Onion planting dates by zone

Day length decides the bulb

An onion makes a bulb when the days reach the length its variety needs. University of Illinois Extension says most varieties are long-day, for northern latitudes, or short-day, for southern ones, so varieties grown in the South do not adapt to the North and vice versa; long-day types may never get the bulbing signal in the Deep South. Texas A&M AgriLife puts short-day onions at 11 to 12 hours of daylight, intermediate-day at 12 to 13 and long-day at 14 to 16.

University of Minnesota Extension says Minnesota’s storage onions are generally long-day types needing 14 or more hours, and that sweet, mild short-day onions grown there will generally make small bulbs; it also says most onion sets sold in Minnesota are short-day. Illinois says short-day varieties may do acceptably in the North if set out very early.

In the South it is the reverse. UF/IFAS Gardening Solutions says Florida gardeners should choose cultivars labeled short-day, such as Granex, the cultivar grown as the Vidalia. Clemson’s Home & Garden Information Center says short-day onions suit all of South Carolina and intermediate-day onions may be grown in the Upstate.

Seed, sets or transplants

Illinois says sets, small bulbs grown the year before, are the simplest way to grow green onions, and that transplants most reliably give large, dry slicing onions. It sorts sets by size: larger than a dime for green onions, since they may bolt, and smaller than a dime for bulbs. Clemson avoids sets more than an inch across for the same reason.

The table’s spring dates are our crop table’s timing for onions: start seed indoors 8 weeks before the median last freeze, set out sets or transplants 3 weeks before it, or sow seed outdoors 4 weeks before it. Our tools carry onions as green onions, so these dates are for spring planting; Minnesota starts transplants earlier, 10 to 12 weeks before planting outside, and plants sets and seed as soon as the soil is workable.

Minnesota plants transplants 2 inches deep and 3 to 4 inches apart in rows 12 to 16 inches apart, and thins direct-sown seedlings to 3 to 4 inches. Illinois plants small sets 1 inch deep, 2 to 4 inches apart, and transplants 1 to 1½ inches deep, 4 to 5 inches apart; it says closer spacing significantly decreases bulb size. Texas A&M AgriLife plants sets or transplants ¾ inch deep and 3 inches apart, and never more than 1 inch deep.

Fall planting in the South

Clemson says onions are usually planted in the fall in South Carolina for a late spring harvest. Its planting table gives sets September 15 to October 15 in the Piedmont and September 15 to November 15 on the coast, onion plants October 1 to November 15 on the coast, and seed September 15 to October 30 on the coast; it also sets out transplants from late December through February, and says seed is not recommended in the Piedmont.

Texas A&M AgriLife sows seed for bulbs ¼ inch deep from October through December, and says onions seeded then or transplanted in January and February should make bulbs in May to July. UF/IFAS plants onions in Florida from September through December, and says they should be ready in four to five months.

Water, weeds and feeding

Minnesota says onions are shallow-rooted and need constant moisture, soaking the soil at least once a week when rain falls short of an inch, and stopping when the bulbs reach full size and the tops begin to fall. Clemson stops watering a week before harvest so the bulbs form their protective scales.

Onions compete poorly with weeds, Illinois says, so hoe shallowly and often; Minnesota warns that injured bulbs are more prone to disease. Clemson says not to hill soil around bulb onions, which can encourage rot, and Illinois hills only green onions.

Minnesota says too much nitrogen gives late maturity, thick necks that are hard to cure, soft bulbs and poor storage, and Clemson that excess nitrogen late in the season delays maturity and causes doubling. Texas A&M AgriLife fertilizes again when the plants have 5 to 6 leaves, saying each leaf forms a ring in the bulb.

Harvest and curing

Minnesota harvests when about half the tops are falling over and dry, Clemson when about three-fourths have fallen, and Illinois when most have, letting them fall naturally: breaking them over early gives smaller bulbs that keep less well. Illinois lifts northern onions in late July or early August.

Curing is essential for storage, Minnesota says: keep the bulbs in a warm, dry, well-ventilated place for two to four weeks until the outer scales are dry and the neck is tight. Illinois gives them full air circulation for 2 to 3 weeks, then cuts the tops 1½ to 2 inches long. Clemson says most short-day onions are very sweet but do not store especially well.

Common problems

Bolting, a flower stalk, Clemson says, follows a prolonged cold spell after good growth, and the larger the plants are when the cold comes, the more of them bolt; the bulbs are then small, with thick necks that are hard to cure. Illinois says to use any plant that has bolted right away.

Minnesota and Clemson name the onion maggot, which bores into the plants, and Clemson onion thrips, which suck sap from the leaves. Minnesota says Fusarium basal rot, Botrytis neck rot and bacterial soft rot infect onions, and that resistant varieties exist for Fusarium basal rot; Clemson says not to plant onions in the same spot more than once every four years.

Common mistakes

• The wrong day length. Illinois says southern varieties do not adapt to the North and vice versa.
• Planting large sets for bulbs. Illinois and Clemson say sets over a dime or an inch across tend to bolt.
• Too much nitrogen. Minnesota says it gives thick necks and poor storage.
• Hilling bulb onions. Clemson says it can encourage rot.
• Skipping the cure. Minnesota says poorly cured onions decay in storage.

We found no onion source for Louisiana, Alabama or Mississippi that we read for this page. The Southern advice here is Clemson’s, UF/IFAS’s and Texas A&M AgriLife’s; the Georgia onion circular did not load.

Questions

Should I grow short-day or long-day onions?

University of Illinois Extension says long-day onions are bred for the North and short-day onions for the South. UF/IFAS says Florida gardeners need short-day onions, Clemson that short-day onions suit all of South Carolina, and University of Minnesota Extension that Minnesota’s storage onions are long-day types.

When should I plant onion sets?

In the North, as soon as the soil can be worked, University of Minnesota Extension says; the table on this page gives our crop table’s date, 3 weeks before each zone’s median last freeze. In South Carolina, Clemson plants sets in the fall as well as late winter.

Why did my onions bolt?

Clemson says onions bolt after a long cold spell following good growth, and that larger plants and sets bolt more often. Illinois says sets larger than a dime are likely to bolt.

When are onions ready to harvest?

When the tops fall over: University of Minnesota Extension waits for about half, Clemson for about three-fourths. Illinois says to let them fall naturally, since breaking them early gives smaller bulbs.

Sources

Freeze dates on this page are NOAA 1991–2020 Climate Normals, for every station the ZIP codes in each zone cite. Zones are the USDA Plant Hardiness Zone Map, 2023. The planting rules are named beside the table. Growing advice is quoted from the extension publications below, each written for its own area.

• University of Minnesota Extension, Growing onions in home gardens , by Marissa Schuh and Cindy Tong (Reviewed 2024). Written for: Minnesota (USDA zones 3a–5a).
• University of Illinois Extension, Onion (Home Vegetable Gardening) . Written for: Illinois (USDA zones 5a–7a).
• Clemson Cooperative Extension, Home & Garden Information Center, Onion, Leek, Shallot, & Garlic (HGIC 1314) (Updated January 20, 2023). Written for: South Carolina (USDA zones 8a–9b).
• UF/IFAS Gardening Solutions, Onions and Shallots (Accessed October 2026). Written for: Florida (USDA zones 8b–11b).
• Texas A&M AgriLife Extension, Onions (Easy Gardening, EHT-037) , by Joseph Masabni . Written for: Texas (USDA zones 6b–10b).

How we write our guides

Published October 27, 2026