How to Grow Onions in Raised Beds

Onions not only add depth to casseroles, soups, and salads, but they are also easy to grow, may deter pests, and are naturally anti-inflammatory. According to the National Onion Association, they are the third most consumed fresh vegetable in the United States, with each person consuming about 18 pounds annually! 

If you’ve ever attempted growing bulbing types, but they turned out to be small and not what the packet photo indicated, it probably wasn’t a result of your gardening skills, but rather the type of seed or when you planted them. They’re fussy about how much sun they receive and planting time, and gardeners across zones will have more success if they overwinter them. Yes, even in colder zones! 

Select the Right Variety 

You’ll want to select varieties that grow well in your growing zone and suit your cooking style. They can be sweet or spicy; you can eat them fresh, roasted, fried, pickled, grilled, or cook them down until they’re sweet and caramelized. They’ll elevate soups, stews, quiches, and stir-fries and are the base of many great meals. They’re a staple ingredient in most kitchens!  

Onions share a genus with garlic, chives, leeks, and shallots, all of which have spicy undertones and are culinarily versatile. Here are some types to choose from. 

Yellow

These are some of the most popular types of onion sold at farmers’ markets and grocery stores. If you strictly grew or purchased yellow bulbs, you’d be fine, as they’re versatile enough to use in most recipes that call for onions. A popular variety is ‘Yellow Sweet Spanish Utah.’ I recommend starting with yellows if growing this crop for the first time. Store them for up to six months in perfect storage conditions. 

Red

A bit sweeter than yellow varieties, red onions are high in vitamin C and quercetin. Many red varieties do not store well, so if you’d like bulbs for winter use, grow these along with a good storage onion and use the reds fresh. Try one of our community’s favorites, ‘Cabernet,’ and check out the recipe for red onion jam in the packet! 

White

Typically used in fresh salsa and cucumber salad, these are crisp, pungent, and delicious. White onions are great for pickling and burgers, wraps, and sandwiches. Try ‘Gladstone’ for an excellent day-neutral onion. 

Sweet 

These are exactly what you might think by their name. They’re sweeter than their red and white cousins and perfect for grilling, caramelizing, and eating raw in a salad. Their shape is a bit more flat than round and they’re light in color. You know some of the sweetest onions if you’re familiar with ‘Vidalia’ and ‘Walla Walla.’ 

Green Bunching Onions or Scallions 

Green onions can be bulbing varieties harvested early or non-bulbing varieties like ‘White Lisbon’ or ‘Tokyo Long White Bunching.’ Squeeze these guys in just about anywhere you have space. They don’t require the same amount of underground space to bulb out, so intercrop these with lettuce, kale, and Swiss chard. They’ll be mature and harvested before they’re shaded out by large leaves. 

You can clump three to five of these together when transplanting, so you can squeeze many into a small space. 

Shallots

Regarding fitting a lot into a small space, shallots are a milder allium and much smaller than their onion cousins. They’re popular in homemade salad dressings, marinades, and French cuisine. ‘Zebrune’ is an exciting mix between an onion and shallot. 

Leeks

Leeks have a mild onion flavor and are widely popular among cold region growers for their extreme tolerance of frost and freezing temperatures. One of the best features is their ability to stay in the ground for long periods, giving you an extended harvest window. 

Pick the Right Location 

Onions need full sun to thrive, so be sure your raised bed receives eight to ten hours daily. Any shade can diminish bulb production and overall yield.

Prepare the Beds

Growing this crop in raised beds is perfect for growers with infertile or clay-like soil. The elevated soil will keep them out of soggy winter soil. The enclosed space will warm up earlier than garden soil, allowing you to work the soil sooner. These alliums prefer well-draining and rich, composted soil with a pH between 6.0 and 6.8. 

The depth of your bed should be at least 15 inches deep so the bulbs have enough space to form, roots can reach down, and water can drain. Remember, the taller your raised bed, the less bending you must do. Check out the wide range of Birdies available and find one that fits your aesthetic needs.

Start Seeds Indoors

For spring planting: Sow seeds indoors 10 to 12 weeks before the last estimated frost in your region to give them a head start on growth and allow them to establish strong roots before transplanting them into chilly spring soil. 

For fall planting: These alliums need four to six weeks of warm temperatures to establish a robust root system, so sow them indoors in late summer. The exact timing will depend on your growing zone and your selected variety.  

Bury seeds about ½ inch deep in high-quality cell trays and organic seed-starting mix. Provide them with good airflow to keep the soil surface from crusting over and algae from forming. 

Keep the soil moist during germination, which will take 4 to 14 days. While a heat mat is unnecessary to start onion seeds, you should keep the tray in a warm place. If you use a heat mat, set it to 65 to 70°F (18 to 21°C). Don’t allow the soil to dry out. Once germination has occurred, place them in sunlight or provide artificial light. 

Note that direct seeding onions is not recommended. 

Use Sets

If you’re not set up to start seeds indoors, you can purchase onion sets and plant them in the ground. Like garlic, onion sets consist of small, immature onions grown the previous year and kept dormant until you plant them. Don’t bury them more than two inches deep; space them at four to six inches. Whether you use onion sets or start them seed is a personal preference. 

Give Them a Haircut

There is some misinformation about whether you should or shouldn’t do this. I’ve trimmed all my onion tops, which have always turned out beautifully, so I’m a big advocate of this method. Trimming the tops before transplanting them forces energy into the root system, which you want before they go into the ground.

The key is trimming them about six weeks before transplanting because doing so much closer to transplant can cause stress and harm the plants. No trimming should occur once the plants are in the ground, as proper photosynthesis requires dense foliage to gather sunlight. 

Trim your seedlings when they’re six to eight inches tall. Use a sharp, clean pair of shears and snip off the thin green tops down to about two inches. Throw the scraps on the compost pile or use them as a garnish in tonight’s meal! 

Harden Off 

Wait until your onions have reached six inches in height after their haircut, then harden them off in preparation for transplant. Proper hardening off will reduce the risk of transplant shock, which is crucial for spring plantings when air and soil temperatures are lower. Avoid hardening off fall-transplanted onions on harshly hot and sunny days. Use shade cloth if necessary. 

Transplant 

Transplant onion seedlings into a shallow hole made with your fingers or a dibbler. Some growers prefer to put one onion plant per transplant hole, while others add three to four. Most varieties bulb out just fine sharing a hole with their onion friends, while others may become misshapen. When transplanting onion sets, ensure the pointed side faces up for proper growth. 

Spacing 

If it’s your first time growing a particular variety, experiment with spacing and record the bulb sizes they yield. 

You should plant four to six inches apart, with six to eight inches between rows.

Pro Tip: If you are trialing tighter spacing, harvest every other one to use as fresh scallions or bulbs. Giving the ones remaining in the ground extra space will allow them to grow larger bulbs.  

Leeks are the outlier here because they require more space underground to form their sought-after, delicious white shank. Drop them in a deep hole and hill them up every week or so as they grow. The more you hill them, the more white they’ll grow. Discard the green foliage before use. 

Maintenance

These alliums are easy to maintain with a bit of love and attention. 

Irrigate

Onion roots are shallow, so the soil cannot dry out in between waterings. We recommend drip irrigation to get water straight to the roots where it’s most needed. They require at least an inch of water weekly, whether by rainfall or irrigation. A watering system for raised beds is crucial to their success. Choose between soaker hoses, an automatic olla watering kit, or a customizable irrigation system. 

Keep Weeds Out

These alliums don’t compete well with weeds, so ensure the soil around them is clear. When practicing companion planting, ensure each has enough space to receive proper sunlight, nutrients, and water.

Pro Tip: Cover your raised bed with a silage tarp a week before planting to smother any young weeds beneath the surface. 

Let’s Talk About Pests

Feed

Fertilize at the time of transplant with a 10-20-10 product. The phosphorus boost encourages the root system, giving them a strong start at life in the soil. If your soil is deficient in phosphorus, your onions may take longer to mature, and bulbs may be small. Give them a shot of nitrogen three weeks later to promote above-ground vegetative growth.

Continue general fertilizer applications every two to three weeks, ceasing when necks soften. Apply liquid fertilizer after rainfall or irrigating to avoid evaporation—side dress with granular feed.

Protect Onions in Winter

I highly suggest giving fall transplanted onions a shot, especially if you’re a northern grower who has started seeds in the spring without great success or yields. 

Like fall-planted garlic, protect your onions from winter weather with a low tunnel or add a thick layer of organic straw. Remove it in the spring when temperatures increase, and add the straw to garden paths to help suppress annual weeds from emerging. 

Harvest 

Onions necks will naturally fall over when the bulbs are mature and ready for harvest. The foliage will become brown and crisp. Jiggle the soil and gently pull up on the onions. If the soil is wet or compact, use a digging fork to loosen it and avoid bulb damage. 

Enjoy onions fresh immediately if they are not a storage variety. 

Curing and Long-Term Storage

Add a single layer of onions with the necks intact to a drying rack away from direct sunlight and extreme temperatures. Provide good airflow for several weeks while they cure. When the skin feels like thin, dry paper, cut into a neck and confirm no moisture remains. 

Remove any remaining soil and the dried tops, and store them in a bushel or cardboard box with air holes. Handle them like you would eggs to avoid damage affecting their storability. You can store some varieties for up to eight months away from direct sunlight in a cool, dry space. Keep temperatures between 45 and 55°F (7 to 13°C). 

Who says green onions are only good fresh? Roughly chop them and dehydrate or freeze them for winter use. 

Rotate Crops

Practicing crop rotation will reduce pest and disease issues. Add a round or two of cover crops to replenish the soil and add biomass. Yes, you can and should cover crop raised beds! 

Choose buckwheat for a quick summer cover, sunflowers or field radishes to help with compaction, or hairy vetch for a nitrogen boost. Some studies have shown successful growth of barley as a living mulch alongside onions. When using these practices, have a termination plan lined up so as not to affect onion bulb size.