Detailed Growing Degree Days (GDD) Requirements by Growth Stage for Onion

Overview

Onion (Allium cepa L.) is strongly influenced by accumulated temperature (Growing Degree Days, GDD) during its growth process. GDD is an essential indicator for predicting and managing arrival and progression at each growth stage and varies with base temperature and regional climate. This report presents required GDD for each major growth stage (germination, vegetative growth, bulb formation, flowering/seed production) under Japanese or similar climates, citing direct values or estimates from existing literature and guidance with calculation basis and sources.

Base Temperature and GDD Calculation

The base temperature (Tbase) most widely used for onion GDD accumulation is 5°C. This is adopted in Japanese agricultural research and major literature (Lancaster et al. 1996, etc.) [1][2].

GDD formula (simple type)
GDD = Σ((daily mean temperature − base temperature) × days)

GDD requirements by growth stage are detailed below.

GDD Requirements by Growth Stage

1. Germination (Sowing to Emergence)

• Optimal germination temperature: 15–20°C
• Days to germination (at optimal temperature): About 7 days
• Reference basis
  • "Optimal germination temperature is 15–20°C; uniform germination in about 7 days at optimal temperature" [3][4]
  • Similar optimal germination temperature and duration reported internationally [5]

GDD estimate (Japanese standard conditions)

• Base temperature: 5°C
• Daily mean temperature: 18°C (midpoint of optimal germination temperature)
• Days: 7

→ GDD = (18 − 5) × 7 = 91°C·days

2. Early Vegetative Growth (Seedling Stage: Sowing to Transplant)

• Seedling period: About 55 days from sowing to 3–4 true leaves (transplant-ready)
• Standard sowing time / mean temperature: Mid-September (about 15°C)
• Reference basis
  • "Seedling period about 55 days; transplant at 3–4 leaves" [1][4]
  • "About 1000°C accumulated temperature to 6th leaf after germination, about 65 days after sowing" [6]

GDD estimate

• Base temperature: 5°C
• Daily mean temperature: 15°C
• Days: 55

→ GDD = (15 − 5) × 55 = 550°C·days

3. Late Vegetative Growth (Post-Transplant to Bulb Initiation)

• Period: Late November to early March (autumn-sown onion)
• Growth slows or stops in winter cold; resumes with rising temperatures in early spring [1][7]
• GDD estimation has large variation (physiological dormancy intervenes); main vegetative GDD demand concentrates in spring bulb formation.

4. Bulb Formation and Enlargement

• Bulb initiation indicator: Increasing day length (after spring equinox), rising mean temperature
• Period from bulb formation to harvest: 40–60 days (varies by cultivar and climate)
• Optimal temperature for bulb formation and enlargement: 15–25°C
• Reference basis
  • "Optimal temperature for bulb formation and enlargement is 15–25°C; bulb formation period is April–June" [1][4]
  • "Onion bulb formation requires at least 600°C·days accumulated temperature (under photoperiod conditions)" [2]
  • Similar trends in Isahaya Bay reclaimed land and Toyama Prefecture surveys [7]
  • Global literature also cites "bulbing requires 600–900 GDD" [8]

GDD estimate

• Base temperature: 5°C
• Daily mean temperature: 20°C (spring average)
• Days: 40–60

→ GDD = (20 − 5) × 40–60 = 600–900°C·days

5. Flowering / Seed Set

• Flowering induction (vernalization):

  • 90–120 days at 5–13°C; low temperature especially at 5–10°C is essential (low-temperature requirement rather than GDD) [9]
  • Seed onions require sufficient cold treatment for flowering

• From flowering to seed harvest:

  • International report: 900°C·days from bolting to seed maturity at base temperature 10°C [10]
  • Converted to base 5°C: about 1.5–2 months (mean 15°C × 60 days = 600°C·days)

GDD estimate (flowering to seed harvest)

• Base temperature: 5–10°C (varies by literature; 5°C is reasonable for Japanese open-field cultivation)
• Daily mean temperature: 15°C (April–June standard)
• Period: 60 days

→ GDD = (15 − 5) × 60 = 600°C·days (900°C·days at 10°C base)

Summary Table of Estimated GDD

Growth Stage	Estimated GDD (°C·days)	Calculation Basis and Conditions
Germination	~90	7 days, daily mean 18°C, base 5°C
Seedling stage (early vegetative growth)	~550	55 days, daily mean 15°C, base 5°C
Bulb formation and enlargement	600–900	40–60 days, daily mean 20°C, base 5°C
Flowering / seed production	600–900	Post-flowering to seed harvest, 60 days, daily mean 15°C, base 5°C; or 900°C·days at base 10°C

Try it in your region

See how these GDD requirements apply to your local weather data. Simulate Onion cultivation →

Notes for Application

• GDD variation and responsiveness differ by region, year, and cultivar (very early, mid-late, etc.); use as standard reference values.
• In actual cultivation, combine weather data with field observation for best growth stage management.
• Cold treatment (vernalization) is essential for flowering/seed production induction but is managed by low-temperature days, not GDD.
• Bulb formation GDD is closely linked to day length (after spring equinox, etc.); integrated management is required, not GDD alone.

Conclusion

This report systematically organizes GDD requirements for each developmental stage of onion under Japanese or similar climatic conditions. Where direct values were unavailable, estimates from emergence days and temperature ranges are shown with formulas and basis. Flexible application considering local climate and cultivar characteristics is needed in the field. Detailed references and manuals are cited for further understanding and practical use.

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Sources

[1] Onion (autumn-sown spring harvest): https://www.ja-nanto.or.jp/farming/document/family-farm/tamanegi.pdf
[2] Bulbing in Onions: Photoperiod and Temperature Requirements and ...: https://academic.oup.com/aob/article-pdf/78/4/423/7982622/780423.pdf
[3] Onion germination days: https://タマネギ栽培.com/タマネギ栽培-qa/タマネギ-発芽日数.html
[4] Takii Onion Cultivation Manual | Vegetable Cultivation Manual: https://www.takii.co.jp/tsk/manual/tamanegi.html
[5] JAPANESE BULB ONION: PRODUCTION, CONSUMPTION, AND ...: https://bibliotekanauki.pl/articles/55789021.pdf
[6] Differences in growth patterns between autumn-sown and spring-sown onion: https://www.jstage.jst.go.jp/article/hrj/22/4/22_267/_pdf/-char/en
[7] Growth progression of mid-late onion in Isahaya Bay reclaimed land: https://www.pref.nagasaki.jp/e-nourin/nougi/theme/research_report/PDF/S8-1.pdf
[8] Growth responses of tropical onion cultivars to photoperiod and temperature based on growing degree days: https://www.researchgate.net/publication/264893889_Growth_responses_of_tropical_onion_cultivars_to_photoperiod_and_temperature_based_on_growing_degree_days
[9] Flowering and Seed Development in Onion—A Review - Scirp.org: https://www.scirp.org/pdf/oalibj_2016031813225045.pdf
[10] ONION FLOWERING AND SEED PRODUCTION - jstor: https://www.jstor.org/stable/45126745