Walla Walla onions demand a long, cool growing season, making early indoor sowing essential for their signature sweetness. If you have been searching for how to grow walla walla onions, you have likely discovered that these aren’t your average storage onions. They are fresh-eating giants that require a bit more patience and attention than standard sets.
These onions are famous for their mild, sweet flavor and their massive size. Some bulbs can easily reach the size of a softball. However, achieving that perfect sweetness starts long before you ever put a seed in the ground. It starts with understanding their unique photoperiod requirements and their need for consistent moisture.
In this guide, we will walk you through every step. We will cover everything from seed starting to harvest. You will learn the specific tricks to get these beauties growing in your own backyard.
Why Walla Walla Onions Are Different
Before you even think about planting, you need to know what you are dealing with. Walla Walla onions are a “long-day” onion variety. This means they need about 14 to 15 hours of daylight to trigger bulb formation.
This makes them perfect for northern gardeners. If you live in the South, these onions will likely just grow tops and never form a decent bulb. They are specifically bred for the Pacific Northwest, but they thrive in any region with cool springs and long summer days.
Another major difference is their storage life. Unlike yellow or red storage onions, Walla Wallas do not keep well. They have a high water content, which gives them that crisp bite, but it also means they spoil quickly. You should plan to eat them within a few months of harvest.
How To Grow Walla Walla Onions
Now, let’s get into the meat of the process. The success of your crop hinges on timing and temperature. You cannot just throw seeds outside in the spring and hope for the best. You must start them indoors during the winter.
Starting Seeds Indoors
Timing is everything here. You need to start your seeds about 10 to 12 weeks before your last expected frost date. For most northern gardeners, this means starting seeds in January or early February.
Walla Walla seeds are tiny and need light to germinate. Do not bury them deep. Press them gently into the surface of a moist seed-starting mix. Then, lightly dust them with a thin layer of vermiculite or soil.
Here are the key steps for successful germination:
- Use a heat mat to keep the soil temperature around 65°F to 70°F.
- Keep the soil consistently moist but not soggy.
- Place the tray under a grow light immediately after sowing.
- Germination usually takes 7 to 10 days.
Once the seeds sprout, move them to a cooler location. A temperature of around 55°F to 60°F is ideal for young seedlings. This prevents them from getting leggy and weak.
Providing The Right Light
Indoor seedlings need a lot of light. A sunny windowsill is rarely enough. You will need to invest in a decent grow light. Keep the light on for about 14 hours a day. Keep the light source just a few inches above the tops of the plants.
If you notice the seedlings are falling over or looking pale, they are not getting enough light. Raise the intensity or lower the light closer to the tray. Strong light now will result in strong transplants later.
Hardening Off The Seedlings
About two weeks before you plan to transplant, you need to start hardening off the plants. This is a critical step that many beginners skip. You cannot move them from a warm, cozy house directly into the cold ground.
Start by placing the trays outside in a sheltered, shady spot for a few hours each day. Gradually increase their exposure to sunlight and wind over the two-week period. This process toughens up the cell walls of the plant, preparing them for the harsh outdoor environment.
Walla Walla onions are cold-hardy. They can survive frosts and even light freezes. However, they do not like extreme temperature swings. Hardening off helps them adjust to those swings.
Preparing The Garden Bed
While your seedlings are growing, you should be preparing the soil. Onions are heavy feeders. They need rich, well-draining soil with a pH between 6.0 and 7.0. They also hate compacted soil.
Choose a location that gets full sun. Since these are long-day onions, they need every bit of sunlight they can get. If your bed is shaded for more than a few hours a day, your bulbs will be small.
Here is how to prepare the bed:
- Remove all weeds and rocks from the area.
- Loosen the soil to a depth of at least 12 inches.
- Mix in 2 to 3 inches of compost or aged manure.
- Add a balanced fertilizer like 10-10-10 according to package directions.
Onions have shallow root systems. They do not compete well with weeds. A clean, fluffy bed is essential for their root development.
Transplanting To The Garden
You can transplant your seedlings outdoors as soon as the soil is workable in the spring. This is usually 3 to 4 weeks before the last frost date. These onions can handle the cold weather without any issues.
Space the plants about 4 to 6 inches apart in rows. The rows should be spaced 12 to 18 inches apart. If you want smaller, “green” onions, you can space them closer together. If you want giant bulbs, give them more room.
When planting, set the seedling slightly deeper than it was in the pot. However, do not bury the green tops. Only the white base and roots should be under the soil. Firm the soil gently around the roots.
Water them thoroughly after planting. This settles the soil and removes any air pockets around the roots.
Caring For Your Onions
Once your onions are in the ground, the real work begins. Consistent care is the secret to growing those massive, sweet bulbs. Let’s break down the essential care routines.
Watering Requirements
Onions need about 1 inch of water per week. If you are growing them in sandy soil, you may need to water more often. The key is to keep the soil consistently moist. Do not let it dry out completely.
However, do not overwater. Soggy soil can lead to root rot and fungal diseases. A good rule of thumb is to water deeply once or twice a week, depending on rainfall. Use a soaker hose or drip irrigation to keep the foliage dry.
Wet foliage can invite diseases like downy mildew. Watering at the base of the plant is always the best practice.
Fertilizing For Sweetness
To get that signature sweetness, your onions need a steady supply of nutrients. Nitrogen is crucial for leaf growth. The more leaves your onion has, the larger the bulb will be.
Start fertilizing about three weeks after transplanting. Use a high-nitrogen fertilizer. You can use a liquid fish emulsion or a granular fertilizer like blood meal. Apply it every two to three weeks.
Stop fertilizing once the bulbs start to swell and push up out of the soil. At this point, excess nitrogen can cause the onions to split or reduce their storage quality. The plant is now focusing its energy on the bulb.
Weeding And Mulching
Weeds are your enemy. They steal nutrients and water from your onions. Since onion roots are shallow, hand-pulling weeds is often the safest method. Be careful not to disturb the onion roots when weeding.
Mulching is a great way to suppress weeds and retain moisture. Use a light layer of straw or grass clippings around the plants. This also helps keep the soil temperature cool, which the onions prefer.
Do not use a thick mulch, as it can trap too much moisture and cause rot. A thin, 1-inch layer is usually sufficient.
Common Pests And Diseases
Walla Walla onions are relatively tough, but they do have a few enemies. Being aware of these issues will help you act quickly if they appear.
Onion maggots are a common pest. They lay eggs in the soil, and the larvae feed on the roots. To prevent them, avoid planting onions in the same spot year after year. Crop rotation is essential.
Thrips are another pest to watch for. These tiny insects suck the sap from the leaves, causing them to turn silver or brown. You can often wash them off with a strong jet of water or use an insecticidal soap.
Here are a few common diseases to look out for:
- Downy Mildew: Causes yellow patches on leaves. Ensure good air circulation.
- Purple Blotch: Causes water-soaked spots on leaves. Avoid overhead watering.
- Botrytis Neck Rot: Affects stored onions. Cure them properly before storage.
Keeping your garden clean and rotating your crops is the best defense against most of these issues.
Harvesting Your Onions
Knowing when to harvest is just as important as knowing how to plant. Walla Walla onions are ready to harvest in mid to late summer. They take about 100 to 120 days to mature from transplanting.
You will know they are ready when the tops begin to fall over. The green leaves will yellow and flop over at the neck. This signals that the bulb has stopped growing and is entering dormancy.
Here is the correct way to harvest:
- Wait for the tops to fall over naturally.
- Loosen the soil with a garden fork.
- Gently pull the onions out of the ground.
- Do not wash them with water.
- Leave them in the garden to dry for a day or two.
If the weather is wet, move them to a dry, shaded area to cure. Since Walla Wallas are not storage onions, you do not need to cure them for weeks like other varieties. A few days of drying is enough to clean them up.
Curing And Storing
After a few days of drying, you can trim the roots and cut the tops down to about 1 inch. Do not wash the onions. Just brush off any loose soil.
Store them in a cool, dry, well-ventilated place. A mesh bag or a wire basket works well. They will last for about 2 to 3 months under these conditions.
Remember, these onions are best eaten fresh. Their sweetness fades the longer they are stored. Try to use them within a month for the best flavor.
Frequently Asked Questions
Can I grow Walla Walla onions from sets instead of seeds?
Yes, you can, but it is harder to find true Walla Walla sets. Most nurseries sell seeds. Growing from seeds gives you a larger, healthier plant. Sets often bolt (go to flower) if they are not handled correctly.
Why are my Walla Walla onions not forming bulbs?
This is usually a daylight issue. They need 14+ hours of daylight. If you live in the south, they will not bulb up. It could also be too much nitrogen, which encourages top growth instead of bulb growth.
How do I know if my onions are sweet enough?
The sweetness is largely determined by the variety and the growing conditions. Consistent moisture and cool temperatures help. The sulfur content in the soil also plays a role. You can taste one raw right after harvest to check.
Can I grow them in containers?
Yes, you can. You need a container that is at least 12 inches deep. Make sure it has good drainage. Container-grown onions will need more frequent watering and feeding than those in the ground.
When is the best time to plant Walla Walla onions?
The best time to plant seeds indoors is in January or February. You transplant them outdoors in early spring, about 3-4 weeks before the last frost. This gives them the long, cool season they need.
Growing these famous onions is a rewarding project. They require a bit of planning and patience, but the payoff is huge. The flavor is unlike anything you can buy at the grocery store. With the right timing and care, you will be enjoying sweet, juicy Walla Walla onions straight from your garden. Just remember to start early and keep the water coming.