How to Plant and Grow Onions from Seed or Sets

Growing flavorful onions in your garden isn’t too difficult and they don’t need much space. But, if you’re new to vegetable gardening or you’ve never tried raising this particular crop before, there are a few essentials to know about how to grow onions. The following tips will give you the keys to a successful experience, such as the best time to plant them, how much sun onions need, and when to harvest them.

When to Plant Onions

About 2-3 weeks before the average last frost date for your area is the best time to plant onions outdoors. Exactly when to plant them depends on how soon you can work the ground in spring. Onions, which are a cool-season vegetable, can survive light frost but it’s best to wait until your low temperatures stay above 28°F.

In warmer regions, onions can be planted in the fall, at least a month before cool weather arrives so they have time to grow some roots. Then, they’ll go dormant over the winter, and finish growing in the spring when the weather warms up again.

How to Plant Onions

You can either start with seeds or you can buy sets, which are tiny onion bulbs. Many gardeners find it easier to work with sets than the small seeds. You’ll get a harvestable crop a little sooner from sets, too. However, a packet of seeds is typically cheaper to buy than a similar number of sets.

Planting Onion Seeds

When planting onions from seeds, you have two options. Start them indoors 6-8 weeks before you would plant them outside, then transplant them to the garden; or you can sow seeds directly in the ground in early spring. If you have a short growing season, you may want to start them indoors to ensure your onions have at least 3 months of cool weather conditions to develop.

Either way, cover seeds with a quarter inch of soil. Sow 2-5 seeds per inch in a row, then thin them out as they grow (you can throw the thinned seedlings into a salad to add a mild onion flavor). If you are growing onions to a large size, thin them so they are 3-4 inches apart. For green onions, they can be spaced about an inch apart. Keep the soil consistently moist when the green tops are actively growing.

Planting Onion Sets

If you want to grow large onions, it’s best to start with sets. If you’re growing sets into scallions or green onions, plant the bulbs 1½ to 2 inches deep and 1 inch apart with the pointed end up. If you’re growing large onions, plant the bulbs ½ inch deep and 4 inches apart. You can also grow them in containers that are at least 16 inches wide and 12 inches deep. Containers or raised beds are a good option for growing onions if you have soil that doesn’t drain well.

Onion Care Tips

To grow and develop properly, onions need plenty of light, water, nutrients, and cool temperatures.

Light

Besides planting in a full sun location, exactly how much sun onions need depends on the variety. Long-day onions such as yellow ‘Stuttgarter’ stop growing their green tops and start forming bulbs when they receive 14 to 16 hours of light per day, making them a great choice for Hardiness Zone 5 and colder. Short-day onions such as ‘Texas Early Grano’ form bulbs when days contain 10 to 12 hours of sunlight, so they’re a good choice for planting in the spring and fall in the lower third of the United States.

Soil and Water

Onions do best in nutrient-rich, loose, well-drained soil. Adding a generous amount of compost to your planting area before installing your onions will help with all 3 of those soil qualities. If you have heavy clay or rocky soil, raised beds or large containers are the way to go.

Keep your onions well watered throughout the growing season until the bulbs reach the size you want. If your plants get stressed from drought, they may bolt, which means they’ll produce a flower stalk. Once an onion bolts, the bulb won’t store well, but you can still pull it up and eat it right away, either fresh or cooked in a recipe. It’s also important to keep onion beds well weeded to minimize competition for water.

Temperature and Humidity

Onions grow their roots and leaves when temperatures are still on the cool side, 55 to 75°F. When bulbs start to form, however, the plants need warm temperatures and prefer low humidity. If there are a lot of cool, overcast days as the bulbs develop, their growth stalls.

Fertilizer

Onions require a steady supply of nutrients as they grow, so at planting, add a balanced vegetable garden fertilizer to the soil to encourage root growth. After about 3 weeks of growth, you’ll want to boost nitrogen to fuel the green tops; a sprinkling of blood meal around the plants every other week works well.

Pests and Problems

You might think onions are too pungent to be bothered by many plant diseases or pests, and this is true when it comes to mammals like deer and voles. But there are several fungal issues such as purple blotch and blight, and insects such as onion maggot and thrips that can cause problems.

Minimize all of these issues by selecting resistant varieties, companion planting, spacing plants properly, rotating your planting spot for onions each year, and cleaning up all plant debris at the end of the season. For a severe issue, apply natural neem oil to control both plant diseases and insect pests.

Harvesting Onions

You can use green onions and scallions right from the garden. For storage-type onions, harvest when the green tops begin to turn yellow and tip over. Then, gently pull up the bulbs and let them cure in the sun (or a warm, dry, sheltered space such as a garage if rain is in the forecast) for a week.

This curing step helps the onions keep longer. Once they have completely dried, trim off the roots and cut the leaves about an inch from the bulb. Store your onions in a cool place (but not the fridge) until you are ready to use them.

Once you’ve experienced how to easy it is to grow onions yourself, you can use them in all your favorite recipes. For example, try caramelizing onions or grilling them for a homemade burger topper. No matter how you choose to eat them, you’ll love being able to enjoy all the fresh, homegrown onions right from the yard.