, you can also 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 dry Mountain climate. Aim to keep the soil consistently moist, but not soggy. The goal is tender, mild stalks, so don't let them dry out. Their shallow roots mean they can dry out quickly.

During the warmer months, from late May through August, you'll likely 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 to avoid wetting the foliage, which can increase the risk of disease in our cool nights.

Signs of underwatering include wilting or yellowing leaves, while overwatering can lead to yellowing leaves and potentially root rot. A light layer of mulch, like straw or wood chips, can help retain moisture in the soil and regulate soil temperature, which is especially helpful given our intense sun.

You can expect your first harvest of green onions starting in early June and continuing through early October. They are typically ready to harvest about 60 days after planting, 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 soil line. If you cut them, they will often regrow for a second harvest. For continuous production, harvest only what you need, leaving the rest to grow.

As our first frost approaches in early October, harvest any remaining green onions. You can chop and freeze them for later use, or transplant them into pots and bring them indoors to extend the harvest a bit longer.

Here are some common problems you might encounter with green onions in the Mountain West:

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: These pests lay their eggs near the base of onion plants, and the larvae burrow into the soil to feed.
• How to fix/prevent it: Use row covers to prevent adult flies from laying eggs. Apply beneficial nematodes to the soil to control larvae. Rotate crops each year to disrupt the pest's life cycle.

Thrips

• What it looks like: Silver streaks or white patches on the leaves, distorted growth, and tiny black or yellow insects on the plant.
• What causes it: Thrips are small, slender insects that suck plant juices, causing damage to leaves and stems.
• How to fix/prevent it: Spray plants with insecticidal soap or neem oil. Encourage beneficial insects like ladybugs and lacewings. Keep the garden free of weeds and debris.

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.
• 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: Pink or reddish roots, stunted growth, and yellowing leaves.
• What causes it: A soilborne fungus that infects the roots, causing them to turn pink and rot.
• How to fix/prevent it: Improve soil drainage. Rotate crops each year. Plant resistant varieties where available. Solarize the soil in the summer to kill the fungus.

Mountain West Specific Challenges: Our moderate-at-altitude heat and low humidity can actually help reduce the risk of some fungal diseases compared to more humid regions. However, our intense sun can stress plants, making them more susceptible to pests. Consistent watering and providing some afternoon shade during the hottest part of the day can help keep your green onions healthy.

Green onions are great companions for many plants in the Mountain West garden. Carrots benefit from green onions because the onion's scent deters carrot root flies. Lettuce thrives alongside green onions, as they provide shade and help retain moisture. Tomatoes and peppers also appreciate green onions nearby, as they can repel aphids and other pests. Beets are another good companion, as green onions can help deter leaf miners.

Avoid planting beans and peas near green onions, as they can inhibit each other's growth. Onions are allelopathic, meaning they release chemicals that can stunt the growth of legumes.