Onion bulbs in a garden bed

Crop guide

How to grow onions

How to grow onions means matching day-length type and cool-season culture to cited geography, then recording what happened in your bed. Cultivar and latitude change bulbing. There is no single trick.

Growing guidance is general information. Timing, spacing, and care depend on cultivar, weather, soil, and local conditions. Check the sources and adjust the plan for your garden.

Steps (range, not a promise)

  1. 01Read cited day-length notes for your latitude — long-day and short-day sheets are not interchangeable.
  2. 02Treat cool-season culture as source-bound, not a national calendar.
  3. 03Log the bed so next season is not a guess.
Onion plants in a home garden bed

Onions in a home garden bed.

Onion rows in a field

The season, on your ground

Save this bed — free

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Cited cool-season culture (not a climate zone)

Separate source assertions — not a BioArva average.

  • Illinois Extension (Illinois): onion is a cool-season vegetable that can be grown successfully throughout most of temperate North America. Season note on their sheet — not a BioArva climate zone.
  • Clemson HGIC 1314 (South Carolina): onions and related shallots, leeks and garlic all grow best during cool weather. Cool-weather context on their sheet — not a BioArva climate zone.
  • Maryland Extension (Maryland): onions are hardy (can withstand heavy spring frosts). Hardiness on their sheet — not a BioArva climate zone.
  • Utah State (Utah): the onion is a cool-season vegetable; their sheet lists full sun and well-drained soils. Site note on their sheet — not a BioArva climate zone.
  • Maryland Extension (Maryland): bulb onions are biennials treated as annuals; their sheet lists two years to complete the life cycle. Life-cycle note on their sheet — not a perennial program.

Cited planting stock (not a product SKU)

Separate source assertions — not a BioArva average.

  • Illinois Extension (Illinois): onions may be grown from sets, transplants, or seeds. Start-method note on their sheet — not a BioArva product SKU.

Cited day-length notes (not a BioArva zone map)

Separate source assertions — not a BioArva average.

  • Illinois Extension (Illinois): onions start bulb formation when the day length is of the proper duration and different varieties require different day lengths to initiate bulbing. Day-length context on their sheet — not a cultivar ranking.
  • Illinois Extension (Illinois): most common varieties fall into long-day (northern latitudes) and short-day (southern latitudes); onion varieties grown in the South are not adaptable to the North and vice versa. Geography on their sheet — not a BioArva zone map.
  • UMN Extension (Minnesota): storage onions grown in Minnesota generally are long-day types that require 14 or more hours of daylight to form bulbs; their sheet lists full sun. Day-length on their sheet — not a BioArva climate zone.
  • UMN Extension (Minnesota): sweet or mild onions are short-day onions; although you can grow them in Minnesota, they will generally develop small bulbs. Geography on their sheet — not a cultivar ranking.
  • Clemson HGIC 1314 (South Carolina): onions are classed as long or short day varieties based on hours of daylight required to make a bulb; short day varieties set bulbs with about 12 hours of daylight and are suitable for growing throughout South Carolina. Day-length on their sheet — not a cultivar ranking.
  • Maryland Extension (Maryland): if the seed catalog lists the variety as long day, it sets bulbs at 15–16 hours of daylight in Northern summers; short-day types at about 12 hours in the deep South for winter production; long-day and intermediate types are listed for the mid-Atlantic. Day-length on their sheet — not a BioArva zone map.
  • Illinois Extension (Illinois): their sheet lists Yellow and White Sweet Spanish as long-day varieties and Yellow and White Bermuda as short-day varieties. Type/geography on their sheet — not a ranking.
  • Clemson’s sheet lists intermediate-day onions for the Upstate and says long-day onions cannot be successfully grown in the South. Source-specific day-length context—not a BioArva zone map.
  • Cornell (New York): their sheet lists long-day varieties which need about 14 hours of light to form bulbs. Day-length on their sheet — not a BioArva zone map.
  • Maryland Extension (Maryland): bulbs enlarge as day length increases. Day-length context on their sheet — not a BioArva climate zone.
  • Penn State Extension (Pennsylvania): bulbing of onions is primarily induced by photoperiod, or day length. Day-length context on their sheet — not a BioArva climate zone.
  • Penn State Extension (Pennsylvania): their sheet lists long-day and intermediate-day onion varieties for the northeastern United States. Geography on their sheet — not a BioArva zone map.
  • UMass Extension (Massachusetts): onions bulb according to day length; their sheet lists long-day and intermediate-day types for New England. Geography on their sheet — not a BioArva zone map.

Cited harvest and curing notes (source comparison, not a protocol)

Separate source assertions — not a BioArva average.

  • UMN Extension (Minnesota): harvest onions when about half the tops are falling over and dry. Harvest context on their sheet — not a yield promise.
  • UMN Extension (Minnesota): curing is essential if you want to store onions; you must dry them in a warm, dry, well-ventilated area. Curing context on their sheet — not a BioArva program.
  • Utah State (Utah): leave bulb onions in the ground until the tops fall over; once the tops fall over, lift the bulbs but leave them in the garden to dry for 1–2 weeks. Harvest/curing context on their sheet — not a BioArva program.

What to record in the Field Book

Save the bed with the crop and the date. You may add a photo and notes. Compare entries you have recorded with each other — not with a diagnosis from this page. If you want an identification, bring the record to local Extension.

What is the trick to growing onions?

There is no single trick. Illinois Extension (Illinois): onions start bulb formation when the day length is of the proper duration and different varieties require different day lengths to initiate bulbing. Day-length context on their sheet — not a cultivar ranking.

Seed for this guide

No seed offer on this page

When a supplier is listed, it will be outbound and disclosed. Nothing to buy here.

How we know — methodology. Ranges and assertions are cited per source, not blended.