Growing Onion Seed: Planting Onion Seeds In The Garden


By: Nikki Tilley, Author of The Bulb-o-licious Garden

Growing onions from seed is both easy and economical. If you know how to grow onions from seeds, either method for planting onion seeds will yield an abundant supply of onion crops. Read on to learn more about onion seed starting.

How to Grow Onions from Seeds

Onion seed starting is easy. Onions grow best in fertile, well-draining soil. This should also be worked with organic matter, such as compost. Onion seeds can be planted directly in the garden bed.

However, when growing onion seed, some people prefer to start them indoors. This can be done in late autumn.

The best time for planting onion seeds outdoors is in spring, as soon as the soil can be worked in your area. Place them about an inch (2.5 cm.) deep in the soil and approximately half an inch or more apart. If planting rows, space them at least one and half to two feet (.45 to .61 m.) apart.

Onion Seed Germination

When it comes to onion seed germination, temperature plays an active role. While typically germination occurs within 7-10 days, soil temperature affects this process. For instance, the cooler the soil temperature, the longer it will take for onion seeds to germinate – up to two weeks.

Warm soil temperatures, on the other hand, can trigger onion seed germination in as little as four days.

Growing Onion Seed Plants

Once seedlings have sufficient leaf growth, thin them down to around 3-4 inches (7.6 to 10 cm.) apart. Transplant onion seedlings that were started indoors about 4-6 weeks before the last expected frost or freeze date, provided the ground is not frozen.

Onion plants have shallow roots and require frequent irrigation throughout the growing season. However, once the tops begin to lay over, usually by late summer, watering should be stopped. At this point, onions can be lifted.

Growing onion seed plants is an easy, inexpensive way to keep an unlimited amount of onions on hand just when you need them.

This article was last updated on


Starting Onions from Seed (OnionSets)

Helping Nebraskans enhance their lives through research-based education.

Starting Onions from Seed

submitted by Sarah Browning, UNL Extension Educator

Planting onions from small bulbs or "sets" is not the best way to grow large onions for storage. Plants grown from sets often begin blooming in mid-summer and stubbornly refuse to stop. Once that happens, onion bulbs don't increase much in size. This happens because onions are biennials. They grow foliage and a bulb the first season, then bloom and set seed in their second growing season. Growing the sets counts as one growing season, although it was definitely a short one, and the plants are primed to reproduce by setting flowers after you plant them in the garden. This makes onion sets a great way to grow green onions, but not the best way to grow onions for long term storage.

Because of onions biennial nature, plants grown from seed or transplants don't bloom the first year and can develop larger bulbs. Many mail-order companies and garden centers now carry onion transplants in spring, but you can also grow your own.

Growing Onion Transplants

Onion transplants can be grown in approximately 10-12 weeks. Sow seeds in late February or early March for planting outdoors in early May. Plant seeds ¾ inches deep in a seed-starting soil blend and keep them evenly moist. Once they sprout, provide the seedlings with bright light from a sunny, south-facing window, or better yet, provide supplemental light with fluorescent fixtures placed a few inches above them for 12-14 hours each day.

Transplant the little, grass-like seedlings outdoors as soon as garden soil is dry enough to work thoroughly and daytime temperatures reach 50° F. Onion transplants will tolerate light frosts. Place them 4 inches apart in wide row plantings. When using "wide" rows plants are not placed single file on one long row, but spaced through a row ranging from 6 to 36 inches across. Use a row width that is convenient for you to reach fro both sides, to make harvesting and weed control easier.

Direct Seeding

Onions can also be direct seeded. This is a good option if you can't find your favorite cultivar as a transplant. Plant seeds as soon as the soil can be worked, usually from mid to late March. Wide row spacing also works well when planting onion seeds. Plant the seeds 1/4 - 1/2 inch deep in the soil. Space rows 12-18 inches apart. Once the plants have 5-10 leaves, they can be thinned so the remaining plants are spaced 3-4 inches apart, and the harvested plants used as green onions.

Growing Onions

Onions grow best in well-drained soil, 6.5 pH, with a high level of organic matter. Raised beds, 4-6 inches high, work well to provide good soil drainage if the native soil is heavy. They also need plenty of sunlight, and regular watering. The installation of drip irrigation the length of the rows makes watering easier and more uniform. Don't be concerned if a large portion of the bulb develops above ground that's normal for onions.

This resource was added February 2014 and appeared in the Lincoln Journal Star Newspaper Sunday edition. For information on reproducing this article or using any photographs or graphics, read the Terms of Use statement

University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office


The Nitty Gritty on How to Plant Onion Seed

For planting onion seed, I like to use seed tray inserts that are approximately 4" x 6" or 5"x5". Draw 4 furrows about a 1/2" deep the length of the tray. Plant the onion seed down the line, maybe 1 seed every quarter inch or so. I use a chop stick to help place them.

I plant approximately 20 or sow seeds in each row.

Than gently cover the seed with soil and press down all the soil in the tray firmly with your palm and fingers. Spray with water, enough water to get the top inch or so of soil damp.

Place in a standard size plant flat without drainage holes. When the flat is full of seed trays, cover with a plastic humidity dome or saran wrap.

These would probably germinate fine without a heating mat as the ideal germination soil temperature is 55-75 degrees. But I have seed mats on hand so I place the flat on the heat mat when done.

If you don't have a heat mat, try placing the flat in a warm area such as on top of the refrigerator. Watch that the soil doesn't dry out, and spray when necessary to keep the seed in a damp soil medium.

Caring for the Seedlings

Once the majority of the seeds have germinated, it is safe to take the plastic dome lid off. This is only used to ensure that the soil stays damp for germination.

From this point onward, water from the bottom, by filling the flat about halfway with water. Onions are heavy feeders, so I like to feed them with Fish Emulsion Fertilizer in the water, using a highly diluted amount of 1-2 TBSN's per gallon of water.

It is a good idea to place a gentle oscillating fan pointed at the seed starts. This helps prevent mold and fungus from growing on the top of the soil which can harm the plants. It also helps to create a stronger stem on the plant.

At this point, they can also be taken off the heat mat if you are using one. Onions don't mind the cold, and it will free up your heat mat for growing another crop that needs it like tomatoes or peppers.

Ideally the plants should also be under grow lights, placed about an 1" above the plants, raising the light as needed to stay close to the tops of the plants.

I keep them under the grow lights for the first few weeks, and then if you live in a mild enough climate like I do, they can move outside in a sunny sheltered area if warm enough. Just watch the weather and bring them in if it gets to freezing or below.

Keeping your Onion Seedlings Tidy

As your onions starts grow, they may get long and scraggly. It is perfectly ok to give them a "haircut" to neaten them up. I trim them down to 2-3" high.

Trim them pretty regularly, every few days or at least once a week. When the plants are flopping over on top of themselves, they don't get as much air circulation which is unhealthy for them and will cause rot.

Trimmed onions on the left, scraggily long onions that need a trim, on the right

Preparing your Onion Seedling for Final Transplant

One of my onion harvests, drying out for the afternoon beginning the curing process.

Keep watering, feeding and trimming them until you are ready to plant out in the garden. This is usually around April 1st or so here in the PNW. Ideally they should be as thick as a pencil, but I have planted them when thinner. Ideally they should also have their third "leaf" in.

If it is going to be really wet out, I suggest waiting as the smaller plants may rot if it is too wet out. Also, be sure to harden the plants off over a week or so, as they slowly need to adjust to living outside. Putting them straight in the garden without hardening them off, will put them into shock which is not good.

Don't forget to consider other onion plant family varieties when you plant onion seed. You could also start leeks, chives, white onions, spanish onions, or choose from a vast assortment of heirloom onions. Which onions will you be growing? Happy planting!


Growing Onions from Seeds

If you're in a milder area, you may be able to get away with starting onion seed directly in the garden. For those of us that have a real winter, plant onions inside 8-10 weeks before your last frost date in spring (earlier is better).

Onion plants take around 100+ days to mature, but you can harvest bulbs before they reach full growth.

Starting the Seeds Inside (for Cooler Climates)

For northern growers who are growing onions from seeds inside, or southern growers who want large onions:

  1. Choose a container with good drainage and enough room for around 3 inches of dirt. I use recycled seedling packs from the nursery, but you can also use recycled plastic tubs from salad or fruit, wooden growing trays or even cleaned fast food trays.
  2. Fill your container with damp potting soil, and mark furrows or divots for your seeds – about 1/4 – 1/2 inch deep. Sprinkle seeds along the furrow or place 1-4 seeds in each cell/divot.
  3. Cover seeds with soil and gently tamp down. Mark trays with variety and planting date.
  4. Cover seedling tray with plastic cover or plastic bag to conserve moisture and place in a warm location (70-75°F (21-24°C)) to stimulate germination. I like to use a seed starting heat mat under my trays. Onions may take a while to sprout, so be patient.
  5. Once the onion seeds have sprouted, remove the cover and place your plants in a well lit location.
  6. Keep soil moist but not wet – no standing water! To give your onions extra TLC, water with a natural liquid fertilizer such as fish emulsion, manure tea or compost tea every two weeks.
  7. As they grow, your baby onions will want to get skinny and flop over into a tangled mess. To grow sturdy plant with strong root systems, use a scissors (or just pinch them with your fingertips) to keep the plants trimmed to around 3 inches tall. (I let them get 4-5 inches as it gets closer to transplanting time.)
  8. Use an oscillating fan to toughen your seedlings

Transplanting your Onions to the Garden

To transplant your onions into the garden (about 1 month before last spring frost):

  1. Harden off your seedlings by setting them outside for 2-3 hours in a sheltered location out of direct sunlight and wind. (I use a cold frame with the lid propped open, or snug them up next to the house.)
  2. Each day, add a little more time outside, until they are outside full time.
  3. When it's time to move the onions to the garden, prep your planting area by making sure it is weed free, well-tilled and amended with the organic matter of your choice (aged manure or compost, or worm castings). Plant onions when frost is out of the soil and nice weather is expected for a few days.
  4. I prefer to plant my onions in triple rows in a grid pattern, with roughly a handspan between plants. Eliot Coleman suggests planting in blocks of four seedlings, with wider spacing between the groups of four (6-7 inches). The onions are supposed to push away from each other as they grow, and it's supposed to make weeding easier. I tried this a couple of times, and my grouped onions always came out smaller than my spaced onions.
  5. Gently remove your onion seedlings from the planting tray and tease them apart, taking care not to damage the roots.
  6. Snug the seedlings into the soil in your desired planting pattern. Water gently to remove air pockets around the roots.
  7. Enjoy your onions fresh out of the garden, or prepare them for storage when mature following the directions in the post, “How to Harvest, Cure and Store Onions“.

Starting the Seeds Outside (for Warmer Climates)

Growing Onions in warmer climates:

  1. Work up your planting area to at least 8 inches deep, and amend with aged compost or manure or worm castings.
  2. When the soil temperature reaches 50F (10 C) in springtime (or cools down in fall), sew one to three seeds per inch (2.5 cm), 1/4 to 1/2 inches (6-13 mm) deep, in rows 4 inches (10 cm) apart.
  3. Begin thinning the seedlings when they reach around 2 inches (5 cm) tall. For green onions (scallions), thin to 1 inch (2.5 cm) apart. For larger bulbs, thin to 4 inches (10 cm) apart. Eat the extra seedlings or transplant (if the roots are intact).

Which are better – onion seeds or onion sets?

You can grow onions from sets or seeds. First we will explain the difference between seeds and sets.

Seeds are collected from the flowering tops of onions. Onions are a biennial, so they normally produce seed in their second year – unless they are stressed out and bolt to seed in their first year.

Onion sets are miniature bulbs grown from seed the year before. If you choose to plant onion sets, go for the smaller ones. Larger sets are more likely to have a flower stalk and bolt to seed.

While there's nothing wrong with using onion sets (my mom often did), I enjoy growing onions from seed because I have so many more different varieties to choose from. They also tend to store better and grow larger than my onions grown from sets.

Long Day, Short Day and Day Neutral Onions

Long Day Onions – generally recommended for northern growers, long day onions need about 14 hours of daylight to form bulbs. Planted in spring, mature in fall. Bulbing is triggered after the summer equinox (June 21) in most northern areas.

Short Day Onions – generally recommended for southern growers, short day onion varieties need roughly equal amounts of darkness and light to set bulbs. Planted in fall, mature in spring.

Day Neutral or Intermediate Onions – will form bulbs regardless of the number of daylight hours. Planted in spring in the north and fall in the south.

If you're not sure what to plant, find out what type of onions are best for your area by contacting your local Cooperative Extension office.


How An Onion Grows

There are two important things to know about onions before selecting how you want to plant them.

Onion sets being planted in the early spring.

One is their growth cycle. The second is understanding which onion varieties are best for your climate.

First, let’s talk about an onions growth cycle.

Onions As A Biennial Crop

Onions are a biennial crop. This means they grow, mature and seed over a two year period.

During the first year, an onion grows from a tiny seed to a bulb. If planted early enough from seed, these bulbs will grow large enough to be harvested and used that year.

But if the bulb is allowed to overwinter, it will resume its growth in year two. It is in this second year that the bulb matures to complete it’s growth cycle.

During the second year of growth, the onion will flower and set seed in the bloom head.

When this happens, it sends up a “bloom” and sets seed on the head of the flower. Thus completing the two year growth cycle.

Now on to the second subject, choosing the right variety for your growing zone.

Selecting The Right Onions For Your Growing Zone

The second important fact is that onion varieties are separated into three distinct categories. All of which are based upon the climate, sunlight, and days needed to mature.

Short Day Onions

Short-day onions are mainly grown in the south and southwest. They are the “warmer” climate onions.

These yellow Granex onions are a wonderful short day onion variety.

Short days need around ten to twelve hours of average daylight to begin forming their bulbs.

Although short day onions can be grown in northern climates, the bulbs do not mature to full size.

Long Day Onions

Long day onions are mainly grown in the northern climates. These onions are planted in the spring from sets or seedlings.

Long day onions need to get between 13 and 16 hours of daylight to begin maturing.

Walla Walla onions are a big favorite among long-day onion growers.

Long day onions cannot be grown in southern areas because the daylight never extends long enough to form or mature bulbs.

Day Neutral Onions

Day-neutral onions are a bit different in that they will form bulbs no matter the hours of sunlight.

These onions can be grown anywhere except the extreme south, where it gets a bit too hot for them to mature.

Day-neutral onions need to be planted in the fall in warm climates, and early spring in the north.

Spring onions are easily grown from onion transplants or sets.

So that all leads us to onion sets vs onion seeds and seedlings. And of course, choosing which is best for you.

Here is a break down of each planting method, along with the advantages and disadvantages of planting each way.

Planting Onion Sets Vs. Onion Seeds & Seedlings – Choosing The Best Method To Plant

Growing Onion Sets

Onion sets are small onions grown from seed the previous year. Instead of being allowed to mature, they are harvested as an immature bulb. Then, they are kept dormant until the following spring and planted.

Planting onion sets in the early spring.

Once planted, they mature into full-grown onions during their second year.

The advantage with bulbs is they already have a head start on their growth. Not only can they be harvested sooner, they can also lead to harvesting larger bulbs.

But, there is a disadvantage. With onion sets, you are limited to very few varieties.

In the world of onions, there are hundreds of available varieties available. But with sets, they are usually found only in only the more common white, yellow and purple varieties.

Onion Seeds And Seedlings

The advantage of growing from seed is that you open yourself up to a wide range of varieties.

Although you can direct seed into the ground in warmer climates, onion seeds take a long time to grow and mature.

Planting seed directly can be a tedious process. And, very hard to weed as they sprout.

That means for warmer climates, they need to be sown in the fall or late winter.

For northern climates, seeds should be started indoors 10 weeks prior to moving outside to transplant.

In essence, this is the process of planting onion transplants or seedlings. And, it is a much better way to plant with seeds.

For starters, it allows the onions to get a head start. In addition, it makes weeding and bed care much easier than trying to see the tiny seeds against any weeds as they sprout.

Here is to growing your own delicious crop of onions this year!

Seed Links :

This Is My Garden is a garden website created by gardeners, publishing two articles every week, 52 weeks a year. This article may contain affiliate links.


Watch the video: Starting Onions from Seed


Previous Article

Information About Pineapples

Next Article

The benefits and harms of walnuts for the human body