To grow onions, you will need the right conditions, including light and temperature. Day length affects onion growth and bulb development, but onion temperature tolerance depends on the variety.
So, are onions heat tolerant? Onions are a cool weather crop, so they are not as heat tolerant as tropical plants, such as tomatoes and peppers. However, certain types of onions are more heat tolerant, such as bunching onions. You can protect onions from high temperatures by using shade cloth to reduce the heat from intense sunlight.
Of course, it helps to know which onion varieties are heat tolerant and how to protect other varieties from heat.
In this article, we’ll talk about how heat and day length affect onions. We’ll also look at heat tolerant onion varieties and how to protect onions from heat.
Let’s get going.
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Are Onions Heat Tolerant?
Onions are a cool season vegetable. That means they can survive in cooler climates, and they can even tolerate some frost.
The ideal temperature for onions depends on their stage of development.
Temperature For Onion Seed Germination
According to the University of California, the minimum temperature for onion seed germination is 32 degrees Fahrenheit (0 degrees Celsius). At this temperature, it will take 136 days for onion seeds to germinate.
The ideal temperature range for onion seed germination is 65 to 85 degrees Fahrenheit (18 to 29 degrees Celsius). In this range, it will take 4 to 6 days for onion seeds to germinate.
The maximum temperature for onions seed germination is 95 degrees Fahrenheit (35 degrees Celsius). At this temperature, it will take 12 to 13 days for onion seeds to germinate.
The table below summarizes temperature ranges and germination times for onion seeds:
| Temp F (C) | Days To Sprout |
|---|---|
| 32 | 136 |
| 65-85 (18-29) | 4-6 |
| 95 (35) | 12-13 |
germination time to soil temperature.
Temperature For Onion Cultivation (Temperature For Growing Onions)
After germination and seedling growth, you will need to transplant onions plants outdoors. The Old Farmer’s Almanac suggests transplanting outside when temperatures are still cool (but not too cold).
The University of California suggests transplanting onions outdoors 3 weeks before last spring frost. Of course, the exact date will depend on where you live.
You can find the last spring frost date with this tool from the Old Farmer’s Almanac.
For example, the last spring frost date in Boston, MA is April 10. If I want to transplant my onions outdoors 3 weeks (21 days) before that date, then I need to count backwards:
- 10 days in April
- 11 days at the end of March
That means I would want to transplant my onions outdoors around March 20.
What Temperature Is Too Cold For Onions?
Onions are a cool weather crop and can tolerate light frost. However, cold tolerance for onions depends on the age of the plant.
For a young plant, any temperature below 28 degrees Fahrenheit (-2 degrees Celsius) can cause damage to the plant. It could also lead to a lack of bulb formation and early bolting (flower and seed production) later in the season.
If the weather forecast calls for cold temperatures, keep your onion plants indoors a little longer to avoid a hard freeze. Another option is to provide some type of outdoor cold protection for your onion plants, such as:
- Cloches
- Row Covers
- Frost Blankets
- Cold Frame
- Greenhouse
You can learn more about cold protection resources in my article here.
More established onion plants can tolerate temperatures down to 20 degrees Fahrenheit (-6 degrees Celsius).
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What Climate Do Onions Grow Best In?
Day length plays a large role in onion growth and bulb development. However, temperature is also an important factor.
The University of California suggests that onion plants like cool temperatures until they are more established. A temperature in the range of 50 to 70 degrees Fahrenheit (10 to 21 degrees Celsius) is ideal for early in the season.
“temperatures under 50 ̊F for 10 days or longer (if seedlings are greater than 1/4-inch stem diameter) followed by warm temperatures, in conjunction with lengthening days, will induce flowering.”
https://casfs.ucsc.edu/documents/for-the-gardener/onion_leaks.pdf
Later in the season, warmer temperatures of over 75 degrees Fahrenheit (24 degrees Celsius) are necessary for onion bulbs to begin forming. Temperatures below 70 degrees Fahrenheit will slow or prevent bulb formation, even when the proper day length is achieved.
To complete their growth, onions need even warmer temperatures of 75 to 85 degrees Fahrenheit (24 to 30 degrees Celsius). The University of Illinois suggests that onions need heat to set and cure properly.
If you plant onions too late in the season (or plant a variety that takes a long time to mature), they will experience cool temperatures in the fall. Mature onions may spoil in cool fall weather, so be sure to plan ahead.
Choose the appropriate onion varieties, based on the length of your growing season and your location. Of course, heat tolerance also depends on the type of onion and the location where it is grown.
The three basic types of onions are:
- Short Day – these onions need 10 hours of daylight to produce bulbs. They grow best in Zone 7 & warmer (Southern U.S.)
- Long Day – these onions need 14 to 15 hours of daylight to produce bulbs. They grow best in Zone 6 & cooler (Northern U.S.)
- Intermediate Day (Day Neutral) – these onions produce bulbs regardless of day length.
After you harvest and cure onions, they will store best at temperatures of 40 to 50 degrees Fahrenheit (4 to 10 degrees Celsius).
The table below summarizes the ideal temperature ranges for onions at various stages of growth.
| Growth Phase | Ideal Temp Range F (C) |
|---|---|
| Seed Germination | 65-85 (18-29) |
| Transplant Outdoors | 28+ (-2+) |
| Plant Establishment | 50-70 (10-21) |
| Bulb Formation | 75+ (24+) |
| Complete Growth | 75-85 (24-30) |
| Storage | 40-50 (4-10) |
ranges for various onion growth stages.
7 Heat Tolerant Onion Varieties
Bunching onions are more heat tolerant than other varieties. Here are a few heat tolerant onion varieties to try growing:
- Egyptian Walking Onion – this onion variety is perennial in Zones 3 to 9, so it can tolerate the heat of Southern states in the U.S. It is also drought tolerant. The plant produces bulbs at both the top and bottom of the plant! Later in the season, the upper bulbs drag the top part of the plant to the ground, where it roots to spread and “walk” across the garden. You can find Egyptian Walking Onions from Baker Creek Heirloom Seeds.
- Feast Onion – this bunching onion variety has green leaves and a white stem. It has good heat tolerance and is best for a summer harvest. They are good for planting close together. You can find Feast Onions from Stokes Seeds.
- Heshiko Bunching Onion – this Japanese bunching onion variety is hardy in Zones 2 to 9, so it can tolerate the heat of Southern states in the U.S. It produces 12 to 14 inch stalks, which take 60 days to grow to maturity (for bunches) or 120 days (for bulbs). You can find Heshiko Bunching Onions from Eden Brothers.
- Nabechan Onion – this hybrid bunching onion variety is sweet and resists bulb formation in hot weather. This Japanese onion variety grows to maturity in 60 days. You can find Nabechan onions from Johnny’s Selected Seeds.
- Tokyo Long Bunching Onion – this heirloom bunching onion variety is heat tolerant, with dark green leaves and white bulbs. It is hardy in Zones 3 to 9, and grows to maturity in 75 days. You can find Tokyo Long Bunching onions from Eden Brothers.
- White Lisbon Onion – this bunching onion variety is heat tolerant and cold tolerant. It grows to maturity in 60 days. You can find White Lisbon onions from Eden Brothers.
- White Spear Onion – this bunching onion variety is heat tolerant and resists bulb formation and leaf curl in midsummer. It has blue-green leaves and matures in 65 days. You can find White Spear onions from Johnny’s Selected Seeds.
How To Protect Onions From Heat
If you want to grow larger onion bulbs, then heat tolerant varieties (which are mostly bunching onions) may not be for you. Instead, you might want to try keeping them a little cooler to avoid the hottest temperatures of midsummer.
One way to keep onions cooler is by using shade cloth. This will protect them from intense sunlight during the hottest part of the day.
Shade cloth is made from knitted or woven material. It filters out sunlight and keeps plants cooler.
Shade cloth can reduce temperature by 10 degrees Fahrenheit (6 degrees Celsius) or more. You can use shade cloth to protect any plant from heat and intense sunlight – not just onions.
You can learn more about shade cloth in my article here.
Conclusion
Now you know more about onion temperature tolerance and how to protect onions from heat. You also know which types to grow, depending on the day length where you live.
You can learn more about onion depth and spacing here.
You can learn about the best onion varieties to grow (short-day, day-neutral, and long-day) here.
You might also be interested in reading my article on planting sprouted onions.
You can learn about when and why onions fall over in this article.
I think you’ll also enjoy this article on what to do with extra onions.
I hope you found this article helpful. If so, please share it with someone who can use the information.
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~Jonathon