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"How to Grow",  All Things Garden

How to Grow Onions: from Seed or Sets to Harvest

As strange as it may sound, onions are one of our favorite crops to grow! They’re highly underrated. It’s easy to grow onions, once you know how! Few pests bother them, and the plants are forgiving and versatile – since you can harvest and use these alliums at many different stages along the way. Not to mention, onions are the ultimate garden-to-table crop. Just about every killer soup, stew, sauce, or stir fry begins with a familiar foundation: sauteed onions and oil (or butter). Toss in some garlic? Now we’re talkin’.

Read along to learn how to grow onions from seed, seedlings, or sets. This article will teach you everything you need to know, including how to choose the varieties that will grow best in your garden, when and how to start onion seeds, plus tips for transplanting and ongoing care. Then we’ll talk about how to harvest, cure, store, and preserve your onions too. Spoiler alert: homegrown onion powder is outstanding!



When to Grow Onions


Onions are generally considered a ‘cool season crop’ because onion seedlings like to start their life when conditions are still cool. Then as the weather warms up, they switch to focus their energy on bulb formation. Therefore, spring is the best time to plant onions in most places. If you hope to grow onions from seed, you’ll need to be prepared to get a jump start and plant seeds during winter. After sprouting onions take three to five months to reach a mature size, depending on climate and variety.

Gardeners with mild winters can also grow onions in the fall for a winter harvest. Here on the temperate Central Coast of California, we can grow onions essentially year-round! In colder climates, it is also possible to plant onion sets in fall but allow them to overwinter and harvest mature onions in the spring to early summer. In that case, plant onions when the soil is still warm – at least a month before your first fall frost date. That gives them time to establish healthy roots before they go dormant for the winter. 

All in all, it may take a little experimentation to figure out the best time to grow onions in your particular climate and garden. One very important key to this puzzle is to choose the right type of onions to grow for your area!


Learn when to plant seeds by using this planting calendar for Zone 9, it has many different vegetables lined up on the left side of the chart and all of the months of the year listed on the top of the chart. Each vegetable has different colored lines that correspond with when to start seeds inside, transplant outdoors, and plant seeds outside, along with corresponding last frost date and first frost date where applicable. The lines start left to right, showing what months you should do each particular task depending on the season and where you live.
Take zone 9 for example. According to our planting calendar, zone 9 can start onions in early winter and transplant (or sow outside) in late winter. Then, zone 9 gardeners can sow more onion seeds outside in late summer to early fall. If you need a planting calendar, get one here! We’ve created free printable calendars for every USDA hardiness zone.


TYPES OF ONIONS


The difference between bulbing onions, green onions, scallions or spring onions


Red, yellow, white, sweet… In addition to the various colors and subtle flavor differences, traditional bulbing onions can be enjoyed in a variety of ways – depending on the age they’re harvested! Before forming large bulbs, immature onions may be picked and used as green onions – also known as scallions. If you allow them to mature a little longer and form a modest (but still underdeveloped) bulb, then they’re considered spring onions! Gather up a cluster of young green onions, spring onions, or scallions, and now you have a handful of bunching onions. Note that there are also varieties of slender non-bulbing onions; a true “green onion”.


DeannaCat is holding many bunching Walla Walla onions with both hands. They have fibrous roots with bulbs that are slender and slightly bulbous with brown and yellow coloration leading to white stalk with green leaves. Below lies some nasturtium growing along with some your turmeric leaves growing from a wine barrel. Grow onions and harvest them at many different stages.
A handful of immature Walla Walla onions – usually grown as large dry onions, but we planned poorly and needed to clear their bed for other priority crops, so… we had Walla Walla spring/green onions! Perfect to eat fresh, or turn into onion powder.


Short-day, long-day, and day-neutral onions


In addition to size and color, different onion varieties are further classified as either short-day, long-day, or day-neutral onions. They’re grouped by the number of daylight hours required to trigger good bulb formation. It is important to choose and grow onion varieties that are a good fit for your location! Otherwise, the onion bulbs may fail to successfully develop. Non-bulbing green onions aren’t as fussy about exact daylight hours, though all onions like plenty of sun!


  • Short-day onion varieties will form bulbs once daylight hours increase to 10 to 12 hours per day. They are ideal for southern gardeners where daylight hours are consistently shorter throughout the growing season compared to the longer days up north. If grown in the north, short-day onions may go to flower too early in the season as the days become increasingly longer – resulting in stunted, small bulbs. Short-day onions are rumored to be sweeter than long-day. Some popular varieties of short day onions include Red Burgundy, GabriellaGrano, White Bermuda, Southern Belle, and ‘Yellow Granex’ (also known as Vidalia).


Not sure which to choose?


As a (very) general rule of thumb, short-day onions grow best south of the 35th parallel, and long-day onions do well north of the 35th parallel. Many onion varieties will specifically state which latitudes they’re best suited for. Ironically, our garden is located almost exactly on the 35th parallel – just a squeak north at 35.1216° N. We grow impressively large Walla Walla onions (a long-day variety) during the early spring to summer. Yet they don’t do as well for us as a fall-planted winter crop when the days get shorter. If you’re also close to the boundary, have fun experimenting with a wide variety!


A diagram of the US with three separate regions that are color coded based on the types of onions that will grow in each area. The northern US is in the long-day area, the middle is intermediate-day and the south is short-day. There is a description of each onion type below the map with needed hours of sunlight, latitude, and growth habit.

Where each type of onion grows best. However, see the black star? That is where we are located, on the Central Coast of California, almost right on the 35th parallel. According to this graphic, we should be growing short day onions…. but we grow stellar long-day onions here too! So, if you’re close to the border of a zone, or if you’re in the central “intermediate” area, don’t be afraid to experiment a bit. Yet if you’re in the far north or far south, stick to long day or short day onions (respectively). Thanks for the great map, Johnnys Seed!


Should I grow onions from seed, seedlings or “sets”?


There are three main ways to grow onions: from seed, seedlings, or by purchasing onion sets. Every gardener has their preference between the three! We like to grow onions from seed, or from seedlings we buy at a local nursery. Growing onions from seed is easy, and allows you to choose the exact varieties you want to grow! Purchasing started seedlings or sets is also a great option, and will give you a quicker start than seed. 

Onion seedlings are like small green onions, recently started from seed (within the last couple months). You can grow your own, or purchase seedlings at your local garden center.

In contrast, onion ‘sets’ are small, dry, immature onion bulbs that were grown from seed the previous year. They were allowed to partially develop, harvested as a tiny bulb, and kept dormant to later sell or plant as “seed” – though they’re not really seed. Because they’re on their second year of life (and onions are biennial), onion sets have the tendency to bolt more readily or result in underdeveloped bulbs. When an onion ‘bolts’, it prematurely sends up a flowering stalk and then forms seed. You can still eat around the tough center of the onion bulb, but it will decline in quality and no longer be good for long-term storage.


Flowering onions shooting towards the sky from a garden bed below. Each stalk has a puffy ball on the top of it which is the onions flower. In the surrounding garden area beans, marigolds, squash, onions, salvia, and sage are growing with many colors of flowers contrasting against the twilight of the evening. Grow onions amongst other vegetable crops to repel many pest insects.
Onion flowers. Sometimes we let a few onions go to flower so the bees can enjoy them, and so we can also enjoy their beauty! Though they won’t store well, the onion can still be consumed or used for onion powder, including the flower stalk.


HOW TO GROW ONIONS FROM SEED


To grow onions from seed, you can either start seeds indoors, sow seeds directly outside, or use the winter sowing method. Planting seeds directly outside is simple. You won’t need to fuss with an indoor seed-starting set-up, or have to transplant seedlings later. However, starting seeds indoors during winter will put you several weeks (to months) ahead of schedule. You’ll have onion seedlings ready to transplant into the garden right away in spring. For northern gardeners with short growing seasons, this is a critical advantage! There, the direct sow method won’t give the onions enough time to mature – so seeds should be started indoors.


Direct sowing onion seeds outside


Plant onion seeds outside in late winter to early spring, once the ground is no longer frozen and can be worked in your area. Read the soil preparation and site selection tips below before you start. Then, sow onion seeds approximately 1/4 to 1/2″ deep and about 4 inches apart. Cover the seeds lightly with soil, and maintain damp to promote germination. Once they sprout, thin to one onion seedling per 4 inches to avoid crowding. (Note that germination rates will be slower and less consistent with this method, and not recommended for northern climates). Check your zone’s planting calendar to see if you can also direct sow onions in the fall. 


A small manilla envelope is tipped on its side, small black onion seeds are spilling out from the depths of the package in a straight row.


Starting onion from seed indoors


Start onion seeds indoors about 8 to 10 weeks before the last spring frost date in your area. Follow the common best practices for indoor seed-starting: use a sterile seedling potting medium, keep the soil damp but not soggy, and provide at least 12 hours of bright light per day after germination. You’ll need a few supplies such as seedling trays, small containers or pots, and a grow light or other supplemental light fixture. Onion seeds will sprout most readily in consistently warm soil (70-75°F, a seedling heat mat will help!) but then can be moved to a cooler spot in the 60s.  

The warm, comfy indoor conditions will help onion seeds sprout and grow quickly. Once they reach 5 or 6 inches (or taller), trim the tips of their greens down to 3 or 4 inches. This encourages thickening, stronger root development, and prevents them from flopping over. Eat those trimmed bits as green onions!

I do not thin my onion seedlings until I transplant them outside. I allow a handful of seedlings to grow together in one cell or small pot, and then very gently pull them apart to plant separately come transplant time. Onions do well with this treatment, so don’t be afraid to sow several seeds per container!

Please visit our Seed Starting 101 article for more details about starting seeds indoors, and our favorite supplies. 


DeannaCat is using scissors to cut the top portion of onion seedling greens, leaving about 5 inches of growth below. There are many onion seedlings in the 6 cell pack which will be separated and divided into individual onion seedlings at transplant.
Trimming the greens on our homegrown onion seedlings. Note how we grow several onions in each little cell/container, and will gently pull them apart later to plant individually.
DeannaCat is holding a bunch of the tops of onions seedlings that resemble chives. Below lies a pair of scissors and the trimmed 6 cell pack of trimmed onion seedlings.
Trimmed onion seedlings = tasty green onions!


Winter sowing method


A final option to grow onions from seed is with the ‘winter sowing method’. This is when you sow seeds inside a covered container, but do so outside. As the name suggests, start your onion seeds outdoors during the winter – between late December to February in most locations. The seeds will stay dormant in the chilly outdoor conditions, and sprout in spring once the conditions are right. Then you can transplant the already hardened-off seedlings to their final location in the garden. Winter sowing combines the pros and cons of the other options: you won’t get quite the same early advantage as starting indoors, but it’s almost as low-maintenance as directly sowing seeds outside. 

To grow onions with the winter sowing method, select a wide shallow container with drainage holes that can be covered with a clear or opaque dome or lid (or, one that already has one). Add at least a couple inches of seed-starting soil to the bottom, evenly sprinkle or sow onion seeds inside, and then lightly cover the seeds with about ¼ to ½ inch of soil. Add at least one or two air holes for ventilation to the cover. 

Set the container in a sunny location, water as needed to maintain the soil damp, and wait for the seeds to germinate. Once the seeds sprout, open the container lid on warm days (but close them back up on cold nights). Keep watered as needed. If snow or a hard frost is a threat, move the container to a protected location or cover it with a blanket for insulation. 


Four plastic milk jugs and a plastic container sit on an outdoor table. The containers are being used to sprout onion seeds for winter sowing. You can grow onions in a variety of ways to work with your climate.
Old milk jugs, salad green tubs, or similar containers are a popular choice for winter sowing. Image courtesy of Joe Lamp’l.


Growing Onions from Sets


Plant onion sets in the garden 2 to 4 weeks before the last frost date in your area. Bury them deep enough that the bulb is mostly under the soil, but with the pointed tip level with the soil surface or just poking out. Space onion sets about 4 to 6 inches apart, depending on the expected mature size of the variety you’re planting.  Speaking of variety, don’t assume all onion sets you find in local stores will grow well in your area! Big box garden centers are notorious for selling very zone-inappropriate varieties. Small locally-owned nurseries should offer a much better selection.

We haven’t experimented with growing onions from sets, and I’ve heard mixed reviews from fellow gardeners. While this method is certainly easy, it can lead to underdeveloped or flowering bulbs more often than starting from seed or seedlings. If you do opt to grow onions from sets, choose the smallest bulbs you can find. Sets around a half-inch in diameter or smaller are ideal. Contrary to what you may think, bigger isn’t better! Experts say that larger onion sets are more likely to halt bulb development and bolt (go to seed and flower) sooner than more petite bulbs. 


A close up image of a hand placing a small onion bulb (a set) in the soil for planting.
Planting onion sets.


How to Plant Onion Seedlings (transplanting)


Before transplanting, indoor-raised seedlings must be gradually acclimated to outdoor conditions through a process called ‘hardening off’. Even though onion seedlings can tolerate chilly temperatures, hardening them off first will prevent shock or damage from the sudden change in environment. Learn more about hardening off here. (You can skip this step if you are growing onions from seedlings you purchased at a nursery, or if you used the winter sowing method.)

  • Transplant homegrown or nursery-bought onion seedlings outdoors about 2 to 3 weeks prior to your areas last spring frost date.
  • Gently pull apart or separate any clusters of seedlings, taking care to break as few roots as possible.
  • Create a slender hole that is deep enough to accommodate the seedling’s roots, and bury the seedling so the base is about a half-inch to an inch deep.
  • Space your onions 4 to 5 inches a part, depending on expected mature size for the given variety. Or, plant them closer if you want to harvest them as green onions or small spring onions.
  • If you haven’t already, cut off the tips of the greens with clean scissors, which helps focus energy on growing a large onion bulb. Take the seedlings down to about 5 inches tall. It’s also okay to pull off the outermost seedling leaves; those usually end up looking a bit sad by now.

Onion seedlings that have been hardened off can withstand a light frost, but need to be protected from a hard freeze. If needed, shelter young onion transplants with frost cover, cloches, buckets, or other protective insulation. Learn more about protecting crops from frost here.


A six way image collage of DeannaCat separating onion seedlings that have been grown together as starts. She then makes holes in the garden beds and plants each onion seedling in its own hole until a few rows of onions have been planted.
Transplanting onion seedlings into the garden. These were seedlings I bought at the local nursery, so they were a tad beat up – and still need to have their tips trimmed. Gently pull apart seedlings, poke their roots down into a hole, and gently pack soil around their base.


Planting Location: Sun, Soil, & Amendments


Choose a location in your garden that receives full sun. Some varieties of green onions can handle partial shade, but all onions grow best in ample sun. If your garden is on the shady side, choose short day varieties to encourage bulb development. Onions grow most happily in moderately rich, well-draining soil. Amend clay soil with organic matter (e.g. compost) or potting soil to improve drainage.

Before planting, amend the soil with a balanced, mild, slow-release fertilizer such as this organic all-purpose blend. I also recommend lightly working in well-aged compost and/or worm castings to the top couple inches of soil. Avoid high-nitrogen fertilizers. Excess nitrogen will encourage ample green top growth, but at the expense of the bulb. During the growing season, we water our onions at least once with homemade compost tea.

It’s also a great idea to inoculate the soil with mycorrhizae, either by applying some to the soil or watering with a mycorrhizae solution after transplanting. Mycorrhizae are beneficial fungi that promote healthy root development and nutrient uptake for all plants. However, onions are especially reliant on root-mycorrhizal partnerships to take up phosphorus that’s essential for their growth!


Many onions are growing with a few leeks growing in the furthest back row.  Peppers are growing next to the onions, in the background lies another garden bed that is full with bok choy, kale and other greens. Beyond that lies a wall of flowering perennial sage and salvia. Grow onions to add dimension and versatility to your garden area.
Happy, healthy young onions in our front yard garden (and some leeks on the far right)


Water


Onions have shallow root systems, so it is important to keep them well-watered. A lack of consistent water will result in smaller onion bulbs. Adding a layer of mulch around the onions is an excellent way to prevent the soil from easily drying out. We love to use compost as mulch! It is dual purpose since it helps feed the soil as well as retain moisture. Remember, good drainage is crucial too! Soggy soil or standing water increases chances of onions rotting.

Related: The Pros and Cons of 8 Popular Types of Garden Mulch



A close up image of large Walla Walla onions growing with a soaker hose weaved in between the onions. Half of the onion bulbs are protruding from the soil while the remaining half lies beneath. The onion greens are still strong and vigorous though they will soon be harvested.
Here you can see our BPA-free soaker hose irrigation weaving between rows of large onions, and a woody compost mulch on top to help retain moisture.


Growing Onions as Companion Plants


Very few pests are attracted to onions. In fact, onions actually repel many pest insects, including aphids, flea beetles, cabbage loopers, ants, carrot rust flies, and more. Therefore, onions make great companion plants in the garden! Onions can be interplanted among other pest-prone crops to help provide natural, organic pest control. Rather than always growing onions in separate designated areas, we’ve had success keeping our Swiss chard plants almost completely aphid-free (usually a big issue) by tucking miscellaneous onions between them. Interested to learn more? Stop by this article, which also includes a handy printable companion planting chart!


Harvesting Onions


To use as green onions or young spring onions, harvest onions any time after they reach the size of a pencil or begin to form bulbs. You can also occasionally cut the tops only to enjoy as green onions, leaving the bulb to continue to grow. To do this, be sure to only harvest a “leaf” or two at time (or just a couple inches from the tips) so there are plenty of greens left behind to photosynthesize and aid in bulb development. 

Once the onion bulbs form, you could technically harvest them whenever you’d like to use one. I love popping outside to harvest a fresh onion for dinner as-needed! Yet for maximum storage potential as dry bulbs, you must allow them to grow until they reach full maturity. Onions are mature and ready to harvest once the main stalk gets weak, thin, and starts to flop over. The greens will also start to turn yellow or brown, especially at the tips. 

If your onion stalks don’t start to fall over on their own but you want to harvest them soon (e.g. if they’ve reached a mature size, freezing weather is in the forecast, etc) you can also manually bend or push the stalks over a few days prior to harvesting to expedite the process.

When the time is right, pull the plants up but leave them lying on top of the soil to dry for a day or two. Onions are susceptible to bruising, so be gentle when you harvest. Avoid accidentally puncturing the skin or flesh – they won’t hold up in storage if damaged. 


Rows of growing onions whose top leaves have bent over or toppled over. They are ready to be harvested, grow onions and harvest them once they show signs of reaching full maturity with leaves that have bent over.
Onion tops flopping over, a sign of maturity and time to harvest.
Many yellow onions lying in a field, their green leaves have withered and turned brown. Grow onions and cure them for best storage.
Onions pulled from the soil, and laying out “in the field” (garden) to dry for a couple days.


How to Cure Onions


Properly curing your onions will enable you to store them at room temperature (in dry storage) for many months after harvest. In very hot, dry climates, those couple days onions spend lying and drying in the field after harvest may be sufficient curing time. Yet most gardeners further cure their onions by relocating them to a dry and well-ventilated area for 7 to 10 days. Cure your onions in a drafty garage, carport, patio, shed or even right outside (as long as it isn’t too humid).

Hang onions from a homemade drying rack, or lay them out flat on a wire rack or screen. Curing onions on a solid tabletop can also suffice as long as there is ample air circulation. Again, take care not to ding them up in the process. As onions cure the roots will shrivel up, and the greens will dry and form a seal at the top of the onion. When ready, trim off the roots and tops before tucking them away for long-term storage.


Many onions have been braided together in bunches and tied with twine around their leaves and left to hand dry on a main drying line. The onion bulbs hanging in the air for maximum circulation.
Many bulbs of onions are protruding from a slotted wooden bench or table. The onion leaves are hidden below the table as the onions are left to cure upside down, their fibrous roots and bulbs pointing upwards. Grow onions and cure them to maximize storage potential.


Storing and Preserving Onions


Store cured onions in a cool, dark, dry location. For example, in wire baskets or cardboard boxes in a spare closet or cabinet, root cellar, basement, or similar temperate location. Dry onions that were not cured as well (or at all) should keep at room temperature for several weeks, but they’ll last longer in the fridge. Be sure to use up any dinged or bruised ones first. Store immature onions (green, spring, scallions, etc) in an airtight container in the refrigerator for maximum freshness.

In addition to long-term dry storage, there are a number of ways to preserve onions. We love to make homemade dried onion powder, especially with immature onions or those that have started to flower and thus can’t be cured. Learn how to make onion powder here – and be sure to use the greens too! Onion powder brings the perfect pop of flavor to sautéed veggies, dips, dressings, sauces, soup, sourdough bread, and more.

Another great option for preserving onions is to roast and freeze them. We use these super-flavorful, lightly caramelized onions in future soup, stew, vegetarian chili, or quiche. You can also use this recipe (the same brine/process) to make easy refrigerator pickled onions!


Hot tip: Do onions make you cry? Try holding a slice of citrus (lemon or lime) in your mouth while you cut them!


An image that shows the front of a food dehydrator with its door off so you can see the inside of the dehydrator. There are the six stainless steel racks from the previous image all stored away in position for drying. The top two racks are full of the onion greens and the bottom four are loaded with the onion bulbs.
Drying onions in our food dehydrator to make onion powder.
A close up image of a hand holding up the mason jar that is full of dried onion powder. the backdrop is a large and dark green houseplant, making the jar of onion powder stand out in contrast. Grow onions to dry your own powder.
Done! Homegrown onion powder is so sweet and fresh, it puts store-bought to shame.


And that is all you need to know to successfully grow onions!


Well friends, I hope you learned a few new tips that will enable you to grow big, beautiful, flavorful onions of your own! In all, they’re quite low-maintenance and fun to grow, especially once you figure out what varieties grow best in your area. Let me know if you have any questions in the comments below! If you found this information to be valuable, please spread the love by pinning or sharing this article. See ya next time!



DeannaCat signature, keep on growing

9 Comments

  • Julie James

    Thank you for such good information! We planted our onions in our greenhouse from dried onion bulbs that we purchased from our local farm store. There were MANY of them in the bag, but were not grown locally. They have all sprouted, and quickly! My question is, do I trim the green tops, when and how much/often? We grew them the same way for the last 2 years, following various opinions on trimming, “spooning” and drying/curing, but we never get them to grow as large as they should or keep as well. So, we’re rather confused as to what we need to do! Thanks for any help you can offer!

    • Aaron (Mr. DeannaCat)

      Hello Julie, we have found that planting onions from bulbs isn’t as reliable as growing onions from seeds or started seedlings from the nursery. Onion seedlings from the nursery are a good option because they usually come in 6 cell pack trays and are over seeded, meaning you can lightly separate each onion seedling and plant it directly in the ground. As far as trimming the greens, we usually only do it one time once they are at least 6 inches long. Hope that helps and good luck this year with your onion harvest!

  • rebekah

    I’m just starting out with gardening and growing veggies and herbs this year – your site has been helpful! We built some deep raised garden beds and have planted some goodies, including onions 🙂

    But… oh no! When I transplanted the seedlings I picked up from the nursery, I think I trimmed the tops a little too much. There’s still green but I think they are shorted than you recommend. Will they survive?? lol Do they just need some extra love and care? What do I do? haha

    • Aaron (Mr. DeannaCat)

      Hi Rebekah, so glad to hear you’ve started gardening and congrats on building some raised beds. Don’t be too worried about your onion seedlings, they should be just fine and will continue to grow. Just water them and treat them as you would the rest of your garden. Good luck and happy gardening!

    • Aaron (Mr. DeannaCat)

      Hello Renee, you can more or less apply the same principles to leeks. Leeks are fairly fuss free once they get going (if you start from seed) and are a joy to grow, harvest, and use. Good luck!

  • Daise

    Thank you so much for all your helpful information. I love that you give the science and reason behind it all!
    My plants have been so much happier since adopting no dig and your pointers!
    From zone 5A, this homesteading Mainer appreciates you!

    • Aaron (Mr. DeannaCat)

      Thanks for reading and being a part of the community, we’re glad you have found some of the information useful! Good luck and happy gardening!

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