Radish — GDD Requirements by Growth Stage

Overview

This report summarizes Growing Degree Days (GDD) required at each major stage of Japanese radish (daikon)—germination, seedling/leaf growth, root enlargement, and bolting/flowering—based on scientific papers, public agencies, and agricultural extension materials. It explains stage definitions, base and optimal temperatures, calculation methods, and available data and estimates.

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1. Growth Stage Definitions

1. Germination — From sowing until cotyledons emerge.
2. Seedling / leaf growth — From germination until 5–6 true leaves expand and root enlargement begins.
3. Root enlargement — From start of root swelling until market/harvest size.
4. Bolting / flowering — After root enlargement ends; flower initiation and bolting (mainly for seed production).

Commercial harvest targets mainly the root enlargement stage; bolting is relevant for breeding and seed crops[1][2][3][4][5].

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2. GDD Base and Optimal Temperatures

2.1 Base Temperature

• Japanese research and weather analyses consistently use 0°C (developmental zero) for daikon GDD[1][2][3][4][5][6][7][8][9][10].

2.2 Optimal Temperature

• Germination: 15–25°C (optimal ~20°C)
• Seedling/leaf growth: 15–20°C
• Root enlargement: 15–21°C (growth disorders and pithiness risk above 23°C)
• Flower induction (bolting): 0–12°C, especially 5–7°C — managed mainly by vernalization (low-temperature exposure days) rather than GDD alone[1][2][3][4][5].

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3. GDD Required by Growth Stage

3.1 Germination

• Limited direct data—estimated: Literature reports stable germination in 3–7 days at 15–20°C[6][7][8][11][12].
• Estimate: 3 days × 20°C = 60°C·days; 7 days × 15°C = 105°C·days → ~60–105°C·days (base 0°C), commonly observed in Japanese production areas.

3.2 Seedling / Leaf Growth

• Definition: Germination to root enlargement start (5–6 true leaves, or ~800°C·days accumulated from sowing in Chiba field surveys)[1].
• Estimate: ~700°C·days for seedling stage (800°C total to root start minus ~60–105°C germination). Field variation is large by weather, cultivar, and year[1][3].

3.3 Root Enlargement

• Chiba model: Root enlargement starts at 800°C·days; market size at 1,200°C·days (base 0°C, mean air temperature)[1].
• Root enlargement stage GDD: ~400°C·days (800°C → 1,200°C).
• Harvest timing: 1,200°C·days is a common guide; ‘Shōgoin’ ~1,000–1,100°C[12]; ‘Fuyujiman’ 1,200°C[1]; most cultivars ~1,000–1,200°C[1][12][3][5].

3.4 Bolting / Flowering

• Managed by low-temperature exposure (0–12°C, often 10–20+ days at 5–7°C) rather than GDD thresholds[2][3][4][5].
• Avoiding bolting in winter daikon depends on managing this vernalization period.

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4. GDD Calculation and Notes

• Formula (base 0°C): GDD = Σ (daily mean temperature − 0°C). Example: 15°C × 5 days = 75°C·days.
• Weather and season shift required days and GDD; multi-year regional data are used in practice.
• Germination and seedling values include estimates; soil moisture, depth, cultivar, and weather strongly affect outcomes. Much data come from Chiba/Kanagawa and cultivars such as ‘Fuyujiman’[1][12][3].

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5. GDD Summary Table

Growth stage	GDD (base 0°C)	Representative measured/estimated values	References
Germination	60–105°C·days	(3–7 days at 15–20°C, estimated)	[1][6][7][8][11]
Seedling / leaf growth	~700°C·days	(Germination to root start: 800°C total)	[1][3][7][8][12]
Root enlargement	400°C·days	(800°C root start → 1,200°C harvest)	[1][12][3]
Market size (cumulative from sowing)	1,000–1,200°C·days	Some cultivar/regional variation	[1][12][3][5]
Bolting / flowering	Low-temperature days	(0–12°C for 10–20 days, etc.)	[2][4][5]

Try it in your region

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

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6. Field Applications of GDD Management

• Sowing timing is adjusted using GDD and mean temperature to avoid poor germination from autumn chill or summer heat[3][6].
• Chiba and Kanagawa prefectural stations predict harvest from local GDD with high accuracy (R² ≥ 0.99): 800°C for root start, 1,200°C for harvest[1][2].
• Bolting is managed by sowing date, cover cultivation, slow-bolting cultivars, and winter soil temperature—not GDD alone[4][5].

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7. Conclusion

• Standard Japanese model (base 0°C, daily mean temperature): germination 60–105°C·days; seedling +700°C·days (800°C cumulative to root start); root enlargement/harvest +400°C·days (1,200°C cumulative).
• GDD thresholds support stage management and harvest prediction; commercial GDD services and prefectural models are widely used.
• Bolting/flowering depends on vernalization (low-temperature exposure), not GDD alone.
• Flexible management using local measured temperatures is recommended given cultivar, environment, and year-to-year variation.

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Sources

[1] Chiba Prefecture: Predicting winter daikon root weight using weather data: https://www.pref.chiba.lg.jp/ninaite/shikenkenkyuu/documents/r5n0312.pdf
[2] Kanagawa Prefecture: Autumn-winter daikon growth model report: https://www.pref.kanagawa.jp/documents/27459/daikon.pdf
[3] Takii daikon cultivation manual: https://www.takii.co.jp/tsk/manual/daikon.html
[4] Daikon cultivation - Takii Net: https://shop.takii.co.jp/selection/daikon2011.html
[5] Harvest-timer vegetable GDD list: https://harvest-timer.com/temperature/
[6] Nakahara Seed daikon cultivation: https://www.nakahara-seed.co.jp/pdf/saibai/01_daikon.pdf
[7] JA Zen-Noh Chiba agri information: https://www.zennoh.or.jp/cb/producer/agriinfo/pdf/agrichiba202103.pdf
[8] Daikon sowing time and temperature - Midori Oyayubi: https://midorioyayubi-hanasaku.com/radish-sowing-time-temperature/
[9] Midori Oyayubi twenty-day radish: https://www.atariya.net/yasai/hatuka.htm
[10] Nou-ka.com germination GDD: https://www.nou-ka.com/infomes_detail.html?info_id=0000000790
[11] Deep Green Permaculture「Daikon – Growing Guide」: https://deepgreenpermaculture.com/2025/08/16/daikon-growing-guide/
[12] Saien Club Shogoin and Miura daikon GDD to harvest: https://saienclub.com/daikon201709-12/