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Bulb Onion Growing: Day Length and Temperature

Onion bulbs1
onions in garden
Onion Growing: Day length or daylight hours stimulates the onion plant to start making a bulb.

To grow good bulb onions you need to know about how day length and temperature affect onion plant growth. Onions are photothermoperiodic; that means they are sensitive to temperature and also to daylight.

Day length or daylight hours—determined by the sun tracking north in the northern hemisphere during the spring and summer and south during the fall and winter—stimulates the onion plant to start making a bulb (and to stop making leafy growth). Each onion variety will form a bulb only after it has received a certain number of hours of daylight each day for a certain number of days. Onions are categorized into long-day (northern), intermediate-day (central), and short-day (southern) varieties.

Temperature stimulates the onion plant to stop making a bulb and begin sending up flower shoots and forming seeds—called bolting. Once an onion reaches a certain size—and again this differs by variety—temperatures of between 40° and 50°F (4° to 10°C), will cause it to bolt. (Onions are cool-season biennial plants–meaning they require two seasons to complete the cycle from seed to seed; the two seasons are separated by winter cold.)

There are many varieties of onions suited for growing bulbs. When you choose a bulb onion variety for your garden (and your growing region), it is important to know how many daylight hours you will have during the growing season and the low and average temperatures during that time.

Onions first form tops—leaves—then (depending upon the variety and day length) start to form bulbs. For example, long-day onions will quit forming tops and begin forming bulbs when the day length reaches 14 to 16 hours while short-day onions will start making bulbs much earlier in the year—when there are only 10 to 12 hours of daylight.

More onion growing tips at How to Grow Onions.

onions in garden
Onion bulb development is highly dependent on daylight.

Daylight Hours and Onion Bulb Formation

  • Onion bulb development is highly dependent on daylight. Onions quit forming leafy tops and begin to form bulbs when the daylight each day reaches a certain length. The amount of daylight needed for an onion plant to begin forming a bulb varies by variety.
  • Be reminded that the length of daylight in your part of the world varies throughout the year depending upon latitude. The variation in daylight is caused by the tilt of the Earth’s axis of rotation as it makes its year-long ecliptic journey around the sun. At the solstice occurring about June 20–22, the north pole is tilted toward the sun, and so the northern hemisphere has days ranging in duration from just over 12 hours in the southern portion of the Northern Hemisphere—closest to the equator–to 24 hours at the North Pole in the Arctic Circle. (At the same time in the southern hemisphere days range in duration from just under 12 hours in the northern portion of the southern hemisphere to no daylight at the South Pole. This is, of course, reversed at the solstice occurs about December 20–22. At the equinox occurring about March 19–21 and again about September 22–23, the poles are neither tilted toward nor away from the sun, and the duration of a day is generally about 12 hours all over the Earth.)
  • In each hemisphere, the higher the latitude (or distance from the Equator), the longer the number of daylight hours during the summer and the shorter the number of daylight hours during the winter.
  • Because location or latitude determines day length, some onion varieties are not suited for some locations.
  • Short-day onions require just 12 to 14 hours of daylight each day to form bulbs. This happens in the southern regions of the northern hemisphere to not greater than latitude 36°N.
  • Intermediate-day onions require 13 to 15 hours of daylight each day to form bulbs. Intermediate day onions grow best between latitudes 35° to 38°N–but many are adapted for production to latitude 42°N.
  • Long-day onions require 14 to 16 hours of daylight each day to form bulbs. Long-day onions do the best north of the 36th parallel or latitude 36°N.
Leaf, root, and bulb development occurs in cool temperatures between 55° to 75°F.

Temperature and Onion Bulb Formation

  • Onion seeds sprout usually within 7 to 10 days. The minimum temperature for sprouting is 55°F (12.8°C); as the soil temperature increases from 55°F to 75°F the percentage of seeds sprouting will increase and the time to emergence decreases.
  • Onions are adapted to a wide range of temperatures and they are frost tolerant. Leaf, root, and bulb development occurs in cool temperatures between 55° to 75°F. Optimal onion leaf growth occurs at 68° to 77°F (20° to 25°C). Once bulbing has begun onions easily tolerate temperatures higher than 75°F.
  • Bolting—the setting of seed and cessation of bulb development–is driven by temperatures between 40° to 50° (7.2° to 10°C). (Plant variety, planting date, plant size, temperature, and duration of temperature all factor into whether and when an onion plant bolts.)

Bulb Onion Growing Tips

  • Long-day onion varieties need fourteen to fifteen hours of daylight to make a bulb; these varieties grow best in northern states and Canada. Northern European and Alaskan varieties need sixteen hours or more. No long-day varieties can receive enough hours of daylight in southern states to make a bulb. If you plant a long-day variety in a southern region your crop will always fail to set bulbs; your crop can be used for scallions (green onions).
  • To grow short-day onions in northern regions, seed them in the fall, or alternatively plant intermediate varieties in late winter.
  • In southern regions, sow short-day onion varieties in spring and also in late autumn in regions where there is little or no frost.
  • In southern regions, sow intermediate-day onion varieties in late winter, about February, for harvest in summer.
Onion planting
Sow onion seeds ¼ inch to ½ inch deep.

Planting Tips for Bulb Onion Growing

  • A space-wise way to grow onions is in multiple rows on beds 36- to 42-inches wide.
  • Sow onion seeds ¼ inch to ½ inch deep.
  • Add phosphorus-rich fertilizer across the planting bed before sowing, about ½ cup per 10-row feet.
  • Plant or thin globe onions to 3 to 4 inches apart to insure adequate bulb expansion. (Thinned plants can be used for scallions or for transplanting to another area of the garden.)
  • Keep soil moist until germination and foliage growth. Use drip or sprinkler irrigation or furrows in the center of the bed.
  • Protect young bulb onion seedlings from freezing temperatures which will kill plants. Onion plants less than a pencil in width should be protected from frost. If a young onion plant survives a freeze, the plant will likely bolt as soon as temperatures begin to rise in the spring. Fall seeded crops are susceptible to bolting the following spring if warm fall temperatures bring on excessive foliage growth and are followed by low winter temperatures and slowed growth.
  • Onion bolting—sending up a flower stalk and setting seed–is caused by temperatures of 40° to 45°F or below. Flowering causes a decrease in bulb size.
  • Once bulbs have fully formed, many gardeners knock or bend over plant tops or stalks to stop foliage growth and allow bulbs to ripen; this practice is not essential according to many tests.

Bulb Onion Varieties

  • Short-day onions include ‘Granex’ or ‘Grano’ type onions, ‘Yellow Bermuda’, ‘White Creole’, ‘Eclipse’, ‘California Early Red’, ‘Ebenezer’, and ‘Early Strasburg’. Short-day onions are generally considered non-storage or short-storage because they are soft-skinned and easily bruised or cut.
  • Intermediate-day onions are derivatives of ‘Sweet Spanish’ types; they include ‘Stockton Red Globe’, ‘Early Yellow Globe’, ‘Australian Brown’, ‘White Portugal’, ‘Southport Yellow Globe’, ‘Red Wethersfield’, ‘Southport Red Globe’, ‘Italian Red’, and ‘Flat Madeira’.
  • Long-day onions include ‘Yellow Globe Danvers’, ‘Yellow Flat Grant’, ‘Yellow Rynsburg’, ‘Zittan Yellow’, and ‘Sweet Spanish’.

More tips at How to Grow Onions.


Comments are closed.
  1. Thanks so much for this article. Haven’t recently moved to a different planting zone, I could not understand what I was doing wrong. Need to get short day onions and get them in early.

      • helo guider?am benard kiprono a farmer from elkeiyo-marakwet county.(from chugor village)need to grow bulbing we experience high me how to start to bigin on april

        • The best time to plant onions in Kenya is during the short rains. The short rains usually last from October to December. After this period, they are usually harvested in January and February when the weather is hot.

  2. I wanted to know if onions of the texas grano variety or similar, can be grown in tropical west african country of Sierra Leone. We have two seasons, dry November-April and rains April to Sept, theirs also hammattan, cool dry breeze in Dec. Your response will be highly appreciated.


    • Sierra Leone, as you know, lies at between latitudes 7 and 10; the Texas Grano should grow in Sierra Leone given the day length. The tropical climate, rainfall, and soil moisture would likely give you difficulties to overcome. Onions grow best in well-drained soil. So plant in raised beds or rows and plant so that your crop comes to harvest by April or May at the latest. Your dry season should be long enough to bring in a crop. Start with a couple of varieties the first year and keep a journal of your success; next year you can grow a larger crop if the first year is good. Let us know how it goes. And thanks for this very interesting question. Perhaps some other readers growing onions in tropical settings can add comments.

      • Hi, I tried to grow onions here in tropical country Indonesia. My fault to let my 2 weeks old seedling outside, I put them below the awning but eventually the heavy rain splashing and kill them. some saved, but not last longer since the weather were unpredictable that days, some days hot one day heavy rain. I have a plan to start again and put the seedling indoors until they strong enough.

        • A row cover or plastic tunnel set atop a raised bed can protect young plants from heavy rain and drenched soil.

  3. Thanks for making this valuable information available for people like us who to enter in onion business. I want to start large scale onion farm in Guinea; Firstly, I want to try the land.

    Could you please advise me about the types of onion seeds? Land preparation; do I have to raise seedbed for planting seed and also for transplanting?

    Please add any tip that you think is necessary for this adventure.

    • Review the articles on onions listed in the Topics Index. Day-length will determine the types of onion you grow. I would choose varieties that you know have had success in your region. Onions want moisture-retentive and well-drained soil; growing onions in raised or mounded beds is one way to do this easily.

    • Choose a day-neutral onion variety for growing in Papua New Guinea. For details on how to grow onions, go to the Topic Index and look for How to Grow Onions under Onions.

  4. Hello im in tropical West Africa and am interested in growing the Texas grano short day onions. Our daylight here is a minimum 10hrs with temperatures above 70F throughout the year. Hamattan, a colder season is only 1 to 2months long. Our weather variation is more associated with rains and no rains, 6months of each. I would like your assistance in determining when to plant my Texas grano seeds,


    • Short day onions should be planted when the temperature 70 to 75°F (20-24C). Seed should germinate in 10 to 14 days. Seedlings should be grown in a cool location in full sun; choose the coolest time of the year in West Africa to start your seeds.
      Transplant your seedlings to the garden when they reach pencil size in diameter (about ¼ inch/ .6 cm). Transplant in the coolest time of the year. Watering is critical to the development of onions. Onions should receive about 1 inch of water per week (2 inches in sandy soils). Water slowly and deeply to produce healthy onions. If heavy rains are problematic in your regions, plant onions in mounded or raised beds and if necessary use row covers to protect the crop from being flooded. Onions should be grown in compost-rich, well-draining soil.

    • Wild onions–like most plants–gradually adapt over many generations to climate and temperature changes.Through natural selection, plants adapt.

  5. Hello Steve,
    I have interest in growing Red onions, not sure what type, in Northern Sudan in Africa where the temperature gets as high as 110 F. What type of red onions do you recommend for success for that type of temp? I am trying to produce crops in the summer where there is a bigger demand during that time of the year. Thank you in advance

    • Standard 20- and 40-watt fluorescent lighting tubes can be used for seed germination and early seedling growth. Place the tube about 6 inches above the seed starting trays and keep the light at that distance as seedlings grow. Once outdoor conditions are right for transplanting move the onion seedlings to the garden. Early use of artificial light will not affect an onion’s natural day length and temperature triggers.

  6. Dear sir

    I am beginning to grow bulb plants:..gladiola, lilies, tulips, hyacinth, iris’ daffodils And other common commercial plants for GUADALAJARA Mexico region. Objective is multiple thousands. My partners is local Mexican with river bottom, rich soil. The climate here is noted for all types of agriculture products…selling billions to China.

    I am 83, and grateful for your response.

    dick tomlinson

    • If you are growing bulbing onions in your region of Mexico, choose short-day varieties. There are many varieties with the word Texas in the name–such as Texas Super Sweet; they grow well in your region. Other varieties include Red Creole, White Bermuda, and Yellow Granex.

    • Onions can be grown in the north and the south. It is important to choose onion varieties that grow best where you live. If you live in a northern region, choose a long-day variety. If you live in a southern region choose a short-day variety. Check at the nearby cooperative extension or a local garden center for onion varieties recommended for your area.

    • The most widely grown red onion in Ethiopia is ‘Adama’ followed by ‘Bombay’ and ‘Nasik’, both are also red. Check with a university or government agronomist for suppliers in your region.

  7. I am from southwestern part of Ethiopia. Altitude 2200m asl in this area what type of red onion can adapt to grow? Rainfall available roundly the year!Rf above 1500mm

    • Choose day-neutral onions. Check at a nearby plant nursery or seed supplier and ask for day-neutral red onions.

  8. Great info! I learned something new. Hopefully I will have better luck with my onion growing attempts next year. This was my first year growing from seed. I live in LA and planted my Red Flat Italians (Intermediate Day) outdoors the end of January and now it is June 23rd and the bulbs are only as big around as my thumb… Should I leave them out longer? There are so many factors to growing things properly!

  9. Hi Steve,

    Thank you for an enlightening article.

    We are planning to plant dehydrator white onion variety in Egypt, between Lat 25-30, can send you temp schedule for the region. I am looking for the right short day variety with 20%+ dry matter I think from CA. Do you have any recommendations?

    • Check with a nearby agriculture university of government agriculture agency for specific recommendations for your region. If there is a farmers’ cooperative nearby, they also may have specific recommendations for your growing area.

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