Onion Plant Guide: How to Grow Onions from Seed, Sets and Transplants

To grow an onion plant that makes a full bulb, pick the type that matches your latitude first. Short-day onions begin bulbing at 10 to 12 hours of daylight and belong in the South; long-day onions wait for 14 to 16 hours and belong in the North; intermediate-day onions sit between. Give them full sun and well-drained soil. Start seed indoors 6 weeks to 3 months before your last frost (the counts vary), or plant sets and young plants in early spring once the ground is workable; where winters are mild, sow short-day onions in fall. Water about an inch a week and lift the bulbs once half or more of the tops have flopped.

When to plant onions

Onions go in weeks before the last spring frost. One seed company starts long-day and intermediate-day onions indoors 6 to 8 weeks before the last frost date; another starts its 'Round Tropea' three months ahead and sets the seedlings out 4 to 6 weeks before last frost. Sets go in early spring once the ground can be worked, and one guide says transplants can go out once temperatures reach 50 F.

Counted back from planting out instead, homegrown transplants need about 8 weeks indoors in two guides and 10 to 12 weeks in two others, one of them written for commercial growers. Here are the dated rules at four weather stations. The 1991 to 2020 climate normals give each station a spring date after which another 32 F night has even odds.

Onion planting dates at four weather stations

Station (latitude)	Type the latitude points to (our reading)	Spring freeze date	Start seed indoors, 6 to 8 weeks before	Or start seed indoors, three months before	Set those plants out, 4 to 6 weeks before
Minneapolis-St. Paul, MN (44.9 N)	Long-day	Apr 23	Feb 26 to Mar 12	Jan 23	Mar 12 to Mar 26
Baltimore (BWI), MD (39.2 N)	Long-day or intermediate-day	Apr 10	Feb 13 to Feb 27	Jan 10	Feb 27 to Mar 13
Raleigh-Durham, NC (35.9 N)	Intermediate-day by the overlapping bands; short-day by the 36-degree line	Mar 31	Feb 3 to Feb 17	Dec 31, 2026	Feb 17 to Mar 3
Dallas-Fort Worth, TX (32.9 N)	Short-day or intermediate-day	Mar 8	Jan 11 to Jan 25	Dec 8, 2026	Jan 25 to Feb 8

Our reading throughout: "last frost" is taken as the station's spring freeze date, after which another freeze remains a coin toss. "Three months" is counted to the same day of the month, our count. Latitudes come from the station files, rounded to a tenth. The two indoor counts come from two seed companies' directions; the set-out column goes with the three-month start. The type column applies the latitude bands in the next section to each station; where the overlapping bands and the 36-degree line disagree, both appear. The 6-to-8-week rule covers long-day and intermediate-day onions; short-day onions in mild winters go in during fall (see below).

A cross-check: in Texas, transplants go out from mid-January to early February, which overlaps the Jan 25 to Feb 8 window the set-out column gives Dallas. Use the station nearest you; a USDA hardiness zone cannot replace a frost date.

Short-day, intermediate-day and long-day onions by latitude

An onion stops making new leaves and starts swelling a bulb once days reach a set length, and that trigger differs by type. Guides draw the lines two ways. One seed company gives overlapping bands: short-day onions at 10 to 12 hours of daylight and 25 to 35 degrees north, intermediate-day at 12 to 14 hours and 32 to 42 degrees, and long-day at 14 to 16 hours and 37 to 47 degrees. Others draw one line at 36 degrees north: short-day types generally below it, long-day types north of it.

Onion day-length bands across our sources

Type	Daylight that starts the bulb	Latitude
Short-day	10 to 12 hours; 11 to 12 in one guide	25 to 35 degrees in the overlapping bands; below 36 degrees by the single line
Intermediate-day	Generally 12 to 15 hours; 12 to 14 or 12 to 13 in some guides	32 to 42 degrees in the overlapping bands; some types suited only to 35 to 40 degrees
Long-day	14 to 16 hours	37 to 47 degrees in the overlapping bands; north of 36 by the single line, some doing better north of 40

The wrong type for your latitude. In Minnesota, short-day onions grow but generally make small bulbs. Our reading of why: they hit their trigger early, before the plants have grown the large, healthy tops a full-size bulb needs. Going the other way, at about 39 degrees north the longest day runs 14 hours 55 minutes, so long-day varieties that want 16-hour days are expected to bulb poorly there.

Other hour counts. Some guides give about 12 hours for short-day types and 15 to 16, or simply 14 or more, for long-day types. For mid-latitudes, one guide lets Mid-Atlantic gardeners try all three groups, with long-day and intermediate-day types probably doing best, and a Master Gardener writing from the same region finds intermediate-day varieties the most reliable there.

Which onion start fits your garden

This flow is our reading of the sources above, not a rule any one of them states. The 36-degree line comes from two of them. Two branches are our own inferences: the split at about 40 degrees north (from the 32 to 42 degree band for intermediate-day onions and the note that some long-day types do better north of 40), and sending cold-winter gardens south of 36 to intermediate-day plants in spring (from the same band and its spring planting note).

Onion seeds, sets or transplants

Each start goes in at its own depth. Onion seed sits 1/4 to 1/2 inch under the soil (one guide says 1/4 inch). Sets, the small bulbs grown the year before, go in point up, 3/4 inch to about an inch deep. Young plants go in 3/4 inch to 2 inches deep, depending on the guide; one says never more than 1 inch for sets or plants.

Which to choose. Guides weigh the starts differently. One finds sets the easiest to handle and the quickest to give green onions, with transplants slower but still good. Another rates transplants the most successful start for most Maryland gardeners, and a seed company finds seedling-grown onions bigger and more disease resistant than set-grown ones, all else being equal. Seed costs the least but takes longer.

A catch with sets. Most sets sold in Minnesota are short-day types, the kind that generally make small bulbs there, so read the day length on any bag of sets or bundle of plants. Sets and transplants planted too early often bolt, more so when they are over 5/8 inch across. If you raise your own seedlings, unruly tops may be clipped to 4 or 5 inches at transplanting.

Onion depth, spacing and timing across our sources

What	Range across the guides
Seed depth	1/4 to 1/2 in outdoors; 1/4 to 3/4 in for an indoor start
Spacing in the row	Thinned seedlings 1 1/2 to 4 in (8 in for large bulbs in one guide; 2 in for green onions); sets and young plants 3 to 8 in
Indoor start	About 8 to 12 weeks before planting out; counted from the last frost, 6 to 8 weeks or three months
Days to maturity	85 to 120 for bulbs; from 50 for green onions; 80 to 120 on the Tropea and cipollini listings
Fall-sown bulbs (TX)	Ready May to July

Planting onions in fall where winters are mild

In the South, onions are a winter crop. In Texas, bulb onions are sown 1/4 inch deep from October through December (the printed version says October), transplants go out from mid-January to early February, and bulbs come from May to July. For short-day types, one seed company sows indoors from mid-September to mid-October and transplants 6 to 8 weeks later, with row cover on nights below 50 F. Short-day onions need a mild winter, Zone 7 or warmer, and usually mature in late spring.

Farther north, a fall start needs special overwintering varieties. In Utah they are sown in mid to late August and generally mature in June or July; a seed company sows them from mid-August to early September, under row cover where winters freeze or in tunnels in colder northern areas. One guide plants sets in fall for perennial or multiplier onions.

Italian onions: Tropea and cipollini

Tropea onions are red Italian onions from Calabria, sold long (torpedo) or round. Listings that name a day length call them intermediate-day to long-day types, and two listings give the long Tropea a range of 35 to 48 degrees. Cipollini, Italian for little onions, are small and flat: one seed company sizes them at 3 inches across at most, and 'Borettana' runs up to 4 inches, normally 3.

• 'Rossa Lunga di Tropea': glossy, thin-skinned torpedo, mild, 110 days; 'Red Long of Tropea', 90 days, is for fresh use, not storage.
• 'Round Tropea' and 'Tropea Rossa Tonda': rounds, the first sold as intermediate to long day, the second as medium long day.
• 'Bianca di Maggio': white cipollini, 80 days, long day, about 35 to 55 degrees; keeps under 3 months.
• 'Borettana': yellow cipollini, 105 days, 38 to 50 degrees; keeps to late winter.

One seed company's growing notes suit cipollini to 40 degrees and higher, while the listings above start at about 35 or 38 degrees. Sow them in bands in early spring, unthinned, and pull them by the bunch. Our reading: south of the Tropea ranges above, as at our Dallas station, these varieties are a gamble.

Sun, soil, water and feeding for onions

Onions want full sun: 6 hours or more of direct light daily, ideally 8 to 10. Give them fertile, well-drained soil and aim for a pH of 6.0 to 7.0 (a commercial guide says 5.8 to 6.6); one guide also wants it high in organic matter. Water roughly an inch a week; once a week is usually enough in spring, though dry, windy spells may call for more. Bulb size rises directly with the water the plants get. Stop watering as bulbs reach full size and the tops start to fall; late water delays curing.

Weeding matters most in the first 2 months, while the plants grow slowly. Hoe shallowly, since injured bulbs are more prone to disease, and do not mound soil on the bulbs, which invites stem rot. Guides time the extra feeding differently: one feeds again at 5 to 6 leaves, since each leaf becomes a ring of the bulb, and another side-dresses 1 to 2 weeks after the bulbs start to swell; in Utah, feeding stops after mid-July. Pull any plant that starts to bolt and use it as a scallion.

What to plant with onions

Set shallow-rooted onions among taproots such as carrots and radishes so they compete less, or alternate onion and pepper rows. In an Iowa study, onion, thyme and nasturtium helped lower looper and cabbageworm damage on broccoli. Keep onions away from beans, which they stunt. More in our companion planting chart.

When to harvest onions, and how to cure them

Harvest bulb onions when the tops fall over: guides wait for about half or about two-thirds of them to go down (a commercial guide says usually one-third to one-half). Once half are down, one seed company pushes over the rest and harvests within a week. Handle them gently, since bruising invites storage rot.

Curing dries the neck and skins so the bulb keeps. It takes 1 to 4 weeks, depending on the guide: from 1 to 2 weeks in an airy attic or porch out of direct sun, to 2 to 4 weeks at 75 to 90 F until the necks are tight. Not every guide keeps them out of the sun: one seed company field-cures in the sun for about 10 days, under a tarp when it rains. Then cut the tops back to a stub of about 1 to 2 inches, or braid them.

Storing onions

Keep onions cold and dry. Near 32 F at 65 to 70 percent humidity, cured bulbs keep for as long as 6 to 7 months in one guide (about 1 to 8 months by variety in a commercial guide), or 2 months or more in ventilated bags in a cool basement or garage. Store them at 32 to 40 F: they start to sprout above 40 F and must not freeze. Use soft or sprouting bulbs first. Globe types tend to keep longer.

Green onions and scallions

Any onion can be eaten green. Cut green onions once the tops are 6 inches tall, or pick them from pencil size until they begin to bulb; they can be ready as early as 50 days from seed. Thinnings count too, and green onions manage on 5 hours of sun a day. For greens alone, grow a bunching type such as 'Evergreen White Bunching', a mild green onion, or 'Evergreen Hardy White' (Allium fistulosum, 65 days), a hardy perennial that can be sown every two weeks.

Onion pests and diseases

Thrips are the most common pest of the onion family. These tiny insects hide where leaf meets bulb and turn leaves silver or gray; a stiff spray of water washes them off. Onion maggots make plants turn yellow and wilt: rotate beds and destroy infested plants. Cure bulbs well against neck rot in storage.

Seeds and supplies

Check that a listing names the day length before you buy, and use fresh onion seed: it is short-lived, so retest year-old seed before sowing and discard anything older. For the Italian types, the four varieties above are sold by the seed companies in our Sources list. For starting your own plants, a seed-starting tray with a clear dome does the job, and a pair of snips trims tops at planting and cuts the cured tops at storage time.

What you'll need for onions

As an Amazon Associate we earn from qualifying purchases.

• 

  TomorrowSeeds Red Torpedo Tropea Onion Seeds

  A glossy, thin-skinned red torpedo, mild; check the listing's day length.

  Check price on Amazon
• 

  Sustainable Seed Co Borettana Cipollini Onion Seeds

  A yellow cipollini that keeps to late winter.

  Check price on Amazon
• 

  Sunutri Seeds Evergreen Hardy White Bunching Onion Seeds

  For greens alone, a bunching type such as 'Evergreen Hardy White', a hardy perennial.

  Check price on Amazon
• 

  Burpee 72 Cell Greenhouse Kit

  For starting your own plants, a seed-starting tray with a clear dome does the job.

  Check price on Amazon
• 

  Corona FS 3214D ComfortGEL Micro Snips

  Snips trim tops at planting and cut the cured tops at storage time.

  Check price on Amazon

Sources we used

We read every page below in October 2026. When two of them differ, the text above gives both figures or the range between them. Seed companies are marked.

• "University of Maryland Extension: Growing Onions in a Home Garden"
• "Onions and day length" (Erica Smith, a Montgomery County Master Gardener, writing on UMD Extension's Maryland Grows blog)
• "University of Minnesota Extension: Growing onions in home gardens"
• "Utah State University Extension: How to Grow Onions in Your Garden"
• "Texas A&M AgriLife Extension: Onions" and its printed sheet
• "Penn State Extension: Onion Production" (commercial)
• "NC State Extension Plant Toolbox: Allium cepa"
• Companion pairs: "Washington State University Extension: Gardening with Companion Plants", "University of Maryland Extension: Vegetable Succession and Companion Planting", "University of Minnesota Extension: Companion planting in home gardens" and "West Virginia University Extension: Companion Planting"
• "Johnny's Selected Seeds: onion growing notes and listings" (seed company)
• "Fedco Seeds: onions and listings" (seed company)
• "Baker Creek: Round Tropea" (seed company)
• "Seeds from Italy: Onion Tropea Rossa Tonda" (seed company)
• "NOAA NCEI: U.S. Climate Normals, 1991-2020"

Related guides

• How to plant potatoes, another crop set out weeks before the last spring frost
• How to plant garlic in fall, cured and stored much like onions
• How to grow peppers, also sown indoors well before spring
• How to grow tomatoes, for the next round of indoor seed starting
• How to grow carrots, a root crop for the same early spring bed
• How to grow radishes, a quick crop to sow between onion rows
• How to grow cilantro, a cool-season herb for spring and fall
• How to grow spinach, another crop sown in early spring
• How to grow leeks, also started from seed indoors well ahead of spring

Frequently asked questions

When should I plant onions?

In early spring in most of the country: start seed indoors 6 weeks to 3 months before the last frost, depending on the guide, and set out sets or young plants once the ground is workable. In mild-winter areas such as Texas, sow in fall (October through December) and transplant from mid-January to early February.

What is the difference between short-day and long-day onions?

The day length that starts the bulb. Short-day onions begin at about 10 to 12 hours of daylight and suit the South; long-day onions need 14 to 16 hours and suit the North. One seed company puts short-day types at 25 to 35 degrees latitude and long-day at 37 to 47, with intermediate-day types between; other guides draw a single line at 36 degrees.

Should I plant onion sets, seeds or transplants?

Guides differ. One finds sets the easiest and quickest, another rates transplants the most successful start for most Maryland gardeners, and a seed company finds seedling-grown onions bigger and more disease resistant. Seed is cheapest but slowest. Whatever you buy, check that its day length suits your latitude.

How deep do you plant onion sets?

About 3/4 inch to 1 inch deep, point up. One guide says never more than 1 inch.

Why are my onions not forming bulbs?

Usually the type does not suit your day length, or the plants went in too late: late spring plantings mostly grow leaves and never bulb. Short-day types in the North generally make small bulbs, and long-day types belong north of about 36 degrees.

When are onions ready to harvest?

When the tops fall over: guides wait for about half or about two-thirds of them. Bulbs take about 85 to 120 days to mature.

How do you cure and store onions?

Dry them somewhere airy for 1 to 4 weeks until the necks are tight (some guides keep them out of direct sun, others cure them in the field), then cut the tops back to a stub of about 1 to 2 inches. Store them cold and dry, near 32 to 40 F, and use any that soften or sprout first.

Can you eat onion thinnings as green onions?

Yes. Sow thickly and eat the thinnings as green onions. Pick green onions from pencil size, or once the tops reach about 6 inches.