I was a little intimidated to grow my own garlic and onion bulbs until I realized how easy they can be to plant and tend—not to mention fun to harvest! Homegrown onions are also super tasty and have become one of my staples I grow every year in my vegetable garden.
There are three ways to end up with onion plants in your garden: onion seeds, onion sets (dried baby bulbs), and onion transplants (little seedlings that look like green onions). Onion sets and transplants are usually purchased at local nurseries and garden centers, while seeds are easy to find online and in stores.
The cheapest way to source your own onions, of course, is by grabbing a packet of seeds. Onions are cool-season crops that take a long time to mature, so you’ll typically sow onion seeds indoors to get a head start on the growing season. We’ll focus today on starting seeds for fall-planted onions in warmer climates.
Note: If you’re in a colder climate, you’ll just apply everything we say to the spring.
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PICK THE RIGHT TYPE OF ONION FOR YOUR CLIMATE
The location of your garden will actually determine not only which type you should grow, but also when you should grow them. That’s because your day length during your spring/summer growing season changes by latitude. The further you move from the equator, the longer your daylight hours are during spring and summer.
Let’s take a quick look at the three different types of onions and some of our favorite varieties within each type so you can make sure you’re growing the best varieties of onion for your garden.
Short-Day Onions
Short-day onions are the best type for warmer climates, specifically plant hardiness zones 7, 8, 9, and 10. This is the type we grow here in Central Texas.
Why are they called short-day onions? you might ask. Well, because they like short days. You know, like those brief winter days when you feel like you hardly saw the sun at all. Short-day onions use those cold, bleak days to get settled in, and then as soon as the sunlight hours start to increase to 10 to 12 hours a day in the early spring/late spring, they’ll form their bulbs. If you live somewhere colder (north of the 35th parallel) and want to grow short-day onions, you’ll find the bulbs to be smaller and the plants will flower quickly due to your longer day length.
Our Favorite Short-Day Onion Varieties to Grow
- Yellow Granex – This is a type of Vidalia onion, and it produces large, super sweet, and crisp yellow onions.
- Texas Early Grano – These extra-large white onions are great cooked or fresh.
- Red Grano – Great all-round heirloom red onion! Perfect for cooking or enjoying raw in salads or as a topping.
Day-Neutral/Intermediate-Day Onions
These onions are ideal for zones 5 and 6, but they can actually be grown just about anywhere. They bulb when the hours of daylight increase to 12 to 14 and the temps are right.
Those of us in warmer and colder climates can grow day-neutral onions as long as we time our planting just right to give these plants the daylight hours they need to produce.
The Best Day-Neutral Varieties of Onions to Grow
- Cabernet Onions – These sweet onions have burgundy skin and are shaped like perfect little globes.
- Flat of Italy Bulb Cipollini – This Italian heirloom produces wine-colored globes with a deliciously sweet flavor.
- Gladstone Onions – These crisp white onions have a delicately sweet flavor that makes them great raw.
Long-Day Onions
This is the best type to grow for gardeners in colder climates, specifically zones 6 and cooler. Long-day onions are typically transplanted to the garden in spring so that they can form their bulbs as the days are growing longer. They need 14 to 16 hours of sunlight a day to form their onion bulbs.
Top Long-Day Onion Varieties
- Walla Walla Onions – These guys are extra sweet and juicy. The only downside is they don’t save very well.
- Sweet Spanish Onions – This type is a yellow globe-shaped onion that’s perfect for caramelizing.
WHEN TO SOW ONION SEEDS IN THE FALL
Since onions take a while to develop from seed, they’re typically started ahead of time.
If you live in the north, you do have the option of starting seeds in the fall to overwinter, but they will need to be protected with row covers from the cold and snow. Most nothern gardeners wait until about 10 to 12 weeks before their average last frost date to start their onion seeds indoors and then transplant them outside once the weather is right. You could also plant seeds directly in the garden once the ground is thawed and can be worked.
I’ve found with short-day onions here in my warm Southern climate, it’s best to start seeds for onion plants in early fall, around late September through November, which is 4 to 8 weeks before our first frost of fall. This gives the seedlings time to settle in and get cozy before they go into winter dormancy.
I have started seeds indoors and moved them out fairly quickly to the garden to overwinter, and I have also sown seeds directly in the garden in a grouping and then separated the seedlings later for their proper spacing in January or February.
HOW TO SOW ONION SEEDS
The following steps will guide you through starting seeds in seed starting trays. If you’re new to indoor seed starting, make sure to read this post for more tips and our recommended supplies.
Follow these 8 steps to sow seeds:
- Step One: Fill your seed starting tray with pre-moistened seed starting mix. Place your tray in a non-draining tray for easy watering.
- Step Two: Drop a pinch full of seeds into each cell. It’s ok to plant onion seeds close together as we will be separating the seedlings later. Use your fingers to gently press each seed into the mix. This ensures good soil contact.
- Step Three: Sprinkle a light layer of seed starting mix on top. Onion seeds don’t really need to be buried since they’re so small. They should only ever be planted ⅛” to ¼” deep.
- Step Four: Fill your non-draining tray with a little bit of water. Place your seed tray inside or outside if the weather is mild. Use a spray bottle to moisten the top of your tray. Avoid pouring water directly overhead, which might displace the seeds.
- Step Five: Cover your tray with a humidity dome or wet kitchen towel to keep the soil consistently moist if starting indoors. Otherwise check soil moisture level often and keep soil moist if tray is outdoors.
- Step Six: If you are starting your seeds indoors and it is below 70°F place the tray on a heat mat or clothes dryer. Make sure your seeds are in a spot in your home that stays above 70°F. Check on seeds daily. Add more water as needed to keep seed starting mix moist while you’re waiting on germination, which typically takes 1 to 2 weeks.
- Step Seven: Remove the humidity dome or towel as soon as you see seeds sprout. You’ll also want to take the tray off the heat mat and begin using a grow light for 12 hours a day. Continue keeping the soil consistently moist. If your tray is outside, place in a sunny location.
- Step Eight: I recommend giving your seedlings more nutrients once they’re about an inch tall since onion seedlings need to grow a while before they’re transplanted. You could water them with diluted seaweed fertilizer or compost tea every other week. If seedlings get top-heavy, trim the green tops back to 3 inches tall. (Use these trimmings as you would chives to add a little oniony flavor to meals.)
HOW TO HARDEN OFF ONION SEEDLINGS
If you started your onion seeds indoors you will need to prepare them to be ready to be outdoors. One week before you plan to transplant your seedlings to the garden, you need to begin hardening them off.
Hardening off just means getting them used to the harsher conditions they might face outdoors. Your onion seedlings will be delicate little things at this point, no thicker than a pencil.
Seed packets recommend doing this about 4 to 6 weeks before your last expected frost date, if planting in the spring for our northern friends. For our southern climate I get my fall seedlings ready for transplanting anytime between November through December.
Even though onions can tolerate freezing temps, you want to baby your seedlings and protect them from too much cold until they’re a little older and wiser. Wait for a sunny day that’s cool but not freezing to begin the hardening off process.
Move your onion seedlings outdoors in a shaded spot for a couple hours. Repeat for an entire week, gradually increasing the amount of sun exposure and the duration the seedlings spend in the great outdoors.
HOW TO PLANT ONION SEEDLINGS
Once your seedlings have been toughened up by the real world a bit, it’s time to move them to their new home. Make sure the spot you’ve chosen for them gets at least 8 hours of direct sunlight a day (onions love full sun if you can give it to them). I recommend growing onions in a raised bed with well-drained soil to give these larger bulbs plenty of room to develop. Raised beds also provide good drainage so you don’t have to worry about anything mildewing or rotting underground.
Loosen the top of the soil down to about 6″ in the planting area and add a fresh layer of compost to give your onion seedlings a little nutritional boost. This compost will be like a housewarming gift to your onion transplants.
Gently remove your seedlings from the seed tray and pull apart into individual seedlings. Try not to break the roots too much. To ease them out of their seed starting tray, squeeze the cells gently and tilt the tray a bit. Plant them immediately to prevent the roots from drying out.
Plant your onions about ¾” to 1″ deep and 4″ apart. That should give each bulb plenty of room to form underground. If you want to give your roots an extra nutritional boost, add a little bit of MicroLife’s Multi-Purpose granular fertilizer to the planting area before placing the transplant. This will help them form a strong root system.
Water your onion bed gently to welcome your little green babies to their new home.
TENDING TIPS FOR OPTIMAL BULB FORMATION
Follow these tips to ensure you onion crop is large and juicy!
Tip #1: Add extra nutrients
Onions are what we call heavy feeders. If you think about the size of the bulbs they have to plump up, it makes sense they’d need a lot of vitamins and minerals to help them out. Once your onions have been growing in the garden for about a month, begin adding an organic fertilizer high in phosphorus, such as MicroLife’s Maximum Blooms, to promote better bulb growth. Make sure to water fertilizer in well. Repeat application every 2 to 3 weeks.
Tip #2: Water consistently
You gotta deliver on the water if you want your onions to form nice, plump bulbs. Onions have pretty shallow root systems, so they need the soil at the top of the garden bed to stay moist (but not soggy or they’ll rot!). Nothing beats drip irrigation at efficiently giving your plants a long soak at the soil level. (If you don’t have drip irrigation set up yet, Garden in Minutes has a great grid system that’s super easy to install.)
If you’re watering by hand, come out in the early morning and aim your water at the roots, not the leaves, for best results.
Tip #3: Leave the bulb top exposed
You don’t need to keep piling soil over the top of the developing bulb (the neck) as you would with other roots like carrots and potatoes. It’s preferable to leave those necks sticking out. When your onions are nearing their time to harvest, start pressing on them to gauge their firmness. When they feel soft, stop applying fertilizer.
Tip #4: Cover onions before a hard freeze
While onions can handle freezing temps they need to be protected from hard freezes. Use frost cloth and/or greenhouse plastic to keep them safe.
HOW TO HARVEST ONIONS
The arrival of spring means it’s onion harvest time. Most onions take about 100 to 160 days to mature from seed, depending, of course, on the onion variety and the weather conditions.
You’ll know your onions are ready for harvest when their tops turn yellow and brown, die back, and flop over. Stop watering your onion bed to give the bulbs time to firm up. Wait for a dry day to harvest.
Use your hands or a gardening fork to dig gently around each bulb and lift it up. I say gently because damaged bulbs can’t be stored. The bulbs now need to be cured in a dry location for 2 to 3 days. Some gardeners cure onions outside under the sun, while others hang them upside down in sheds or garages. If you opt for outdoors, cover the bulbs to prevent sun scald and to keep dry.
Once your onions are cured and dried, you can cut the onion tops down to an inch or so, remove most of the roots, and brush off any remaining dirt. They’re now ready to be stored in a dry, dark place.
We hope you enjoy your homegrown onions for many months! Let us know in the comments if you have any questions.
