Comprehensive Temperature Requirements by Growth Stage for Onion

Overview

Onion (Allium cepa) prefers cool climates, and temperature management at each growth stage greatly affects quality and yield. Based on Japanese agricultural manuals, official institution materials, and international agronomic papers, this report organizes optimal minimum and maximum temperatures and stress/injury temperatures for germination, vegetative growth, bulb formation, and flowering (bolting/seed production).

Below are parameter definitions, an overview table of temperature indicators, and detailed explanations by growth stage.

Parameter Definitions

• optimal_min / optimal_max: Optimal minimum and maximum temperature for that stage
• base_temperature: Physiological minimum growth temperature; growth stops below this
• low_stress_threshold / high_stress_threshold: Lower and upper temperatures at which clear stress or injury to growth and function appears
• max_temperature: Absolute physiological tolerance upper limit for that stage
• frost_threshold: Minimum air temperature causing frost injury or survival limit (including seedling and post-transplant cold tolerance)
• sterility_risk_threshold: Temperature threshold at which pollen and other reproductive functions fail during flowering, causing sterility (flowering stage only)

(Temperature indicators are generally daily mean air temperature; stress thresholds are mainly based on experiments and field observations.)

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Temperature Parameters by Growth Stage (Summary Table)

Stage	optimal_min	optimal_max	base_temperature	low_stress_threshold	high_stress_threshold	max_temperature	frost_threshold	sterility_risk_threshold (flowering only)	References
Germination	15°C	20°C	7–8°C	Below 10°C	Above 25°C	30°C	−5 to −8°C (post-germination seedlings)	—	[1][3][5][7]
Vegetative growth	10°C	25°C	5–7°C	Below 10°C	Above 25°C	30°C	−5 to −8°C	—	[2][4][6][8]
Bulb formation	15°C	20–25°C	5–7°C	Below 15°C	Above 25°C	30°C	Around 0°C (slight tolerance)	—	[9][10][11]
Flowering / seed production	4–10°C*	24–25°C	~5°C	Insufficient cold*	Above 30–32°C	Above 32°C	−5°C	30–32°C	[12][13][14]

*Note: Flowering induction (flower bud differentiation/vernalization) requires 4–10°C low temperature for 4–6 weeks

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Details by Growth Stage

Germination

• Optimal temperature is 15–20°C. Uniform germination in about 7 days at around 20°C.
• Physiological lower limit (base temperature) is 7–8°C; international standards of 6–7°C also exist.
• Below 10°C, germination speed drops sharply. Above 25°C, germination rate declines; around 30°C, germination disorders occur.
• Seeds themselves are frost-resistant, but post-germination seedlings (sprouts) may tolerate −5 to −8°C.
• For improved germination rate and uniformity, night temperature during sowing to germination should be managed (maintain optimum).
• References: [1][3][5][7]

Vegetative Growth (True Leaf Expansion to Early Post-Transplant)

• Optimal temperature is 10–25°C; some reports show peak growth rate at 13–24°C.
• Base temperature is 5–7°C; regional and cultivar differences are small.
• Below 10°C, leaf blade growth is delayed. Above 25°C, leaf color, growth, and photosynthetic rate decrease markedly.
• Well-developed seedlings tolerate −5 to −8°C, but prolonged cold carries physiological disorder risk.
• References: [1][5][6][8]

Bulb Formation

• Long days combined with 15–20°C promote bulb enlargement.
• Optimal range is 15–20 to 25°C. Below this, bulb enlargement stalls; below 15°C, response is slow.
• Above 25°C and especially above 30°C, poor enlargement, bulb quality decline, and yield reduction occur.
• Frost threshold is rarely stated explicitly, but cold tolerance declines during active bulb swelling; around 0°C is the practical limit.
• References: [1][3][5][8]

Flowering / Seed Production (Bolting, Seed Formation)

• Flower bud differentiation requires low-temperature stimulation called "vernalization" (4–10°C for about 4–6 weeks).
• Flowering, pollination, and seed formation are optimal at around 24–25°C. Above 30°C, pollen germination rate drops sharply with sterility risk.
• When flowering and pollination coincide with high temperature (30–32°C), seed number and quality decline greatly (female and male sterility). Especially pronounced in F1 seed production and certain cultivars.
• Cold tolerance around −5°C is confirmed before and after flowering, but flowers and buds are less tolerant than aboveground parts and bulbs.
• Insufficient cold (inadequate low-temperature response) prevents flower bud differentiation and bolting does not occur.
• References: [2][5][8][9]

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Notes and Regional Differences

• Parameters vary somewhat by cultivar, region, sowing time, and environment, but major Japanese cultivars fall within the ranges above.
• Stress thresholds may also vary with water, fertilizer, and excess nitrogen.
• Sterility risk threshold during flowering is mainly important for seed producers and breeding; less problematic in typical bulb harvest cultivation.
• International papers may define temperature differently (mean, maximum, minimum, or duration at a given temperature).

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Try it in your region

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Conclusion

Stable high-quality onion production depends on maintaining appropriate temperature ranges at each growth stage and avoiding extreme environments (especially heat stress, low/high temperature at germination, and high temperature during seed production). Inappropriate temperatures from germination through vegetative growth cause early growth delay and delayed bulb enlargement; fine environmental adjustment using regional climate characteristics, sowing/transplant timing, and mulching is recommended.

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Sources

[1] Takii Onion Cultivation Manual | Vegetable Cultivation Manual: https://www.takii.co.jp/tsk/manual/tamanegi.html
[2] Section 16 Onion, MAFF (Miyagi Prefecture Agricultural Management Guide): https://www.maff.go.jp/j/seisan/kankyo/hozen_type/h_sehi_kizyun/pdf/miyagi_yasai18_19.pdf
[3] Spring-sown Onion Cultivation Manual in Tohoku - NARO: https://www.naro.go.jp/publicity_report/publication/files/Onion_Manual.pdf
[4] Base and upper temperature thresholds to support the calculation of GDD - Agricultural Water Management, 2025, ScienceDirect: https://www.sciencedirect.com/science/article/pii/S037837742500469X
[5] Onion - BSI Bio: https://bsikagaku.jp/cultivation/onion.pdf
[6] Scientific Cultivation of Onion (Allium cepa L.) - ResearchGate: https://www.researchgate.net/publication/339351977_Scientific_Cultivation_of_Onion_Allium_cepa_L
[7] Protecting onion from bolting: flower bud, temperature, and fertilization management - note: https://note.com/rapid_sedum6222/n/nd1c5526b4d04
[8] From the Green Consultation Center: Onion growth temperature and bolting - Kumamoto Zoo and Botanical Garden: https://www.ezooko.jp/imgkiji/pub/detail.aspx?c_id=31&id=559
[9] Agricultural Experiment Station Research Report - Akita Prefecture (Onion pollen/flowering temperature): https://www.pref.akita.lg.jp/uploads/public/archive_0000008683_00/48.pdf