How to Grow Onions from Seed
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How to grow onions from seed takes patience, because the crop needs a long season and the right day length before the bulbs form. Seed is the least expensive route, but the Iowa State guide calls it the most difficult for most home gardeners. The guides below agree on the basic steps: start early, sow shallow, thin on purpose, and cure the bulbs before storage. They differ on timing by region, so this guide sets the figures side by side. Nothing here recommends a product.
Quick answer
- Start seed indoors eight to ten weeks before you plan to set the plants outdoors, in bright light and moderate to cool temperatures, as the Iowa State guide suggests.
- Or sow outdoors in spring as soon as the ground can be worked, covering the seed with one-half to three-quarters inch of soil.
- Choose varieties that match your day length, since bulb formation depends on it.
- Expect most cultivars to take 90 to 120 days from planting to maturity, then let the bulbs dry before storage.
Seed is the hardest route, and the sources say so
The Iowa State guide is direct about the trade-off. It says “Growing onions from seeds is the most difficult planting method for most home gardeners”, though seed is the least expensive option. Seed also ages quickly. The same guide says “Onion seed typically loses vigor quickly, and new seeds should be purchased each year”, so buy fresh seed for each season rather than saving old packets.
The Illinois guide puts the choice in a wider frame. It says “Onions may be grown from sets, transplants, or seeds”, and it leaves the gardener to choose by cost, use and availability. Seed gives you the widest range of varieties, which is its main advantage. Sets and transplants are simpler, but they limit what you can grow.
Start indoors eight to ten weeks ahead
Indoor starts give seed the lead time it needs. The Iowa State guide advises starting seed indoors about “eight to 10 weeks before you intend to plant them outdoors.” It adds that seed needs bright light and cool temperatures to grow well, and it recommends 55 to 65 degrees Fahrenheit for best growth.
The same guide notes that onions “can be started indoors from seed and transplanted out in the garden”. Transplants from an indoor start go out once the weather allows, and the outdoor work then follows the planting plan below. Once they are in the ground, see vegetables to plant in raised beds for other crops that share the bed. Plan the indoor sowing backward from your outdoor planting date, not from the calendar alone.
Sow outdoors when the ground can be worked
Outdoor sowing is simpler, but it needs more time in the garden. The Iowa State guide says onion seed “can also be sown directly in the garden” as soon as the ground can be worked in spring. Plant the seed in rows and cover the seeds with one-half to three-quarters inch of soil, which is the depth to keep in mind when you cover the row.
The Clemson guide gives a different plan for fall. It says to “Plant onion seed directly in the garden in fall” in coastal and central parts of its state, and it sows eight to 12 seeds per foot of row at a depth of one-half to three-quarters of an inch. The same guide says seed “is not a recommended method for growing onions in the Piedmont area”, so a fall sowing depends on where you live.
Thin the seedlings to the spacing you want
Thinning sets the size of the bulb you get. The Iowa State guide says that once the seedlings are 2 to 4 inches tall, you should thin the planting. The Clemson guide gives a choice: for larger bulbs, it advises that you thin in spring to 4 inches apart, and then eat the thinned plants as green onions.
The table below gives the spacing from each guide. Where a guide gives no figure for that case, the cell says not stated.
| Source | Seed depth or sowing | Spacing or thinning | Row spacing |
|---|---|---|---|
| Clemson (South Carolina) | ½ to ¾ in, 8 to 12 seeds per foot | Thin to 4 in in spring for larger bulbs | not stated |
| Iowa State | ½ to ¾ in of soil over the seed | Thin at 2 to 4 in tall; plants 4 to 6 in apart for storage onions | 12 to 15 in |
| Illinois (transplants) | not stated for seed | 4 to 5 in apart in the row for large bulbs | not stated |
The gaps between sources are real. The Clemson figure is a thinning target, while the Iowa State figure is a spacing for the plants you keep. The Illinois figure applies to transplants, not to seed. Choose the figure that matches how you started the crop.
Set the bed in full sun
Onions want light and room. The ACES blog says to “Plant in full sun (6 to 8 hours).” The Iowa State guide is more specific about the soil: onions “perform best in well-drained, slightly acidic (soil pH 6-7), fertile soils in full sun.” Choose a bed that drains well and gets full sun for most of the day.
For a raised bed, the soil depth and mix matter as much as the sun. For a guide to working out how much soil a bed needs, see how much soil a raised bed garden needs. Consistent moisture in the bed matters as much as drainage, because the Clemson guide ties steady moisture to root growth and bulb size.
Day length decides the bulb
Bulb formation depends on day length, and that is the most important variety choice you will make. The Iowa State guide says “Onions rely on photoperiod or day length to determine when bulb development begins.” The Illinois guide explains that onions “start bulb formation when the day length is of the proper duration”, and it warns that long-day types “may not get the bulbing signal in the Deep South”.
The Clemson guide reaches the same point from another side. Short-day varieties suit South Carolina, because they set bulbs with about 12 hours of daylight. If you live farther north, check which day-length class your cultivar belongs to before you sow, since a short-day type may not form good bulbs in a long-day region.
Feed, weed and water
Onions need more feed than many vegetables. The Iowa State guide says “onions require higher fertility levels than most other vegetables”, and it suggests about two pounds of all-purpose garden fertilizer per 100 square feet before planting. Four to five weeks after planting, side-dress the row, placing the fertilizer in a narrow band about two to three inches from the base of the plants.
Weeds are a problem because onions compete poorly with them. The Illinois guide says “Onions have shallow roots and compete poorly with weeds and grasses.” The Iowa State guide adds that onions “are shallow-rooted and require careful cultivation around plants to avoid injury to the bulbs.” Hoe shallowly, or pull weeds by hand near the bulbs.
Water matters most during bulbing. The Clemson guide says “Proper soil moisture is critical for continuous root growth”, and it adds that the plant needs moisture for the foliage and the bulb. It also advises that you stop watering about a week before harvest, so the outer scales can form and protect the bulb.
Bolting after cold weather
Cold snaps at the wrong time can cause bolting. The Clemson guide says “Onions will bolt (produce a flower stalk) if exposed to a prolonged cold period”, and it adds that “Planting too early can result in increased seed stem production.” The Iowa State guide describes the same problem: “Onions occasionally bolt” in response to stress or cold, and once that starts, the quality of the bulb falls quickly.
The Iowa State guide advises harvesting bolting onions at the first sign of bolting and using them promptly, since they do not store well. Plant on the calendar the guides describe, rather than as early as the weather allows, to lower the risk.
Harvest, cure and store
Harvest when the tops tell you to. The Iowa State guide says dry or storage onions should be harvested “when the tops have fallen over and start to turn brown.” It also says that most cultivars mature 90-120 days after planting, which gives you a window to plan from. The Clemson guide describes the same stage: harvest when about three-quarters of the tops have fallen over.
Curing comes next. The Illinois guide says “Full air circulation for 2 to 3 weeks” is needed for complete drying and curing. The Clemson guide suggests cutting the tops one to one and a half inches above the bulb and drying the bulbs in a shaded area before storage. Store the cured bulbs in a cool, dry, well-ventilated place.
Pests and disease
The Clemson guide names onion maggot and onion thrips as the most damaging insect pests, and it says good sanitation is necessary to avoid maggots. This guide gives no treatment advice for any pest or disease. For a control decision, read the label of the product you plan to use and check with your local extension office.
What we could not verify
The sources agree on the basic steps but differ on timing and spacing, and the differences matter for planning. The Clemson guide is written for South Carolina, where fall seeding is possible in some areas and not others, while the Iowa State guide gives an early April to early May window for outdoor sowing. The pages do not give a single day count from sowing to harvest for seed-grown onions.
What the sources do not say
None of the four pages gives a number of days from seed sowing to harvest. The Iowa State figure of 90 to 120 days runs from planting, which may be a transplant or a set, not from seed. The Illinois page’s spacing applies to transplants, and the ACES blog is short, adding a sun figure and a container spacing. Use the ranges here as starting points, and check your own frost dates and day-length class before you sow.
How we chose
The guide relies on three university extension pages, from Clemson, Illinois and Iowa State, plus a short Alabama Cooperative Extension System article. All four were opened with curl on 2026-10-23, and all returned HTTP 200. A North Dakota newsletter was also opened, but its layout made the text unreliable for quotation, so it was not used. The guide uses only what these pages state, and each figure carries its source. It names no seed varieties or products, and it treats no retail listing as a source.
Frequently asked questions
How long does it take onions to mature from seed?
The Iowa State guide says most cultivars mature 90 to 120 days after planting. That count starts at planting, not at sowing, so a seed started indoors will take longer on the calendar than the figure suggests. Plan from the date you set the plants out.
Can you plant onion seed in the fall?
In some places, yes. The Clemson guide says to plant onion seed in fall in coastal and central areas of its state, but it says seed is not a recommended method in the Piedmont. Check whether your area has the winter conditions a fall sowing needs before you try it.
Why are my onions small?
Timing and moisture are the usual causes. The Clemson guide lists small bulbs from late planting or dry soil among common problems, and it describes bolting as producing small bulbs with large necks. If you want larger bulbs, its advice is to thin to 4 inches in spring.
Do onions need a lot of fertilizer?
Yes, more than many vegetables. The Iowa State guide gives a rate for the bed before planting and a side-dressing afterward. Follow the rate on your fertilizer label and adjust it to a soil test if you have one.
What should you do if onions start to flower?
Harvest them early. The Iowa State guide says bolting onions “should be harvested at the first sign of bolting” and used promptly, because they do not store well. Keep the rest of the bed watered and weeded until the other bulbs mature.
Sources
- https://hgic.clemson.edu/factsheet/onion-leek-shallot-garlic/ (opened 2026-10-23)
- https://extension.illinois.edu/gardening/onion (opened 2026-10-23)
- https://yardandgarden.extension.iastate.edu/how-to/all-about-onions (opened 2026-10-23)
- https://www.aces.edu/blog/topics/lawn-garden/grow-more-onions/ (opened 2026-10-23)
