How To Grow Walla Walla Onions

If you want sweet, juicy onions that are perfect for salads and grilling, learning how to grow Walla Walla onions is a great project. These famous sweet onions thrive with a long, cool growing season, and with the right steps, you can harvest your own from the garden.

This guide walks you through everything from choosing seeds to storing your harvest. The process is straightforward, even for beginners, as long as you pay attention to timing and a few key needs.

Success with these onions starts with understanding their life cycle. They are long-day onions, which means they need the extended daylight hours of northern summers to form large bulbs. Planting at the correct time is the most critical factor for a good yield.

What You’ll Need to Get Started

Gathering your supplies ahead of time makes the planting process smooth. You won’t need anything too fancy, just a few basics.

Walla Walla Onion Seeds or Sets: Seeds offer more variety and are typically more economical. Sets (small bulbs) can give you a head start but ensure they are specifically Walla Walla, as some sets are for short-day types.
Garden Bed or Containers: A sunny spot with well-draining soil is essential. If your soil is heavy clay, raised beds are a excellent option.
Compost or Well-Rotted Manure: These onions are heavy feeders and need rich, fertile soil.
Balanced Organic Fertilizer: Look for something with a balanced N-P-K ratio, like 10-10-10.
Mulch: Straw or shredded leaves help retain moisture and suppress weeds.

Step 1: Timing Your Planting Perfectly

This is the step where most gardeners make a mistake. Walla Walla onions have a long growing season—about 180 days from seed. They grow through cool weather to bulb up as days lengthen.

In Cold Climates (USDA zones 6 and below): Start seeds indoors 10-12 weeks before your last spring frost date. Transplant seedlings outdoors 4-6 weeks before the last frost.
In Milder Climates (USDA zones 7 and above): You can sow seeds directly in the garden in late summer or early fall for a late spring harvest the following year. Alternatively, plant sets in very early spring.

Getting them in the ground early allows them to establish strong roots and leaves before the bulb-forming signal of long days.

Step 2: Preparing Your Soil

Onions need loose, nutrient-rich soil to expand and grow large. Poor soil leads to small bulbs.

Choose a location that gets full sun—at least 6-8 hours of direct light daily.
Work the soil to a depth of about 12 inches, breaking up any large clumps.
Mix in a generous 3-4 inch layer of compost or aged manure. Add a balanced fertilizer according to package directions and incorporate it well.
Rake the bed smooth, removing any stones or debris that could obstruct bulb growth.

Step 3: Planting Seeds and Seedlings

If Starting from Seeds Indoors:

Sow seeds in trays about 1/4 inch deep. Keep the soil moist and warm (around 70°F) for germination. Once they sprout, provide plenty of light—a grow light is best to prevent leggy seedlings. Trim the tops back to about 3 inches tall a few weeks before transplanting to encourage sturdier growth.

Transplanting Outdoors:

When seedlings are pencil-thick and outdoor soil is workable, it’s time to harden them off. This means exposing them to outdoor conditions gradually over 7-10 days. Then, plant them in the prepared bed.

Space plants 4-6 inches apart in rows that are 12-18 inches apart. The wider spacing allows for larger bulbs.
Plant them so the white base is just barely covered with soil. The green shoot should be completely above ground.
Water them in thoroughly to settle the soil around the roots.

If Planting Sets or Direct Seeding:

Plant sets just deep enough to cover the bulb, with the tip showing. For direct seeding, sow seeds 1/2 inch deep and 1 inch apart, thinning later to the proper spacing. The thinned greens are delicious in salads!

Step 4: Watering, Feeding, and Care

Consistent care through the season is simple but vital. Onions have shallow roots, so they dry out fast.

Watering: Provide about 1 inch of water per week. The soil should be consistently moist but never soggy. Reduce watering as the bulbs mature and the tops begin to yellow and fall over.
Fertilizing: Feed every 2-3 weeks with a nitrogen-rich fertilizer until the bulbs begin to form (you’ll see the soil start to crack and push away). Then, stop fertilizing.
Weeding: Keep the area weed-free. Weeds compete fiercely for nutrients and water. Mulching heavily after planting will save you alot of work.
Pest Watch: Onion thrips and maggots can be a problem. Use row covers early to prevent maggot flies. A strong spray of water can dislodge thrips. Good garden cleanliness helps prevent issues.

Step 5: Knowing When and How to Harvest

Harvesting at the right time ensures the best flavor and storage potential. Patience is key here.

In mid to late summer, you’ll notice the green tops start to turn yellow and flop over. This is the onion’s natural sign that it’s done growing. Once about half to three-quarters of the tops have fallen over, stop watering completely.

Wait for a dry, sunny spell. Then, gently lift the bulbs from the soil with a garden fork, being careful not to bruise them. Let them lay right on the garden soil for 1-2 days to begin curing, weather permitting.

Step 6: Curing and Storing Your Harvest

Curing is the process of drying the outer layers to form a protective papery skin. Without proper curing, your onions will rot quickly.

After their initial sun bath, move the onions to a warm, dry, well-ventilated place out of direct sun. A garage, covered porch, or shed is perfect.
Spread them out in a single layer or hang them in bunches. Good air circulation is crucial.
Let them cure for 2-4 weeks until the necks are completely tight and dry and the outer skins are papery.
Trim the roots and cut the tops down to about 1 inch, unless you plan to braid them.

Store cured onions in a cool, dry, dark place in mesh bags or single layers. Check them occasionally and use any that feel soft first. Remember, Walla Wallas are a sweet, high-moisture onion and won’t keep as long as storage varieties—plan to use them within a few months.

Troubleshooting Common Problems

Bulbs Are Too Small

This usually means the plants were crowded, planted too late, or didn’t get enough nutrients or water during the leafy growth stage. Ensure proper spacing and follow the fertilizing schedule.

Onions Bolting (Sending Up a Flower Stalk)

Bolting is often caused by temperature fluctuations, like a sudden cold snap after warm weather. Once an onion bolts, the bulb stops growing and won’t store well. Use the bulb soon after harvest. Choosing the right planting time for your climate is the best prevention.

Soft Bulbs or Rot in Storage

This indicates the onions weren’t cured thoroughly or were damaged during harvest. Always handle them gently and ensure they are fully cured before trimming and storing. Discard any that show signs of spoilage quickly so it doesn’t spread.

FAQ

Can I grow Walla Walla onions in the south?
It can be challenging. Since they are long-day onions, they require the long summer days of northern latitudes to form bulbs. In the deep south, short-day onion varieties are a more reliable choice.

How do you grow Walla Walla sweet onions in containers?
Yes, you can! Use a container at least 12 inches deep and wide. Ensure it has excellent drainage holes. Use a high-quality potting mix and be even more vigilant about watering, as containers dry out faster. Follow the same spacing and feeding guidelines.

What makes Walla Walla onions so sweet?
Their high water content and lower levels of pyruvic acid (the compound that makes onions pungent) give them their mild, sweet flavor. The unique growing conditions of the Walla Walla valley region contribute, but you can get very sweet onions from your own garden with the right care.

Can I save seeds from my Walla Walla onions?
Onions are biennial, meaning they produce seeds in their second year. To save seeds, you would need to select your best bulbs, store them over winter, and replant them in spring to flower and set seed. It’s a fun project but requires extra space and time. Most gardeners find it easier to buy fresh seeds each year.

Growing your own Walla Walla onions is incredibly satisfying. The key takeaways are to plant early in rich soil, keep them well-watered and weeded, and be patient for the harvest. With this simple step-by-step guide, you’re well on your way to enjoying a homegrown crop of these sweet, versatile onions.