Onions are classified as a cool-season crop, thriving in moderate temperatures and sensitive to heat extremes. Temperature is a determining factor for every stage of the onion’s life cycle, from seed germination to final bulb development. Success relies on managing ambient and soil temperatures to meet the plant’s specific needs. Incorrect temperatures can cause the plant to fail to produce a usable bulb or prematurely flower, a process known as bolting.
Temperature Requirements for Different Onion Varieties
The temperature requirements for an onion are closely linked to its sensitivity to day length. This photoperiodic response dictates where a specific onion type can be successfully grown. Short-day onions initiate bulbing when daylight reaches 10 to 12 hours. These varieties are best suited for southern regions and require mild winter temperatures to grow, typically planted in the fall in warmer climates.
Intermediate-day, or day-neutral, varieties form bulbs with a day length of about 12 to 14 hours. They are ideal for the middle latitudes of the United States, allowing for a wider geographical planting range. Long-day onions require the longest daylight period, needing 14 to 16 hours to trigger bulbing, making them the choice for northern regions. Choosing the correct variety ensures the plant develops sufficient foliage before the long days and warm temperatures signal the switch to bulb formation.
Initial Planting and Seedling Establishment Temperatures
The initial phase of growth, whether from seed, sets, or transplants, is highly dependent on soil temperature. Onion seeds can begin to germinate when the soil temperature is as low as 40°F, but emergence is slow at this point. The ideal soil temperature range for quick and consistent germination is between 60°F and 75°F. At this optimal warmth, seeds can germinate in three to five days, while cooler soil temperatures of 50°F can extend the germination period to two weeks.
Once established, young onion seedlings are quite hardy and can tolerate light frosts and snow. They exhibit good cold tolerance and can survive brief exposures to temperatures as low as 20°F. However, prolonged exposure to freezing temperatures can cause damage and lead to the bulb rotting after thawing.
Ideal Temperatures for Vegetative Growth
The vegetative growth stage is when the plant focuses on producing a large, healthy top. Onions thrive best when the air temperature is consistently cool, with an optimal range for leaf production between 55°F and 75°F. Sustained growth and better yields are achieved when the plant remains in this cool, leaf-producing phase for a longer period.
Temperatures that fall below 50°F for ten days or more can be detrimental, especially if followed by a rapid warm-up. This combination of prolonged chilling followed by increasing temperatures can trigger premature flowering, or bolting. Bolting diverts energy away from the bulb and ruins the crop. Conversely, when temperatures exceed the ideal range, growth can slow, and the plant may experience increased water stress.
Temperatures That Trigger Bulb Development
The switch from vegetative growth to bulb development is accelerated by warm temperatures. Once the plant reaches its required photoperiod, consistently warm temperatures—typically above 75°F and up to 85°F—help accelerate the bulbing process. If the necessary warm temperature threshold is not met, the plant may continue to produce foliage but will not form a large, usable storage bulb. High daily mean temperatures are favorable for accelerating the bulb-swelling process.
After the onion is harvested, a final temperature stage is required for curing, which dries the outer skin and neck for long-term storage. The ideal temperature for this curing process is warm, between 75°F and 90°F, with good air circulation for several weeks.