How To Grow Walla Walla Onions

If you want to learn how to grow Walla Walla onions, you’re in the right place. Growing sweet, mild Walla Walla onions requires a long, cool growing season and specific planting techniques for success. These famous sweet onions are a garden treasure, but they do need the right conditions to form their large, juicy bulbs.

This guide will walk you through every step. We’ll cover planting, care, harvesting, and storage. With the right knowledge, you can enjoy a bountiful harvest of these sweet onions from your own backyard.

Walla Walla onions are a long-day variety that originated in the Walla Walla Valley of Washington State. They are known for their exceptionally mild flavor and high water content, which makes them perfect for fresh eating. Unlike storage onions, they are best enjoyed soon after harvest, though proper curing can extend their shelf life.

The key to success is understanding their need for a long, cool growing period. They are typically planted in early spring as soon as the soil can be worked. In many regions, they are grown over the winter for a summer harvest. Let’s break down the process into manageable steps.

Understanding Your Climate And Season

Walla Walla onions need about 180 days to reach full maturity from seed. This long growing season dictates your planting strategy. They thrive in cool conditions at the start and finish with warmer weather for bulbing.

Gardeners in northern states with cold winters often plant seeds indoors very early. They transplant seedlings in spring for a late summer harvest. In milder climates with frost-free winters, seeds are sown in late summer or fall. The onions grow through the winter and are harvested the following early summer.

Check your USDA hardiness zone and local frost dates. This is the first step to planning your planting schedule correctly. Getting the timing wrong is a common reason for small bulbs or bolting.

Determining Your Planting Window

Your planting date depends on your method: seeds, sets, or transplants.

From Seed Indoors: Start seeds 10-12 weeks before your last expected spring frost date. This is often in late winter.
From Seed Outdoors: Sow seeds directly in the garden 4-6 weeks before the last frost, as soon as soil is workable.
From Sets or Transplants: Plant these in the garden 4-6 weeks before the last spring frost date.
Fall Planting (Mild Climates): Sow seeds directly in late summer or early fall, about 8 weeks before the first fall frost.

Choosing The Right Planting Site And Soil

Onions are heavy feeders that demand excellent soil. The planting site can make or break your harvest.

Choose a location that gets full, direct sunlight for at least 6-8 hours per day. Good air circulation helps prevent fungal diseases. Avoid areas where other alliums (like garlic or leeks) were grown recently to reduce pest and disease risk.

The soil must be loose, well-draining, and rich in organic matter. Walla Wallas need consistent moisture but will rot in soggy, compacted earth.

Preparing Your Garden Bed

Soil preparation should begin a few weeks before planting. Follow these steps for the best foundation.

Clear the area of weeds and debris.
Loosen the soil to a depth of at least 12 inches using a garden fork or tiller.
Mix in 2-4 inches of well-rotted compost or aged manure. This improves drainage, fertility, and moisture retention.
Test your soil pH. Onions prefer a slightly acidic to neutral pH, between 6.0 and 7.0. Amend with garden lime if your soil is too acidic.
Incorporate a balanced, slow-release organic fertilizer or a formula higher in phosphorus (the middle number) to encourage root and bulb development.

Planting Walla Walla Onions Correctly

Proper planting depth and spacing are crucial. Crowded onions will compete for nutrients and produce smaller bulbs.

You can plant Walla Wallas from seed, sets (small bulbs), or nursery-grown transplants. Starting from seed offers the most variety and is most economical, but transplants give you a head start.

Planting From Seed Indoors

Starting seeds indoors gives you control over the early growth phase.

Fill seed trays or pots with a sterile seed-starting mix.
Sow seeds about 1/4 inch deep, placing 2-3 seeds per cell.
Keep the soil consistently moist and warm (65-75°F) for germination, which takes 7-10 days.
Once sprouted, provide plenty of light from a sunny window or grow lights to prevent leggy seedlings.
When seedlings are a few inches tall and have a couple of sets of leaves, thin to the strongest one per cell.
Harden off seedlings for 7-10 days before transplanting them outside.

Direct Seeding and Transplanting Outdoors

When transplanting or direct sowing, handle the delicate plants with care.

Spacing: Space plants 4-6 inches apart in rows that are 12-18 inches apart. This gives each bulb room to expand.
Depth: Plant transplants or sets so the base of the green stem is level with the soil surface. Do not bury them too deeply.
Direct Seeding: Sow seeds 1/4 inch deep and 1 inch apart. Once seedlings are established, thin them to the final 4-6 inch spacing, using the thinned green onions in your kitchen.

Water the bed thoroughly immediately after planting to settle the soil around the roots.

Essential Care During The Growing Season

Consistent care is what transforms small plants into large, sweet bulbs. The main priorities are water, nutrients, and weed control.

Watering and Mulching

Onions have shallow root systems and need consistent moisture. Uneven watering leads to split or pungent bulbs.

Provide 1-2 inches of water per week, including rainfall. Water deeply to encourage roots to grow downward.
Use a soaker hose or drip irrigation to keep foliage dry and prevent disease.
Apply a 2-3 inch layer of organic mulch, like straw or shredded leaves, around the plants. This conserves moisture, suppresses weeds, and keeps soil temperatures cool.
Reduce watering in the final few weeks before harvest to allow the bulbs to mature and the outer skins to tighten.

Fertilizing for Optimal Growth

Feed your onions regularly but avoid over-fertilizing with nitrogen, which promotes top growth at the expense of the bulb.

At Planting: Use a balanced fertilizer or one higher in phosphorus (like a 5-10-10) worked into the soil.
Early Growth (4-6 weeks after planting): Side-dress with a nitrogen source, such as blood meal or a balanced organic fertilizer, to support green leaf development.
Bulb Initiation: When days lengthen and the base of the stem begins to swell, side-dress again with a fertilizer lower in nitrogen and higher in potassium to support bulb growth.
Stop fertilizing once the bulbs have significantly enlarged, about a month before harvest.

Weed Control and Other Maintenance

Weeds compete fiercely with onions for water and nutrients. Hand-pull weeds carefully to avoid disturbing the onion’s shallow roots. The mulch layer you applied will do most of the work for you.

As bulbs begin to form, you may notice the soil crack around them. Gently push soil back over the exposed shoulders, but avoid covering the plant itself. Exposed bulbs can get sunscald.

Managing Common Pests And Diseases

Walla Walla onions can be affected by a few specific issues. Vigilance and good garden hygiene are your best defenses.

Common Onion Pests

Onion Thrips: Tiny insects that cause silvery streaks on leaves. Blast them off with water or use insecticidal soap.
Onion Maggots: The larvae of a fly that burrows into bulbs. Use floating row covers at planting to prevent egg-laying. Rotate crops each year.
Slugs and Snails: They chew on leaves, especially in damp, mulched beds. Use organic bait or traps to control them.

Preventing Fungal Diseases

Diseases like downy mildew and botrytis neck rot are often caused by wet foliage and poor air circulation.

Water at the base of plants, not overhead.
Ensure proper spacing for good air flow.
Remove and destroy any plants showing signs of disease immediately.
Avoid planting onions in the same spot more than once every three to four years.

Knowing When And How To Harvest

Harvesting at the right time ensures the best flavor and storage potential. Watch for these visual cues.

In late summer, the onion tops will naturally begin to yellow, flop over, and dry out. This is the plant signaling that it is done growing. Once about half to three-quarters of the tops have fallen over, it’s time to stop watering and prepare for harvest.

The Harvesting Process

Choose a dry, sunny day for harvesting.
Gently loosen the soil around the bulb with a garden fork, being careful not to pierce it.
Lift the bulbs from the soil and brush off any loose dirt. Do not wash them.
If the weather is dry and warm, you can leave the bulbs on the soil surface for 1-2 days to begin drying. If rain is forecast, move them to a protected area immediately.

Curing And Storing Your Harvest

Curing is the most critical step for preparing Walla Walla onions for storage. It dries the outer layers to form a protective wrapper.

Spread the harvested onions in a single layer in a warm, dry, well-ventilated place out of direct sun. A garage, covered porch, or shed is ideal. Allow them to cure for 2-3 weeks until the necks are completely tight and dry and the outer skins are papery.

Once cured, trim the roots and cut the dried tops down to about 1 inch above the bulb. Gently wipe off any remaining soil.

Best Practices for Storage

Remember, Walla Wallas are sweet onions with high water content. They will not store as long as pungent storage varieties.

Store cured onions in a cool (35-45°F), dry, dark place with good air circulation.
Use mesh bags, nylon stockings, or slatted crates. Never store them in plastic bags, as this traps moisture and causes rot.
Check stored onions regularly and use any that show signs of softening or sprouting first.
With proper curing and storage, you can expect your Walla Walla onions to last for 1-3 months.

Saving Seeds For Next Season

Walla Walla onions are biennial. They produce a bulb in the first year and a flower stalk and seeds in the second. To save seeds, you must overwinter some bulbs.

Select the best, healthiest bulbs from your harvest. Replant them in the garden the following spring. They will send up a tall flower stalk. Allow the flower head to dry completely on the plant. Then cut it off, place it in a paper bag, and shake to release the seeds. Store the black seeds in a cool, dry place for next year’s planting.

Frequently Asked Questions

How Long Do Walla Walla Onions Take To Grow?

From seed to harvest, Walla Walla onions typically require a long growing season of approximately 180 days. This is why timing your planting correctly for your climate is so essential. Starting seeds indoors is often necessary in regions with shorter summers.

Can You Grow Walla Walla Onions From An Onion?

You cannot regrow a full bulb from a harvested onion. However, if you plant a Walla Walla onion set (a small, dormant bulb) or a transplant, it will grow into a new, full-sized bulb. The most reliable method is to use seeds, sets, or transplants purchased for gardening.

What Is The Best Fertilizer For Walla Walla Onions?

Onions benefit from a balanced fertilizer at planting and a side-dressing of a nitrogen source during early leaf growth. When bulbing begins, switch to a fertilizer with less nitrogen and more potassium, such as a 0-10-10 mix. Avoid excessive nitrogen late in the season, as it can hinder bulb formation and reduce storability.

Why Are My Walla Walla Onions So Small?

Small bulbs are usually caused by incorrect planting time, overcrowding, insufficient water, or poor soil nutrition. Ensure you plant early enough for a long growing season, thin seedlings to proper spacing, provide consistent moisture, and fertilize appropriately throughout the growth stages. Competition from weeds is another common culprit.

How Do You Prevent Walla Walla Onions From Bolting?

Bolting (premature flowering) is often triggered by temperature fluctuations, particularly a period of cold weather after growth has started. To minimize the risk, plant at the correct time for your zone, use mulsh to moderate soil temperature, and choose healthy transplants that are not root-bound. If a plant bolts, harvest and use that onion immediately, as the bulb will not continue to grow and its storage quality is poor.