Survey report on the cumulative temperature-GDD-required amount at each growth stage of lettuce

Overview

In this report, we investigated in detail the cumulative temperature (Growing Degree Days, GDD) required for each growth stage of lettuce (germination stage, seedling stage, growing stage/heading stage, bolting stage, flowering stage, and fruiting stage) based on Japanese and English scientific literature and gardening manuals. At stages where the main integrated temperature values ​​are directly shown, the values ​​are specified, and at stages where direct data cannot be obtained, estimated values ​​are presented based on the actual temperature, period, lettuce base temperature, etc. We also organized the reference temperature and optimal temperature when integrated temperature is used.

Growth stage classification and overall characteristics of lettuce

• The growth stages of lettuce are mainly classified into the following four:
  • Germination stage
  • Seedling stage
  • Growing stage/heading stage (from leaf formation to head-heading and harvesting)
  • Bolt bolting/flowering/fruiting stage (reproductive growth stage)
• Gumulated temperature (GDD) is mainly used to manage the period from germination to harvest and predict the timing of head-heading/harvesting. On the other hand, the bolting and flowering stages are strongly influenced mainly by temperature and photoperiod conditions, and there is little literature describing specific thresholds based on cumulative temperature.
• The standard temperature for lettuce (GDD calculation standard temperature) is 2 to 5°C, and the optimal temperature is 15 to 22°C (germination and seedling growth period) and 18 to 23°C (growth period) [1][2][3][4][5][6].

Cumulative temperature (GDD) value for each growth stage

1. Germination period

• Optimal temperature: 15-22°C (especially 18-20°C is best).
• Duration: 2 to 7 days (minimum 2 days, up to 10 days if conditions are bad).
• Reference temperature: 2-5°C.
• Required cumulative temperature (estimated):
  • Example: 18°C (average temperature), period 5 days, reference temperature 2°C:
    (18-2) × 5 = 80°C/day
  • If the period is 2-7 days, and the temperature is 15-20°C, the recommended temperature is 20-100°C/day.[2][4][5][7][8]
• Note: Both low and high temperatures tend to reduce the germination rate, so it is important to manage at the optimal temperature.

2. Seedling stage

• Optimal temperature: 15-22°C.
• Duration: 2-3 weeks (14-21 days).
• Required cumulative temperature (estimated) (e.g. 18°C, reference temperature 2°C, 21 days):
  • (18-2) × 21 = 336°C/day
  • For reference temperature 5°C: (18-5) × 21 = 273°C/day
• Reference: This stage is when the initial growth of roots and leaves progresses, and several true leaves appear [1][6].

3. Growing season/Heading season (Planting~Harvesting)

a) Planting to before the start of head formation (period of increase in the number of leaves)

• The start of head formation is from around 12 to 13 true leaves[1].
• Shizuoka Agricultural and Forestry Research Institute Report (2020):
  • It has been directly measured that lettuce during the harsh winter season requires an cumulative temperature of 751°C/day (standard temperature 2°C)** from "planting to harvest (heading harvested after approximately 38 true leaves)" [9].
  • The relationship between increase in leaf age and cumulative temperature has also been expressed mathematically using a new prediction formula (y=0.0584x–5.8537; y: leaf age, x: cumulative temperature) [9].
• For normal spring crops, it usually takes about 30 to 40 days from planting to harvest.

b) Start of head - Harvest

• From the gardening manual of Takii seeds and seedlings:
  • Early maturing varieties: 350-380°C/day
  • Medium-maturing varieties: 400°C/day
  • Late-maturing varieties: 450-500°C/day
  • *The standard temperature is often not specified, but 0-5°C is common [1][2][6][9].
• The above values ​​are cited extensively in popularization manuals throughout Japan and are highly reliable.
• **Cumulative temperature (overall) from planting to harvest** is based on a range of 751°C/day (2°C standard, winter) to 800°C/day [9][10].

4. Bolting, flowering, fruiting period

• There is very little literature describing the direct threshold/required amount of integrated temperature.
• Main factors:
  • High temperatures (over 24-25°C during the day and over 15°C at night) and long day conditions induce bolting and flowering [3][11][12][13].
  • For bolting, the effect of sudden high temperatures and photoperiod is more dominant than the cumulative temperature during the growing season exceeding a certain threshold.
• There are almost no numerical descriptions of bolting period management using GDD, including foreign literature[12][13].

Practical points of integrated temperature management and notes on its use

• Management using cumulative temperature is extremely effective for predicting the appropriate harvest time, leaf age, and shipping plan[9][10].
• The increase in leaf age and cumulative temperature are almost proportional, and when the number of leaves reaches about 40, the head matures.[10]
• Maintaining the optimum temperature range for each growth stage makes it easier to prevent sudden growth problems.
• For bolting, Cultivation environment (temperature/day length) control is more important than GDD management.

Summary table of each stage

Growth stage	Optimum temperature	Period	Cumulative temperature (GDD, Standard temperature 2~5°C)
Germination period	18~20°C	2~7 days	20~100°C/day (estimated)
Seedling stage	15~22°C	14~21 days	273~336°C/day (estimated)
Growing season/heading stage	18~23°C	30~40 days	Start of head to harvest: 350-500°C/day Planting to harvest: Approximately 751°C/day (standard 2°C, winter)
Bolting, flowering, and fruiting period	Occurs at 24°C or higher	--	Specific GDD value is unknown. Mainly triggered by high temperatures and long days

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Overview

In the growth management of lettuce, cumulative temperature (GDD) is mainly used to predict the period from planting to head-heading and harvesting, and in Japanese horticultural literature, 350 to 500°C/day (depending on variety and cropping type) is often used, especially for head-heading types. Although it is possible to estimate the GDD from germination to seedling stage, a specific GDD threshold for bolting and flowering stages cannot currently be found. Overall, it is essential to manage temperature and photoperiod at each growth stage in addition to integrated temperature control, and environmental control is especially important for preventing moss bolting.

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Sources

1. Takii Lettuce Cultivation Manual
2. BSI Biological Science Institute Lettuce
3. Asahi Agria vegetable cultivation lettuce
4. Garden News How to grow and cultivate lettuce
5. GardenStory.jp Lettuce characteristics and cultivation methods
6. Cultivated Earth - Lettuce Growth Timeline (English)
7. Takii seeds and seedlings - guideline for suitable temperature for germination
8. Spider Farmer Lettuce Growth Stages
9. Shizuoka Prefectural Agriculture and Forestry Research Institute Lettuce growth prediction technology using effective integrated temperature
10. The Norinchukin Research Institute - Number of lettuce leaves is 40
11. Epic Gardening Lettuce Bolting
12. Fine Gardening Why Lettuce Bolts
13. MRCC Growing Degree Day Description (English)