Grow Your Own Organic Onions

Onions are a must-grow in the cool season. They're pretty straightforward to grow, but the best thing about them is their ability to help protect your garden from pests. For 100 percent organic pest control, plant onions near leafy greens and plants that are prone to aphid attacks.

I tend to lump onions in with root crops like radishes, carrots, and beets, but onions are technically bulbs. And bulbs aren’t roots at all but modified stems that contain underground buds with overlapping leaves. Onions are in the Amaryllidaceae family alongside other bulbs like garlic and leeks. So if you're familiar with my Leaves, Roots, and Fruit system, you'll see some overlap with root crops and then a couple things unique to bulbs.

Let's get into how to grow onions organically.

When to Grow Onions

Onions are cool season plants that love temps between 45 and 80°F. They're frost tolerant and can be planted in the spring as soon as the soil can be worked (typically when the temps go above 45°F). A good timeline to follow is about 45 to 60 days before your last frost date. Onions actually need this time in cool to cold weather to get established and start sending up leaves so they can put on their best bulbs for you later.

In warmer climates, onions can be planted in the fall and overwintered. You'll want to get them planted about 4 to 6 weeks before your first frost date so they have time to get settled. They'll go dormant during the coldest part of the year and then start to grow again once the soil warms up a bit. They'll be ready for harvest in the spring. 

In colder climates, onions can still be planted in the fall if they'll be protected under a cold frame for the winter.

The Best Types of Onions to Grow

Onions are what we call photoperiodic. That's just a fancy way of saying their growth is triggered by the number of daylight hours. You should grow the best onion type for your climate. Don't worry—you'll have plenty of tasty options for red onions, yellow onions, white onions, and maybe even some pink specialties within each group.

Short-Day Onions

If you live in a warmer area (typically zone 7 or higher in the United States), you'll grow short-day onions throughout your winter months and harvest them in the spring. These onions only need the days to be 10 to 12 hours long to form bulbs.

If you live in a colder climate and try to grow short-day onions, your plants will try to form bulbs too soon, which will actually stunt their growth.

Intermediate/Day-Neutral Onions

This onion variety is ideal for those of you in moderate climates. They form their bulbs when the days are 12 to 14 hours long.

Long-Day Onions

This type grows best in colder climates (zone 6 or colder). They'll be planted later in the spring and won't grow until the days are at least 14 hours long. When I lived in Chicago, I'd plant onions in March and harvest them in July or August.

Where to Grow Onions

The most important thing is to pick a sunny spot. Bulbs need full sun, so anything from 6 to 12 hours of direct sunlight a day. You can grow onions in a row garden or raised bed. Just make sure there aren't taller plants nearby that will block sunlight from your growing bulbs.

If you're growing in the ground, dig about 6 to 12 inches down and amend the soil with some compost before planting to make it nice and loose. Remove any rocks or debris. The soil in a raised bed is typically looser than in the ground, so you don't need to dig down unless it's looking compacted.

I love to have onions growing in my raised beds because they're excellent companion plants for all my leafy greens.

Growing Onions in Containers

Another option is to grow onions in a container or grow bag. Pick something at least 10 inches deep and wide so you can grow several bulbs together. Make sure your container has at least one good drainage hole in the bottom so your bulbs don't rot. Fill your container with sandy loam soil or a mix of potting soil and compost.

The Best Onion Companion Plants

I love to interplant onions with lettuce, spinach, and arugula to protect them from pests. Onions put off a smell that confuses pests like aphids. They just smell the onion instead of all the yummy stuff. Plus, these greens have really shallow root systems and will be done in about 60 days, long before your bulbs start filling out.

I wouldn't interplant onions with anything that needs to form a taproot because they'll both be competing for space underground.

3 Different Ways to Grow Onions

You can start onions from seed or set, or you can buy bunches of small plants from a seed company or grower.

Growing Onions from Seed

I'll go over how to start onions by seed in a bit. It's the cheapest way to end up with however many onion plants you want to grow, and you don't need any special supplies or anything. Onions grow really easily from seed, so don't be intimidated.

You can direct sow onion seeds in your garden, but I've had way better luck starting them by seed indoors. Onions need that slight head start indoors to form a complete bulb before the end of the season.

Growing Onions from Sets

Onion sets are basically mini onion bulbs (think those pearl onions you can buy at the store). What happened there was the grower grew them to a certain stage before pulling them up, drying them out, and shipping them to you. When you plant them, they start growing again (aren't plants so cool?).

Because their growth was started and then interrupted, onion sets only need about 14 weeks to grow to maturity. They're also a bit hardier than onions grown from seed, so they can better withstand frost and other less-than-ideal growing conditions.

You can buy onion sets online or from your local nursery.

Growing Onions from Starts

You can also just buy little containers of small onion plants from your local nursery. They're usually started by local farmers in non-heated greenhouses. They'll be growing really close together, and you just separate them as you plant them in your garden.

I know it's normally not great to buy pots that have been over-seeded, but it is the best way to go for onions. Otherwise, you'll pay like $3 for a single onion when you could get an entire bag of onions from the store for that price.

How to Start Onions by Seed

Onions will look a little different than most of the other veggies you start indoors. That's because you can sow them really close together. They don't need to be in individual cells or anything, and then you'll just separate them out when you're planting them in the garden. This method has always worked really well for me.

Onions like cool soil, so you won't need a heat mat to start them indoors. You will need a shallow container, some organic seed starting mix, and grow lights. Then follow these steps.

Step one

Fill your container about 4/5 of the way up with damp seed starting mix.

Step two

Onion seeds look like itty bitty pieces of charcoal—good luck seeing them on the soil! (If you do direct sow onion seeds, I recommend mixing them with some sand so you can see where you've already dropped them.)

Take a handful of seeds in your palm and scatter them over the surface of the mix. Again, they won't be growing to maturity like this; we're just growing them this way until they're ready to move out to the garden (and they really don't stay indoors for a very long time at all).

Once your seeds are scattered, press down on them with your fingertips to make sure they have good soil contact. Cover the seeds with just a little bit of seed starting mix. (The seed companies usually recommend a seed depth of 1/4 inch for onions. That's really not that deep at all, so I typically just cover this type of seed instead of burying them.)

Step three

You can cover your little container if you'd like to maintain the moisture level in the soil while you're waiting on your onions to sprout (that usually takes about 7 to 14 days). As soon as you see green, it's time to turn on those grow lights. Leave the lights on for 12 hours a day, and make sure to keep them right over the sprouts. (Don't worry. Those lights are nothing compared to the power of the sun. Lights that are too far away from your little seedlings can be a deal breaker.)

Believe it or not, those skinny little sprouts wearing their black seeds as caps are soon going to become huge onion bulbs!

When to Start Onion Seeds

The best time to start your onions indoors is about 6 to 8 weeks before your last frost date.

Here in Nashville, I typically start my onions around January 1st, even though my last anticipated frost isn't until April. It's because I'm so excited for the upcoming growing season.

If you happen to start your onions a little too soon and aren't able to move them out to the garden space yet because the ground is still hard, you can graduate them up to a bigger container, or you can just give the tops a little trim with a pair of scissors. This allows them to focus their efforts down on the roots instead of trying to stretch tall (which means they'll get too leggy). You don't want your onions to be heavy on top because the whole point of growing an onion is the bulb, right?

How to Plant Onion Seedlings

As soon as your soil can be worked, you can plant onion seedlings in your garden. Seedlings should be ready to go a couple of weeks after starting them indoors.

Use a hori hori or trowel to make a nice, wide trench 2 to 3 inches deep in the soil. Take some organic compost and sprinkle it along the bottom of the trench before planting. This will give your onions a great nutritional start.

Use your fingers to carefully pinch your onion babies out of the tray. Don't separate them out until you're ready to plant so the roots aren't exposed for too long.

Space each little onion about 4 inches apart in the trench. Picture a full onion in your head, and make sure you're leaving at least that much space between each seedling. Use your fingers to push some soil around the little seedlings once you've placed them all. Support them with more soil if needed so that they're standing upright and the white part is completely underground.

Water your seedlings in and watch them over the next couple of weeks while they get adjusted.

It's hard to believe, but each of those tiny little seedlings will eventually become a big onion bulb.

How to Plant Onion Sets

Like seedlings, you can plant onion sets as soon as your soil can be worked in the spring.

Use a hori hori or trowel to make a nice, wide trench 2 to 3 inches deep in the soil. Fill the bottom of the trench with organic compost to ensure your onions have all the nutrients they need to start growing again (they're probably pretty hungry after that long nap!).

Place your sets in the trench with the pointy end up, spacing them about 4 inches apart. Make sure to stagger your rows if you're doing more than one. Push some soil back to fill in your trench. Water the planting area to tell your sets it's time to wake up and start growing.

How to Tend Onion Plants

Onions are pretty much set-'em-and-forget-'em plants. You could use a little fertilizer as they start to grow if you want. Avoid nitrogen-rich fertilizer because that will give you more leaves, and we don't want more leaves. We want bigger bulbs.

If you're growing your onions in the ground, make sure to keep the area weed-free. Many gardeners choose to mulch around their onions to suppress weeds and retain moisture, but I'm not a fan of mulch. If you skip the mulch, use frost cloth to protect newly planted onions from any late-season freezes.

Water about 1 inch per week if you're not getting rainfall.

Even though I often lump garlic and onions in with root crops, you'll treat your bulbs a bit differently once they start swelling underground. You don't need to push any soil around their base if the bulb presses above the soil. It's perfectly fine for the top of the bulb to be exposed.

When Are Onions Ready to Be Harvested?

Onion bulbs require more time to form and be ready for harvest than simple root crops like radishes and beets. We're talking 3, 4, maybe even 5 months from planting time. That puts spring-planted onions ready for harvest around midsummer. It takes time to swell and grow and form all those layers, okay?

Don't worry—the wait is worth it. One tiny seed produces one large, flavorful onion. 

The exact time to harvest will depend on how large you want the bulb to be, the method you used to plant your onions, and how much space they had in the garden. You'll know your onions are approaching harvest time when the leaves start turning yellow and falling over. Wait until they're brown. Then it's time to dig up your underground treasure!

If your onion plant starts sending up flower stalks, it's definitely time to harvest. Your plant has stopped forming its bulb and is now 100 percent focused on making more of itself for the future. Once you get to this point, your onions won't store very well; enjoy them within a couple of days. Another option would be to leave the bulb in your garden so you can save onion seeds for next year.

How to Harvest Onions

Wait until dry weather to harvest your onions. While I like to water the garden the night before a root crop harvest, you'll want the soil to be dry when you pull your onions up. If your onions are wet at harvest time, they won't cure as well and are more likely to rot in storage.

Use a long, thin trowel or a hori hori knife to loosen the soil around the bulbs. Grab the leaves at the base and pull up. Make sure to handle your onions carefully. They may not look as delicate as tomatoes or peaches, but your bulbs can bruise too. And bruised onions won't enjoy a very long shelf life.

Cut the roots and then the tops down to 1 to 2 inches. (Leave the greens if you're planning on being fancy and braiding your onions together).

How to Cure and Store Onions

Your onions are ready to be enjoyed immediately, but if you want them to last a while in your pantry, you'll need to cure them first. Spread out your harvest somewhere cool and dry for a few days. This could be outdoors if there's no rain in the forecast, or somewhere more sheltered like a garage or shed.

Once they're cured, store your onions in a cool, dry spot, away from anything that produces ethylene gas (think potatoes, bananas, apples, and pears). The gas can encourage your bulbs to sprout prematurely, and in return, your onions can actually change the flavor of other produce.

One good way to keep your onions away from other foods is by gently placing your onions in a mesh bag and hanging them up Strega Nona-style.

If any of your onions sprout before you can enjoy them, plant them in a pot with some soil. Put the pot somewhere sunny and water regularly. You won't grow more bulbs, but you'll get lots of tasty onion greens you can cut like chives and toss into your dishes.

Time to Plant Some Onions

Whether you're growing onions all by themselves or interplanting them with your salad garden, it's time to dig in and plant your onions for this coming season!

Source

• University of Nebraska - Lincoln: Institute of Agriculture and Natural Resources