When to Plant Onions: Fall in the South, Spring in the North

Plant onion sets and transplants in the four weeks before your average last spring frost, unless you live in the Deep South, where onions go in during fall and winter. That puts spring planting around March 13 to April 10 in Chicago and March 26 to April 23 in Minneapolis, while Texas A&M sets transplants out in mid-January to early February. Which onions you buy depends on your latitude, not your frost date.

Most guides give the three onion types and a list of hours, and stop there. We computed the day-length type for 40,567 ZIP codes in the lower 48 states from their latitude, checked the hour labels against the actual length of the day, and put the published Southern and Northern planting windows next to NOAA frost dates for 27 cities.

Onion planting dates for 27 US cities

For spring planting, each window runs from four weeks before the median last frost to the median date itself. That four-week count-back is the same plant-out date our guide to starting onion seeds indoors uses, so start seeds 8 to 10 weeks before the first date. Frost dates come from NOAA’s 1991–2020 normals for the station nearest each ZIP code.¹ Where a local extension service publishes a fall or winter window, the table shows that window instead.

Onions are hardy plants that can take heavy spring frosts, according to the University of Maryland Extension,¹⁷ so the median frost date is not a hard limit. For any other ZIP code, look up your frost dates and their odds.

Short-day, intermediate or long-day: go by latitude

An onion starts forming its bulb when the days reach a certain length, and that length differs by type. NC State Extension puts short-day onions at about 10 to 12 hours of daylight, intermediate-day onions at 12 to 14 and long-day onions at 14 to 16.⁴ Because summer days get longer the farther north you are, the type has to match your latitude. The University of Maryland Extension’s garden blog draws the line here:

Short-day onions are grown in the South, generally below latitude 36° N.

Long-day onions do best north of that line, it adds. Johnny’s Selected Seeds gives overlapping bands instead: short-day at 25 to 35°, intermediate at 32 to 42° and long-day at 37 to 47°.³ We placed every ZIP code in the lower 48 on those bands.

• 30% of ZIP codes lie south of 36°N. That covers Florida, the Gulf Coast, most of Texas and Georgia, and the southern deserts.
• 10% sit between 35 and 37°N, where Johnny’s lists only intermediate types. That band crosses 13 states, including Arkansas, Kentucky, North Carolina, Oklahoma, Tennessee and Virginia.
• 66% fall inside Johnny’s intermediate band of 32 to 42°, so intermediate onions are the safe choice for most of the country. The University of Maryland finds them the most reliable in Maryland.²

Seven states straddle the 36°N line: Arkansas, California, Nevada, New Mexico, North Carolina, Oklahoma and Tennessee. In those states the right type can change within the state.

Why the hour labels don’t add up

Extension services don’t agree on the hours. We collected the ranges from twelve sources:

Sources: NC State,⁴ Texas A&M,⁵ Ohio State,¹⁵ UGA,⁸ Nebraska,¹³ University of Maryland,^17,2 Minnesota,¹¹ Oregon State,¹⁶ Clemson,⁹ MOFGA¹⁹ and Johnny’s.³

Some of those numbers can’t be literal. We computed day length from latitude, sunrise to sunset, and our figure for Rockville, Maryland matches the University of Maryland’s: 14 hours 55 minutes at the June solstice.²

By that calculation, daylight never reaches 16 hours south of about 47.7°N, which is only 1.6% of lower-48 ZIP codes. Columbus tops out at 15 hours 1 minute, yet Ohio State, which gives long-day onions 16 or more hours, recommends long-day cultivars for Ohio.¹⁵ Daylight reaches 15 hours only north of about 39.8°N. Read the hours as catalog labels; the latitude bands above are the practical guide.

Southern onions: plant in fall and winter

In the Deep South, short-day onions grow through the mild winter and bulb as the days lengthen in spring. Extension calendars there follow the season, not the last frost:

• Texas: Texas A&M seeds bulb onions in October and plants sets or transplants from mid-January to early February.⁶ Onions seeded from October to December or transplanted in January and February should make bulbs from May to July.⁵
• Louisiana: LSU AgCenter plants bulb-onion transplants from mid-December through January, for harvest around May.⁷
• Georgia: UGA’s George Boyhan advises home gardeners who direct-seed to wait until mid-October, and says transplants are sold in November and December.⁸
• South Carolina: Clemson sows seed in early fall and sets transplants from late December through February.⁹
• Florida: UF/IFAS plants from September through December.¹⁰

Measured against NOAA frost dates, those windows open anywhere from a few weeks to six months before the median last frost. Google’s AI Overview for Texas says transplants go in about 4 to 6 weeks before the last spring freeze. That roughly fits Dallas and Austin, but in Houston the Texas A&M window runs from about two and a half weeks before the median last frost to five days after it. Use the local calendar; a frost count-back doesn’t work here.

Too early in fall makes onions bolt

Fall planting has its own risk. NC State warns that planting too early in the fall can increase bolting, the premature flower stalk that leaves small bulbs with thick necks.⁴ Size at planting matters too. LSU AgCenter says transplants that are too large are likely to bolt, and too-small ones may not survive; it suggests plants thinner than a pencil.⁷

Northern onions: plant in spring, about four weeks before the last frost

North of the Gulf states, onions go in early in spring. The University of Maryland says to plant as soon as you can cultivate the garden.¹⁷ Early planting buys leaves, and leaves make the bulb. Texas A&M says each leaf forms a ring in the bulb.⁵ Oregon State’s vegetable breeder puts it this way:

The size of the bulb is dependent on the size of the plant when bulbing begins

The plants handle cold. Our frost-cover guide lists onions with the hardy crops, which take the mid to low 20s°F. That matters because at the start of the four-week window, a hard freeze of 28°F is still likely. Across 37,564 ZIP codes, the median last 28°F night comes 16 days after the window opens; our explainer on frost versus freeze covers the difference.

How the four-week rule compares with Northern calendars

We checked our window against five published Northern calendars. Two agree. Three of the coldest-region calendars start later, at or just after the median last frost:

The 50°F rule explains part of the gap. The University of Minnesota says transplants can go in when temperatures reach 50°F,¹¹ and Nebraska Extension says daytime temperatures.¹³ The two readings give very different dates.

Across 28,209 ZIP codes where winter highs drop below 50°F, the normal daily high reaches 50°F a median 39 days before the median last frost, earlier than our window opens. The normal daily mean reaches 50°F a median 11 days before the frost date, near the end of the window. In Minneapolis the mean reaches 50°F on April 25, just ahead of Minnesota’s May. In the Upper Midwest and Plains, follow the later date.

Sets, transplants or seed

The form you plant changes the timing a little and the result a lot:

• Sets (small dry bulbs) are the easiest, but Nebraska Extension says plants from sets often bloom in midsummer and stop growing their bulbs, because a set is already in its second season.¹² Small sets bolt less. Illinois prefers sets smaller than a dime,¹⁸ and Utah State says sets over ⅝ inch bolt more often.¹⁴ Check the type on the bag, too. The University of Minnesota notes that most sets sold in Minnesota are short-day onions.¹¹
• Transplants (bundles of young plants) give the best results for most Maryland gardeners, according to the University of Maryland.¹⁷ Utah State sets out sets and transplants at the same time, in April.¹⁴
• Seed sown directly in the garden goes in as soon as the soil can be worked: late March to April in Ohio¹⁵ and mid to late March around Lincoln, Nebraska.¹²

Planting too early has a cost of its own. The University of Maryland warns that exposure to cold after planting can trigger a seed stalk,¹⁷ and Utah State says sets and transplants often bolt if planted too early.¹⁴ Clemson explains why: onions bolt after a prolonged cold period that follows good growth, and the larger the plants, the higher the rate of bolting.⁹ That is why the window opens four weeks before the median frost and not earlier.

Garlic follows a different rule: it goes in around the first fall frost, as our garlic planting guide explains. Potatoes go in about when onions do, timed by soil temperature in our potato guide. For the rest of the fall garden, see fall vegetable planting dates and the other vegetable guides.