I know, I know—it’s only December. But in a matter of weeks it’ll be time to start thinking about next year’s garden… and yes, even planting it. For me, that ceremonial moment always begins with onions. Onion seeds are the very first thing I start each year, usually in February but sometimes even earlier depending on how antsy I am to get my hands dirty.
If you’ve never grown onions from seed before, there’s no need to be intimidated. Starting your own onion seeds is easy, surprisingly satisfying, and the payoff is tremendous. So cozy up—it's time to start dreaming about next year’s garden, one tiny black seed at a time.
Growing onions from seed is a rewarding process that will have you harvesting delicious homegrown onions with surprising ease.
Why Start Onions From Seed?
Most gardeners stick with sets or purchased transplants simply because that’s what’s available locally. But growing onions from seed gives you several big advantages:
1. Bigger, Better Bulbs
Seed-grown onions are younger and have not been pre-stressed. Sets, on the other hand, are essentially tiny dormant onions that already think their lifecycle is well underway. Because of this, onions grown from seed produce larger, better-formed bulbs and are far less likely to bolt prematurely.
2. It’s Far Cheaper
A packet of seeds costs less than a bundle of store-bought starts and produces far more plants—often hundreds. If you grow a lot of onions (or want to start), seeds stretch your gardening dollars farther than anything else.
3. Endless Variety Options
Garden centers may offer two or three onion varieties on a good day. With seeds, the world opens up: sweet onions, long-storing onions, red onions, candy types, torpedoes, heirlooms, and regional specialties. If you want an onion tailored to your climate and cooking style, seed is the way in.
Seldom found in garden centers, heirloom onions like Walla Walla can be easily grown from seed. often producing larger bulbs than can be achieved from purchased starts.
Choosing the Right Onion Variety
Onions are daylength sensitive, meaning they begin forming bulbs only when daylight reaches a specific number of hours. Choose a variety mismatched to your latitude, and you’ll end up with bulbs no bigger than golf balls.
There are three main categories:
Short-Day Onions
- Begin bulbing at 10–12 hours of daylight
- Best for Southern growers (roughly below latitude 35°)
- Mature early and often sweet
Examples: Texas Early Grano, Red Creole, White Grano, Red Burgundy
Intermediate-Day Onions
- Bulb at 12–14 hours of daylight
- Adaptable for much of the U.S., including Midwest
Examples: Australian Brown, Walla Walla, Red Cipollini, Borettana
Long-Day Onions
- Bulb at 14–16 hours of daylight
- Best for Northern growers (roughly above latitude 40°)
- Generally produce the best storage onions
Examples: Ailsa Craig, White Sweet Spanish, Red Wethersfield, Long Red Florence
Selecting the correct daylength type ensures your onions bulb at the right time, ensuring you'll get the largest bulbs.
Red Creole is a short-day onion best suited to Southern climates. Although it can be grown in the north (these plants were grown in Iowa), it will produce bulbs that are smaller in size.
The Setup: What You’ll Need
Before you start, gather the basics. Onion seeds don’t require anything fancy, but having the right tools on hand makes a big difference in germination and early growth.
Materials List
- Seed-starting mix — Look for a fine-textured blend with good moisture retention (peat-based mixes work beautifully).
- Seed tray or open flat — I prefer a flat with no dividers so the seedlings can grow densely and be lifted out easily later.
- Humidity dome — Helps maintain even moisture while the seeds germinate.
- Heat mat — Onions germinate best with soil temps around 70–75°F, so bottom heat speeds things along.
- Grow light — A simple led light works; the important part is bright, consistent light directly above the tray.
Once everything is assembled, fill your tray with moistened seed-starting mix and gently level the surface. You’re ready to sow.
These Yellow Sweet Spanish onions are one of the main crop varieties we grow every year, starting the seeds in January or February.
Sowing Onion Seeds
Onion seeds are tiny but incredibly forgiving. Here’s exactly how to plant them:
Step 1: Sprinkle Seeds on the Surface
Scatter the seeds evenly across the top of the soil. There’s no need to space them perfectly—onions don’t mind growing shoulder-to-shoulder at this stage.
Step 2: Press Seeds In
Use a flat hand or small board to gently press the seeds into the surface. Onion seeds germinate best with light contact but not deep covering.
Step 3: Mist or Water Gently
Give the tray a light watering to settle the seeds. (We’ll talk more about watering technique in the seedling-care section.)
Step 4: Cover and Warm
Place the humidity dome over the tray and set it on a heat mat. This keeps humidity high and temperature steady, encouraging fast, even germination.
Step 5: Add Light
Set a grow light several inches above the dome. Onions don’t require high heat after germination, but they do need bright, consistent light from day one.
Red Wethersfield, a long-day variety, is our go-to red onion. You'll be hard pressed to find starts of it, but starting your own is easier than you think.
Caring for Onion Seedlings
Once your onion seeds sprout, the real magic happens. They’ll start out looking thin, spindly, and a little chaotic (perfectly normal!), but with a bit of simple weekly care, they’ll bulk up into sturdy little transplants.
Watering
Keep the soil consistently moist but never soggy. A good rule of thumb is to water when the top of the soil just begins to lighten in color. When watering, use a gentle shower to avoid flattening the seedlings.
Light & Growing Conditions
As soon as the seeds have germinated, remove the heat mat and dome. From this point forward, onions prefer cooler temperatures—around 55–65°F if you can manage it. Keep the grow light several inches above the seedlings. Bright, close light helps prevent the seedlings from stretching too much.
Trimming onion seedlings reduced tangling and encourages the stems to grow thick and strong.
The First Haircut
About three to four weeks in, your onions will start leaning, tangling, and generally looking like a mess. This is your cue for the first haircut. Using clean scissors, trim the tops back to 1–2 inches tall.
You’re not harming them—quite the opposite. Trimming:
- encourages thicker, sturdier stems
- prevents the seedlings from collapsing into a mat
- allows the inner leaves to grow freely
- keeps the stand neat and manageable
Bonus: the trimmings taste just like onion chives. Don’t waste them.
Weekly Trims
From this point on, give your onions a quick trim about once a week, always cutting them back to 1–2 inches tall. Each haircut encourages the seedlings to strengthen and thicken at the base, which leads to more robust transplants later on.
Transplanting Onion Seedlings
By the time transplanting day arrives, your onion seedlings will still look a bit spindly—nothing like the thick, pencil-sized starts you may be used to buying. That’s perfectly fine. Home-grown onion transplants are younger, fresher, and far less stressed than store-bought sets, and they’ll take off quickly once they’re in the ground.
| A Note on Hardening Off |
| Onions are surprisingly tough, and I’ll admit I’m a little lazy about hardening mine off—I’ve never been bitten by skipping the full week-long process. Still, it’s good practice to give them at least a day or two outdoors before planting. Onions can go into the garden as soon as the soil can be worked, and I usually aim to transplant mine about a month before our last frost, which for us is early April. |
Trim One Last Time
Before transplanting, give your onions one final haircut, this time trimming them to 2–3 inches tall. This keeps the tops tidy and helps the seedlings stand upright during planting.
Prepare the Row
Till or loosen the soil and mark out your row. Onions prefer a loose, well-worked bed so their bulbs can expand without obstruction.
Onion seedlings ready for transplant.
Lift and Loosen the Seedlings
Carefully remove the entire sheet of seedlings from the tray. They’ll be growing in a dense mat at this point, so gently tease them apart and pile them loosely beside you. Don’t worry if they flop over—they’re tougher than they look.
My Sharpie Method
Working along your marked row:
- Plunge a Sharpie into the soil at the spot you'd like to plant.
- In one quick motion, pull the Sharpie out and simultaneously lower the seedling into the hole.
- Aim to bury about an inch of stem—you don’t need perfect precision here. Onions readily adapt and will anchor themselves quickly.
This creates a perfectly sized planting hole every time, and the rhythm of “poke–pull–plant” makes the process go fast.
The simple, but effective Sharpie method makes quick work of transplanting hundreds of onion seedlings
Firm the Soil Around the Row
Once an entire row is planted, gently press soil against the bases of the stems from each side. The goal is simply to ensure good soil contact—not to bury them deeply.
Your onions may still look a little floppy for the first week or so, but they’ll perk up quickly. Once they adjust, you’ll notice steady growth and a gradual thickening at the base as the plants prepare to bulb.
Spacing Your Onions
I garden in simple 3-foot-wide raised beds—not the wooden, boxed-in kind, but shallow beds just a few inches higher than the walkways. That little bit of elevation is enough to define the space and discourage wandering feet, and it makes planning and cultivating incredibly straightforward.
When it comes to onions, I like to plant my rows across the bed, perpendicular to the path. This lets me stand comfortably with my feet shoulder-width apart and work the hoe straight out in front of me—a small ergonomic gift to future me.
Here’s the spacing system I use:
- Row spacing: 12 inches apart
- Seedlings within the row: 2 inches apart
As the onions begin to size up, I harvest every other plant as spring onions—first for salads, then later for cooking as they develop more body. This progressive thinning leaves the remaining onions with about 4 inches between plants, just right for bulbing varieties.
My onion soldiers, standing in single-file. Spacing here is critical—it allows the roots room to expand.
Cultivating Your Onion Bed (and Why It Matters)
Remember when we talked about daylength being the key to growing good onions? Here’s where that really pays off. Onions are intensely daylength sensitive. Early in the season, before daylength hits its trigger point, the plant is focused entirely on building its leaf structure—the solar panels that will ultimately feed bulb development. Once daylength reaches the variety’s threshold, the plant stops making leaves and shifts all its energy into bulbing.
And this part is critical:
Your final bulb size is largely determined by how big the tops are before that trigger flips.
After bulb initiation, the plant is done growing foliage. What you have is what you get.
This is why early-season care—fertilizing, weeding, keeping the soil loose—is so important. If you let the onion patch get away from you and only rescue it right as the bulbs begin to form, it will be too late. You’ll end up with small bulbs no matter how perfect the weather is afterward.
Weeding & Cultivating
I typically like to weed my onions every one to two weeks, but if I lay down mulch, I can often get away with weeding just twice a season. Onions are shallow rooted, so use a hoe carefully—light, shallow strokes are all you need. The goal is to cut weeds off at the surface, not to disturb the soil deeply.
In the early weeks, a few grassy weeds will inevitably slip through within the row. That’s normal. Give it a little time and it becomes easy to distinguish them from the narrow, tubular onion leaves. Hand-pull the offenders when you see them.
For onions to reach their maximum size, the bed must be kept as weed-free as possible. Extra attention early in the season will pay big dividends later.
Fertilizing for Strong Tops (and Bigger Bulbs)
I hold off on fertilizing my onions for the first few weeks after transplanting. Their roots are still tiny at that stage—too small to take up an appreciable amount of nutrients—so the effort (and the fertilizer) is largely wasted. Instead, I wait until I can tell the plants have really taken hold: they’re standing upright, putting on noticeable growth, and beginning to look more like little onions than threadlike seedlings.
Once they reach that point, I make two fertilizer applications:
- First feeding: about 4 weeks after transplanting
- Second feeding: roughly 2 weeks later
A standard 10-10-10 granular fertilizer works perfectly, or if you prefer to garden organically, a complete organic fertilizer (COF) does the job just as well. Apply the fertilizer in a thin band beside the row, keeping it a few inches away from the stems to prevent burning.
These early feedings help the plants build the strong leaf structure they need before daylength triggers bulb formation. After that, the window closes—so well-timed fertilizer is one of the best tools you have for growing truly impressive onions.
Watering
Onions like consistent moisture, especially early in the season while they’re building foliage. Aim for soil that stays evenly damp but never waterlogged. Because onions are shallow rooted, they dry out faster than deeper crops, so a light watering every few days—or a deeper soaking once a week, depending on your soil—usually keeps them happy.
The most important time to maintain steady moisture is before bulb initiation, when the plants are putting all their energy into leaf production. Once the bulbs begin to size up, reducing water slightly can help them cure more cleanly later on, but avoid letting the soil swing between extremes. Even, moderate moisture makes for healthier tops and smoother, more uniform bulbs.
Letting up on watering later in the season will encourage the onions to dry down, or cure. This increases their storage life and makes them less susceptible to mold and rot.
Harvesting & Curing Onions
Judging harvest time for onions is one of the easiest tasks in the garden—they quite literally go to sleep when they’re done. When about half the tops have fallen over, that’s your signal. At this point, I gather up all my onions and get them ready for bed.
Harvesting
Pull the plants gently from the soil and lay them out in a warm, dry place out of direct sunlight. A porch, shed, or shaded table works beautifully. Let them cure for about one week, or until the leaves and necks are fully dry.
Trimming or Braiding
Once cured, you can trim the tops—just be sure not to cut into the bulb itself. Leave a couple inches of stem to keep the onion sealed and protected. If you’re feeling fancy, you can braid the tops, but I usually opt for the simpler route.
Storing Your Onions
I place my cured onions in mesh bags and hang them from hooks in my pantry. Airflow is key to preventing rot, and hanging keeps the bulbs dry and visible.
Keep in mind that onion varieties differ widely in their storage abilities.
- Long keepers like Australian Brown can last for several months.
- Short keepers like Ailsa Craig need to be enjoyed within a few weeks.
When I have a bumper crop of the shorter-lived types, I slice them thin with a mandoline and freeze them. They’re not crisp after thawing, but they’re perfect for soups, sautés, and any cooked dish—an absolute gift on busy evenings.
Common Question: Can I Replant My Unused Bulbs?
In short: no. Onions are biennials, which means their second year is devoted to producing a flower stalk—not another bulb. If you replant leftover onions, they’ll simply bolt and bloom. They’re pretty enough in the garden (and they produce seeds!), but they won’t form the large, storage-worthy bulbs you’re after. If you want large, fresh onions every year, you'll need to plant seeds every year.
With a little attention and an early start, growing your own onions from seed just might be the most rewarding task you do this gardening season.
Final Thoughts
Growing onions from seed might feel like jumping into the season early, but it’s one of the most rewarding traditions in the garden. A few trays started in winter become sturdy transplants by spring, and by midsummer you’ll have rows of beautiful bulbs—each one grown from scratch.
Once you understand daylength, give the seedlings their weekly trims, and stay ahead of early weeds, onions are remarkably straightforward. And unlike sets or store-bought starts, seed-grown onions reward you with bigger bulbs, better varieties, and a closer connection to the whole growing cycle.
Ready to grow your own onions from seed? Check out our extensive selection of heirloom onion seeds and find the one that's perfect for your region. Mobile users, be sure to scroll to the bottom to filter varieties for your region's daylength.
