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 day light.
Day length or day light 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 day light 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 day light hours you will have during the growth 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.
Day Light Hours and Onion Bulb Formation:
- Onion bulb development is highly dependent on day light. Onions quit forming leafy tops and begin to form bulbs when the day light each day reaches a certain length. The amount of day light needed for an onion plant to begin forming a bulb varies by variety.
- Be reminded that the length of day light in your part of the world varies throughout the year depending upon latitude. The variation in day light 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 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 day light at the South Pole. This is, of course, reversed at the solstice occurring 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 day light hours during the summer and the shorter the number of day light 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 day light 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 day light 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 day light each day to form bulbs. Long-day onions do best north of the 36th parallel or latitude 36°N.
Temperature and Onion Bulb Formation:
- Onion seeds sprout usually within 7 to 10 days. 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 Variety Growing Tips:
- Long-day onion varieties need fourteen to fifteen hours of day light 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 day light 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.
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.