When to Plant Onions

Why timing matters so much for onions

Onions bulb when the days reach a set length, whatever the size of the plant. Each leaf the plant has grown by then becomes a ring of the bulb. Planting date therefore determines bulb size: it sets how many weeks the plant has to grow leaves before the trigger arrives. Plant late and you get small bulbs. Utah State warns that onions planted too late in spring may never bulb at all.

Planting too early has its own risk. Clemson explains that a long cold spell after a period of good growth can make onions bolt, and that the bigger the plant when the cold arrives, the more likely it is to bolt. The goal is to get young plants in early enough to grow plenty of leaves, but not so early, or so large, that a cold snap sends them to seed.

Cold-winter regions: early spring

From the northern tier down through the Mid-Atlantic, onions are one of the first crops to go in. Sources differ slightly on how early. Vermont and Montana Extension plant sets about 2 to 4 weeks before the last frost. Vermont’s garden-planting guide and the University of Rhode Island calendar treat onion sets as very hardy and plant them 4 to 6 weeks before. The dates for your zone above reflect this range. Nebraska Extension’s practical test is soil dry enough to work and daytime highs around 50°F. Young transplants shrug off light frost.

Direct-sown seed goes in as soon as the soil can be worked. Onion seed will germinate in cold soil, just slowly. Oregon State’s germination table shows about 13 days at a 50°F soil temperature, 31 days at 41°F and about 4 days at 77°F.

For transplants you grow yourself, count back from your outdoor planting date. Maryland Extension and Utah State allow about 8 weeks of indoor growth, the Maryland Grows video 8 to 12 weeks, and Nebraska 10 to 12. In practice, that means sowing indoors in mid-to-late winter in most northern gardens.

Utah State notes that sets and transplants planted a little later in spring, in late April in Utah, are less likely to bolt. That is a trade-off between bolting and bulb size, so don’t push it much later.

Which type to plant where

• The North (the upper Midwest, Northeast, northern Plains and Northwest): long-day onions, planted in spring. Iowa State calls them the best choice for the upper Midwest.
• The transition band (the Mid-Atlantic, Virginia, Kentucky, Tennessee): intermediate-day onions in spring. A Maryland Grows article calls them the most reliable choice in Maryland, and Maryland Extension adds that long-day types can also do well in the Mid-Atlantic. Tennessee Extension recommends intermediate types for Tennessee. It says short-day onions are typically grown south of 35° N and can bulb too early farther north. The Maryland article puts the dividing line near 36°.
• The Deep South, Gulf Coast and Florida: short-day onions, planted in fall. Clemson says long-day onions cannot be grown well anywhere in the South.

Warm-winter regions: fall and winter

Where winters are mild, onions grow through the cool months and bulb as spring days lengthen. Regional extension calendars give these windows:

• Florida: UF/IFAS plants short-day bulb onions from September through December statewide. Its vegetable guide narrows the window to October in Central and South Florida.
• Texas: Texas A&M sows seed from October through December and sets out transplants in January and February, for bulbs from May into July.
• South Carolina: Clemson sows seed in fall in the Coastal and Central regions only, not the Piedmont. Its planting table lists fall windows for sets and transplants, and its text also allows sets in late winter and transplants from late December through February.
• Phoenix and the low desert: the University of Arizona calendar sows seed in October and November and plants sets from December into early February. Seed-grown onions take about 7 to 8 months there and sets 4 to 5.

In these regions, the planting window balances two risks rather than following a frost date. Clemson cautions that late plantings are more exposed to cold injury, while plantings made too early are more likely to bolt.

The middle of the country in fall

Fall-planting intermediate onions sounds appealing in places like Tennessee. Tennessee Extension says some growers transplant in September or October and protect the plants through winter. It also says the method hasn’t been tested enough there to recommend reliably. For a dependable crop in the transition band, plant in spring.