Scoot over leafy greens! Onions are a staple cool-season vegetable with so many uses in the kitchen.
If you’re looking for something new to try in the garden, give onions a shot. You may be surprised at how rewarding the process is.
In this post, I’ll go over how to plant onions and care for them until harvest.
Let’s dive in!
Why Plant Onions
It seems that a majority of recipes require a chopped onion – and for good reason! Onions provide a savory taste to any dish.
They’re good for you, too.
Onions are low in calories, but they’re packed with vitamins and minerals. It’s a great way to add vitamin B6, vitamin C, and manganese to your diet.
If you’ve tried starting onion seeds in the past with no luck, don’t give up hope. With the right variety for your zone and a little soil amendment, you’ll be on your way to mastering this vegetable.
There are also a few different ways to plant onions, so don’t rule them out! Onions can be direct sown, transplanted, or planted as a set (bulb).
Types of Onions
Before I dive into how to plant onions, let’s look at the different types you can grow in a home garden.
There are several different categories of onions. Usually, you’ll see them grouped by their color and growth habit. The common types are white, yellow, bunching (green), sweet, red, and shallots.
You may also see onions grouped by which area they grow best in.
- Long Day Onions are planted in the spring for a harvest in mid-summer. These are best for northern zones.
- Intermediate Day Onions are planted in late winter for a harvest in early summer. These are best for central growing zones, but they can overlap the other types.
- Short Day Onions are planted in the fall for a spring harvest. These are best for southern growing zones.
Popular Varieties to Plant
Here are a few popular varieties to try at home:
- Savannah Sweet Onion – This short-day variety produces sweet onions that are large to jumbo in size. Savannah Sweet is early-maturing, so you can enjoy them first thing in spring.
- Warrior Bunching Onion – This green onion can be planted in fall or early spring. Warrior Bunching Onion grows 1/2″ bulbs, but the green tops are long and crisp. You can even leave this variety in the garden a little longer if you want larger greens.
- Walla Walla Onion – A long-day sweet onion, Walla Walla grows jumbo onions weighing two to three pounds at maturity. The flavor profile is mild – perfect for adding to any recipe.
- Red Creole Onion – Red Creole is a short-day, red onion that’s crispy and flavorful. It also has an impressive shelf life of two to three months. Chop it, and add it to a salad.
- Early White Grano Onion – This short-day white onion has a sweet flavor, great for caramelizing. It’s cold-hardy and grows well in the chilly spring. Early White Grano also has good resistance to Pink Root Disease.
How to Start Onions Indoors
In this post, I want to focus specifically on how to plant onions by seed and transplanting. You can purchase bulbs (or sets), but the varieties tend to be limited.
And onions are easy to start indoors for transplanting.
What You’ll Need
You’ll need proper seed starting equipment for starting onion seeds.
Here are my top recommendations:
To stock up on all your favorite onion varieties, check out the onion seed selection at Hoss Tools. They carry quality seeds with great germination rates.
If you need more options, True Leaf Market also carries a wide variety of onion seeds.
Starting Onion Seeds
Onions grow underground, similarly to other root vegetables. So, it’s a good idea to use pots that are at least four inches deep and wide. This allows sufficient room for seedling growth.
Although, starting onion seeds in smaller seed starting trays is fine for bunching onions.
First, fill several pots with a lightweight seed starting mix. You can premoisten the mix to prevent washing seeds away before they germinate.
Make a 1/4″ indention in the mix for your seeds. Place the seed in the hole and lightly cover it with the mix. Don’t forget to label each pot with the coordinating onion variety!
Place the pots on a heat germination mat under a full-spectrum grow light. The onion seeds should germinate within seven to 10 days.
Keep your seedlings consistently moist until it’s time to transplant.
Before your onion seedlings make the big transition to the garden, they need to be hardened off. This will get your seedlings used to being in outdoor weather conditions.
Every day for a week, you should set your seedlings outdoors. Start with one hour on the first day and gradually increase that time.
Hardening off prevents transplant shock. It also strengthens seedlings to withstand any wind or rainy weather.
To plant onion seedling transplants, space them in the ground every four to five inches. The bulb should be placed no more than two inches deep.
How to Plant Onions Outdoors
Learning how to start onions outdoors is another great option for growing onion plants.
Starting onion seeds can be done in spring, fall, or winter, depending on the type. Refer to the onion variety packet to figure out the best time to sow seeds.
Choose a Planting Site
Onion plants need lots of light to grow successfully. While short-day onions need 11 to 12 hours of light per day, long-day onions need around 14 hours of light per day. So, a full-sun area is a necessity.
Since most onion varieties are growing bulbs beneath the soil surface, soil quality is also very important.
You should be starting onion seeds in soil that’s loose, loamy, and well-draining. This will allow the onion to grow without any hindrance.
If you’ve had trouble in the past, you may have heavy clay soil.
To amend the soil, mix composted organic matter in the top six inches of soil. This will increase drainage and improve the texture and nutrient content in the soil.
Also, avoid planting in low-lying areas that hold water.
Starting Onion Seeds
Onion seeds need three to four inches of space per plant. An easy way to sow onion seeds is by creating a line in your garden. The line should be approximately 1/4 inch deep.
Place a seed every three or four inches, then lightly cover the row with soil.
If you plan on planting multiple rows of onions, space them 12 to 18 inches apart.
Caring for Onion Plants
Mastering how to plant onions doesn’t stop at sowing seeds! Let’s look at how to care for onions until harvest time.
Keeping onions consistently moist is one of the best ways to grow healthy bulbs.
Onions should receive at least one inch of water per week.
Watch your local weather. If you’re going through a dry spell, make sure to water your garden area by hand. If you’re unsure, feel the top inch of soil for dryness.
Nitrogen is essential in onion development, along with phosphorus and potassium. To give onions a boost in growth, apply a balanced or nitrogen-based fertilizer.
Hoss’ Ammonium Sulfate Fertilizer is a great way to improve onion size and flavor.
Or, try out Hoss’ All-In-One Vegetable Fertilizer, which can be applied to a majority of garden vegetables.
Common Pest and Diseases
Sometimes, pests and diseases can get in the way of starting onion seeds successfully. Luckily, your onion plants can usually be saved if spotted in time! Or, at least, you’ll know what to look for next year.
Pink Root is a fungal disease that affects onion roots and bulbs. The most common symptom is the appearance of pink roots. This leads to root damage and decreased bulb size.
Pink Root is spread through water and contaminated gardening equipment. To avoid this disease, clean your tools after every use and try to rotate crops in the garden every season.
Downy Mildew is a common disease that damages onion leaves and, eventually, bulbs. Symptoms include yellowish-brown spots on onion foliage. Foliage will eventually wilt and fall over.
This disease is common in wet, humid areas with little air circulation. Avoid planting onions too close together and remove any surrounding weeds. It’s best to rotate crops in previously affected areas.
Thrips are tiny winged insects that appear yellowish-brown in color. They cause damage to leaves by feeding on them. You’ll often see wilting or brown foliage as a result of thrips.
Eventually, thrips may cause enough damage to decrease bulb size. To prevent thrips, avoid planting close to grain fields. You can apply an garden insecticide to already affected plants.
When and How to Harvest Onions
Harvesting is the simplest part of how to start onions!
Onions can be pulled from the ground before maturity if you wish to use the greens. But, if you want mature bulbs, you should wait until the recommended maturity time.
Onions can take anywhere from 60 to 175 to mature. Green, or bunching, onions typically take the least amount of time.
The foliage on mature onions will start to brown and fall over.
Small to medium bulbs can usually just be pulled up from the soil.
For extra-large bulbs, gently dig around the bulb and loosen the soil around it. Then, pull the bulb up from the ground.
After harvesting onions, lay them out for a few days in a warm, covered area. This allows the bulbs to cure, extending their shelf-life.
After curing, you can trim any remaining greens or roots from the onion bulb.
When stored correctly, onions can last several months. Keep onions in a cool, dark, and dry place. Ideal temperatures for onion storage range from 45 to 55 degrees.
Keeping onions in a lighted area can cause them to sprout, while humid areas can invite mold. Try not to keep onions in a Ziplock bag to avoid trapping moisture. Wire baskets work perfectly for onion storage.
Wrapping Up How to Plant Onions
Learning how to start onions can be trickier than other vegetables. But pulling a mature onion bulb from the garden is beyond rewarding. So, don’t leave out onions on your to-grow list this year!
Round out your cool season garden. Visit the Seed Starting page on our website for more posts like this on how to start your favorite vegetable