Want to know what really brings tears of joy? A bumper crop of homegrown onions! Whether you prefer sweet, sharp, red, or yellow, onions are one of the easiest (and most rewarding) veggies to grow. They don’t take up much space, they’re pest-resistant, and they store for months. From planting seeds or sets to harvesting big, beautiful bulbs, we’ve got the juicy details to help you master the art of onion growing. Let’s get growing—it’s about to get aromatic in here!
Should I Grow Onions from Seeds or Sets?
You can grow onions from seed or small bulbs called “sets.” If seeding, start indoors and then transplant outdoors.
We recommend onion sets for beginners. They are far easier to grow, give you a jump-start on the season, and have a higher success rate than direct-sown seeds or transplants.
Onion sets look like small bulbs sold at gardening stores; in just 14 weeks, they mature into full-size bulbs. Choose onion sets with bulbs 3/4 of an inch in diameter; larger ones tend to produce stiff necks and go to seed.
Onions grown from seed: are the most economical, though more work. If growing from seed, it’s recommended to start indoors, especially for cooler climates. Seeding outside results in slow and patchy germination. Another benefit of growing from seed is that you can choose more interesting and different varieties versus ordering onion sets.
Onions need to grow in full sun and in a spot where they won’t be shaded by other vegetables. The more energy they can get from sunlight, the larger their bulbs can grow. Soil must be well-draining and loose—sandy-loam to silt-loam, not heavy clay or compact. Mix aged manure or compost into the soil in the fall or early spring to improve texture. Soil pH should be between 6.9 and 6.8. Have your soil tested. Onions don’t grow in high-acid soils.
When to Plant Onions
Plant onion sets (dormant bulbs) that can be planted directly into the ground in early spring, as soon as the ground can be worked and the danger of hard freezes has passed. This is typically around 2 to 4 weeks before your last expected spring frost date.
Start onion seeds indoors: 8 to 10 weeks before your area’s last spring frost date, which typically means planting them in late winter. Some gardeners will start even earlier to grow seeds to roughly the same bulb size. Check out our tips for growing onions from seed.
Transplant seedlings or plant starters outside in early spring when the soil is workable, about 2 to 4 weeks before the last spring frost. Onions require soil at least 50°F to germinate, so they should be started indoors about 6 weeks before transplanting to the garden and no longer likely to dip below 28°F (-2°C). This means snow has melted, the days are warmer, and nights are still near freezing. For us in the northern zones, we’re talking April or early May.
A fall-planted crop of onions needs at least 4 to 6 weeks of warm temperatures to become established in the ground. They will remain dormant during the cool season; as the temperatures and soil warm again in early spring, the onions return to life.
Photo credit: YuriyS/GettyImages
How to Plant Onions Indoors
Whether you’re planting seeds or sets, get a shallow tray and fill it with compost to the top. Modular trays with a small section for each bulb are ideal.
Plant seeds 1/4 to 1/2 inch deep, or plant the onion bulbs so that the pointy end is just peeking over the top of the soil.
Lightly water the tray and let it drain through.
Set onions in a warm place, such as a well-lit windowsill.
Thin to 3 to 4 inches.
How to Plant Onions Outdoors
Onion plants are heavy feeders and require constant nourishment to produce big bulbs. At planting time, add nitrogen fertilizer. Many organic gardeners will add an inch of compost to the bottom of each row before planting. Or, dig a trench in the soil about 2 inches deep and 3 inches wide, and then fill the trench back in with about an inch of compost.
If planting onion sets outdoors, bury them 2 to 6 inches apart, gently pressing them 1 to 2 inches deep into loose soil. (Use the closer spacing if you want to pull immature onions as scallions.) Don’t bury them more than 2 inches under the soil. Onions mustn’t be planted too deep, as this can affect bulb development.
If you are direct seeding outdoors, sow seeds 1/4 to 1/2 inch deep once the soil is workable.
Once onions begin to grow, thin them to 3 to 4 inches apart. Plants thinned from the row to create room can be used as green onions. Space the seedlings in rows 12 to 18 inches apart.
If you are transplanting from indoors, don’t forget toharden off the young plants for at least a week.
Mulch with straw between rows to help retain moisture and stifle weeds.
Growing
We find it helpful to think of onions as a leaf crop (like lettuce or kale) rather than a root crop (like beets or carrots). Fostering healthy foliage growth ensures the plants have enough energy to form large bulbs.
Ensure immature bulbs stay covered with light mulch to protect them, retain moisture, suppress weeds, and allow air circulation.
Do not cover emerging onions.
Fertilize every few weeks with nitrogen to get big bulbs. Stop fertilizing when the onions push the soil away, and the bulbing process has started. Do not put the soil back around the onions; the bulb needs to emerge above the soil.
Generally, onion plants do not need consistent watering if light mulch is used. About 1 inch of water per square foot per week, including rainwater, is sufficient. If you want sweeter onions, water more. To deter bolting, water them often during hot spells.
To deter thrips, intercrop onions with tomatoes or carrots in closely alternating rows.
See our video demo to see how to plant and grow perfect onions!
Growing Onions in Containers
You can also grow onions in containers! Prepare a container that is at least 10 inches deep and as wide as possible (to allow for space between maturing onions). A 5-gallon bucket will support 4 to 6 onions, and a tub many more. Place the container where it will receive the maximum hours of direct sunlight per day. (It will be heavy to move once filled and planted.)
Provide a well-draining potting mix that is rich in organic matter; add compost or well-rotted manure. Mix in a complete fertilizer, per the package guidance on container size, or fertilize with diluted fish emulsion or compost tea.
Types
Onion varieties are categorized based on whether they’re “long-day”, “short-day”, or “day-neutral” varieties. This is because they are “photoperiodic,” meaning that hey grow in response to day length—the number of daylight hours needed to trigger bulb formation.
Short-day onions grow best in the southern United States, between 25° and 35° north latitude, and start bulbs when day length reaches 10 to 12 hours. The earlier that short-day onions are planted, the larger they get. In the South, onions can be planted in the fall and overwintered. Since they continue to grow throughout the winter, they will be ready for harvest in the spring.
Intermediate, or day-neutral, onions are best suited to the middle tier of U.S. states, approximately between latitudes 32° and 42°, and start bulbs when day length reaches 12 to 14 hours.
Long-day onions grow best in the northern states, between latitudes 37° and 47°, and start bulbs when day length reaches 14 to 16 hours.
Do not try to fool Mother Nature: Short-day onions planted in a long-day zone will result in small bulbs—not an early harvest.
Pull any onions that send up flower stalks. This means that the bulbs have stopped growing. These onions will not store well, but can be used in recipes within a few days.
When onions start to mature, the tops (foliage) become yellow and begin to fall over. At that point, bend the tops down or even stomp on the foliage to speed up the final ripening process.
Loosen the soil around the bulbs to encourage drying.
Harvest by late summer in dry weather. (Wet-harvested onions do not cure well and might rot in storage.)
When the tops are brown, pull the onions. Handle them carefully, as the slightest bruise (now and in storage) will encourage rot.
Cut the roots and trim the tops back to 1 or 2 inches (but leave the tops on if you are planning to braid the onions together).
Onion’s skin very thin, Mild winter coming in; Onion’s skin thick and tough, Coming winter cold and rough.
Photo by Rootstocks/Getty Images
How to Store Onions
Set onions on dry ground for a few days to cure, weather permitting, or in a protected place such as a garage or barn.
Once cured, hang onions in a mesh bag or nylon stocking; spread up to two layers deep in a box; or braid and hang them in a cool, dry, well-ventilated area. The ideal temperature range for storage is 40 to 60°F (4 to 15°C). Do not store in a refrigerator, as conditions will be too damp.
Check periodically for sprouting or rotting onions and remove them.
Don’t store onions with apples or pears, as the ethylene gas produced by the fruits will interrupt the onions’ dormancy. Onions may also spoil the flavor of these fruits (as well as potatoes).
A pungent onion will store longer than a sweet onion. Sweet onions have a high water content and do not keep well. Eat the sweet varieties first and save the more pungent onions for later.
To avoid bruising, store them so that they do not touch each other—for example, in clean old pantyhose. Slip in the onions one at a time, tying a knot between each one. Hang them in a cool, dry place.
Check out this video to learn how to harvest and store onions.
Gardening Products
Pests/Diseases
Thrips: To control thrips—tiny insects about as fat as a sewing needle—take a dark piece of paper into the garden and knock the onion tops against it; if thrips are present, you will spot their tan-colored bodies on the paper. A couple of treatments with insecticidal soap kills them. Follow the package directions. Spray the plants twice, 3 days apart, and the thrips should disappear. To prevent future infestations, consider using row covers.
Onion Maggots: Cover your emerging onion crop with fine mesh netting or row covers. Seal it by mounding the soil around the edges. The onion maggot fly likes to lay its eggs at the base of plants, so the netting should prevent that. You should also keep mulch away because the insects like decaying organic matter, and make sure you completely harvest your onions as the season progresses. Onion maggots are usually a problem in very rainy periods, so these precautions may be unnecessary if you have a dry season.
White Rot: This very serious disease starts with infected plant material. Avoid transplants grown in soil-based compost. Stick to starting from onion seeds, onion sets, and bulbs bought from inspected producers. With white rot, the foliage will yellow and wilt; below ground, white, fluffy fungal growth appears on the base of the bulb. Unfortunately, once rot sets in, there is no solution. Dig up the crop and dispose of it in the trash (do not compost). Avoid growing onions in the same location in future seasons, as the dirt will remain infected for many years.
Wit and Wisdom
In terms of location, practice crop rotation with onions. Don’t plant them in the same location year after year, as this can encourage the spread of diseases that affect the crop. Learn more about crop rotation.
Can You Plant a Sprouted Onion?
Yes, you can plant a sprouted onion, though you won’t get more onions from it. You will get lots of tasty green sprouts, however! Here’s how to do it:
Fill a pot with potting soil and make a hole in the middle about the onion’s depth and width.
Place the onion in the hole and cover with soil.
Water and put the pot in a sunny spot.
Harvest the green sprouts as needed for cooking.
If you get a sprout with a flower, wait until the flower goes to seed. Save the seeds for planting in the spring.
Fun Onion Facts and Folklore
Onions were worshiped in ancient Egypt, where their concentric circles were seen as symbols of eternal life.
In the Middle Ages, onion juice was believed to cure baldness, snakebite, and headaches.
A generation or two ago, children were treated with a poultice of mashed onions applied as a paste to cover a wound.
A whole onion eaten at bedtime was prescribed to break a cold by morning, and sliced onions were placed on the soles of the feet to draw out fever.
Early settlers made cough syrup by steeping raw onion slices in honey overnight.
A raw onion rubbed on a bee sting or insect bite will relieve the pain and itching.
To make onions taste milder, soak them in milk or pour boiling water over the slices and let them stand for 20 minutes. Rinse with cold water.
Chopping onions can sometimes look daunting: There’s the skin and the layers… Where to begin? Check out our tips for chopping onions in four easy steps. Onion skins actually have several health benefits, too, so don’t throw them out!
Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann
Contact your local Agriculture Extension Office for a soil test kit. The results will provide a wide spectrum of information and advice on steps to take to change the soil to improve it.
Our onions have beautiful tops but the size of the bulb is about the same as when we planted them. I read here that depth of planting appears to be critical and we may have planted too deep but we followed instructions on the package - we used onion sets, red and yellow. It appears we may need more nitrogen but are there any other options for us with fall coming in the not too distant future to increase the size of the onion bulb. We're in region 5. Suggestions would be appreciated. In advance, thanks for your assistance.
I think your Bulb Size problem is you're using onion sets... Use the onion plants if you want large storage onions like you buy at the grocery store... I've been growing a garden for almost 50 years... We only use onion sets for green onions They never get large bulbs only about 1-inch bulbs... We always use onion plants you can buy at your local garden centers in a bunch with about 60 plants per bunch... There is a good site online called Dixondale Farms with all the plants and growing info you would need to know to grow nice large onions... Google it...
Glad you asked, Dick! I had the same outcome when I pulled a couple of reds recently. There are several factors involved and most have to do with soil. pH is critical; it should be between 6.0 and 7.0 (even 6.8). And it should have a couple of inches of composted organic matter in it. A good slow-release fertilizer helps, too. Or you could fertilize ever couple of weeks or so. Good drainage is essential…and so is consistently regular watering. And weeding. You could be right about nitrogen; do a soil test. And consider that onions are light/sun-sensitive. You need to know that they are suited to your latitude (day length).
Does any of this sound like something/s you could have done differently?? (It does to me!)
The area at the base of the stem, where it meets the bulb, is indeed vulnerable. If the bulbs are still small and far from ready to harvest, you might want to cover them with a light mulch that will simultaneously protect it, keep moisture in, suppress weeds, and allow air circulation.