Summary of temperature requirements by growth stage of lettuce (Lactuca sativa)

Overview This is a comprehensive and systematic arrangement of the temperature requirements for each growth stage (germination, seedlings, growth/heading, bolting/flowering) of Sativa) based on major academic papers, agricultural technology materials, Japanese research results, etc. The temperature parameters required at each stage (optimal_min, optimal_max, base_temperature, low_stress_threshold, high_stress_threshold, max_temperature, frost_threshold, sterility_risk_threshold only in the flowering stage) are shown, and currently known knowledge and data gaps are also noted.

List of temperature parameters for each growth stage

1. Germination

• optimal_min: 16°C
• optimal_max: 22°C
• base_temperature: Not explicitly stated. The practical minimum limit for germination initiation is approximately 7°C [1][2]
• low_stress_threshold: Germination rate and speed decrease significantly below approximately 7°C [2]
• high_stress_threshold: Germination will be inhibited when the temperature exceeds about 22°C (especially above 28°C) [2][3]
• max_temperature: Around 28°C (germination will be inhibited and dormant above that) [2][3]
• frost_threshold: Seeds are resistant to light frost, but detailed frost damage risk values are not specified. [1]
• sterility_risk_threshold: Not applicable

Germination is best at 16-22°C. If the temperature exceeds 28°C, the seeds will enter a state of "germination dormancy" and germination will be severely inhibited. Germination is possible at temperatures close to 7°C, but it takes much longer [1][2][3][4].

2. Seedling stage

• optimal_min: 10°C (night)
• optimal_max: 20°C (day) [5][6]
• base_temperature: No clear description, growth slows below about 8°C [6] 36-40°C (upper limit of experimental stress assessment) [7]
• frost_threshold: Tolerates light frost down to -2°C (28°F), but covering recommended [1]
• sterility_risk_threshold: Not applicable

Seedlings are susceptible to both cold damage (below 8°C) and high temperature (above 24°C), but growth is stable with day and night temperature control [5][7].

3. Vegetative & Heading

• optimal_min: 13°C (minimum growth limit) [1][5]
• optimal_max: 20°C (or up to 22°C) [5]
• base_temperature: Around 7°C (growth stagnates below 7°C) [1][5]
• low_stress_threshold: Risk of growth/heading failure at below 7°C [1]
• high_stress_threshold: Risk of bolting and quality deterioration above 24°C [5]
• max_temperature: Over 24°C to 25°C (progressive bolting with continued use) [5]
• frost_threshold: Adult plants are resistant to around -2°C (28°F). Tissue damage below -2°C [1]
• sterility_risk_threshold: Not applicable

A cool climate (13-20°C) is best during the growth and head-heading stages. If the temperature continues to exceed 24°C during the day, the risk of developing bolting and bitterness rapidly increases [1][5][8].

4. Bolting/Flowering/Reproductive

• optimal_min: 15°C
• optimal_max: 22°C [5][9]
• base_temperature: No clear description. Generally, it is around 7°C as in the vegetative stage [1]
• low_stress_threshold: Below 10°C, flowering is delayed and growth is impaired [9]
• high_stress_threshold: Bolting and flowering is promoted and seed quality decreases at 24°C to over 25°C [4][9]
• max_temperature: Severe reproductive damage above 28°C (clear limit value not specified in literature)
• frost_threshold: Inhibition of flowering and fruiting below 0°C [9]
• sterility_risk_threshold: undetermined. It is said that seed set and quality are significantly reduced under high temperature conditions (above 28°C), but a ``lethal sterility threshold'' is not specified (the paper states that ``the quantitative threshold is unknown'') [4][10].

Very sensitive to high temperatures during the bolting and flowering stages. At temperatures above 24-25°C, bolting is rapidly accelerated, and seed quantity and germination rate also decrease rapidly. A clear "threshold of sterility" has not been found for lettuce, and poor fruit set and failure under high temperatures become a problem [4][10][11].

---

Explanation of each parameter and supplementary references

• optimal_min / optimal_max: Temperature range in which the most active germination, growth, bulb formation, and flowering can be expected (mainly from an agricultural practical/productivity perspective).
• base_temperature: Guideline for the lowest temperature at which growth can begin (no clear research values ​​are established, around 7°C is a rule of thumb) [1][5].
• low_stress_threshold / high_stress_threshold: Guideline values ​​beyond which growth delays, physiological disorders, moss bolting induction, and quality deterioration are likely to occur rapidly.
• max_temperature: Practical production is difficult if this temperature range is exceeded for a long time.
• frost_threshold: Short-term damage temperature. Young seedlings and flowering stages are particularly sensitive to temperatures below 0°C [1][9].
• sterility_risk_threshold: Threshold that indicates poor pollination and fruit set due to high temperatures, but in the case of lettuce, the "lethal" threshold is not clear and is expressed as a "drastic decrease in seed set rate and germination rate" [4][10].
• Note: Each threshold value varies depending on the variety and cultivation conditions. In particular, the development of summer-friendly and high-temperature varieties is progressing [8][12].

---

List of temperature parameters by growth stage

< td>-2°C

Growth stage	optimal_min	optimal_max	base_temperature	low_stress_threshold	high_stress_threshold	max_temperature	frost_threshold	sterilit y_risk_threshold (flowering period only)	References
Germination period	16°C	22°C	Approx. 7°C	7°C	22°C~	28°C	—	—	[1][2][3]
Seedling stage	10°C< /td>	20°C	About 8°C	8°C	24°C~	36~40°C	-2°C	—	[4][5][6]
Growth/heading period	13°C	20-22°C	Approx. 7°C	7°C	24-25°C~	24°C~	—	[7][8][9]
Bolting/flowering period	15°C	22°C	About 7°C	10°C< /td>	24～25°C～	28°C～	0°C	—(28°C or above)	[10][11]

---

Data Gaps and Notes

• There are few experimentally measured values for the “base_temperature” of lettuce even in agricultural data from other countries, and there are many empirical rules around 7°C.
• There are very few quantitative papers describing “sterility_risk_threshold” (sterility risk threshold) worldwide, and it is essentially expressed as “sudden decline in seed formation/germination rate under high temperature”, and no clear threshold has been set [4][10].
• Including Japanese primary materials, seedling stress experiments have been carried out at strict low temperatures (8°C/4°C, night/day) and high temperatures (40°C/36°C), but there is a slight range in estimating the temperature at which actual damage occurs [7].

---

Try it in your region

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

References

Sources

[1] Clemson HGIC Lettuce | Home & Garden Information Center: https://hgic.clemson.edu/factsheet/lettuce/
[2] PubMed Central Genetic Variation for Thermotolerance in Lettuce Seed Germination (ERF1): https://pmc.ncbi.nlm.nih.gov/articles/PMC4704578/
[3] SFGate Optimal Germination Temperature for Lettuce: https://homeguides.sfgate.com/optimal-germination-temperature-lettuce-72929.html
[4] PubMed Central Inflorescence Development and the Role of LsFT in Regulating Bolting in Lettuce: https://pmc.ncbi.nlm.nih.gov/articles/PMC5778503/
[5] Best Temperature Conditions for Growing Lettuce - Cultivated Earth: https://cultivatedearth.com/en/lettuce/ideal-temperature-for-lettuce/
[6] Growing Lettuce | HOrT COCO-UC Master Gardener Program: https://www.hst-j.org/articles/pdf/w1dx/kshs-2025-043-05-6.pdf
[8] Lettuce Growth Stages: 6 Stages Explained in Detail - Spider Farmer: https://www.spider-farmer.com/blog/lettuce-growth-stages-6-stages-explained-in-detail/
[9] Crop Improvement and Protection Research - Publication : USDA ARS: https://www.ars.usda.gov/research/publications/publication/?seqNo115=424031
[10] Heat Threatens Open-Field Lettuce Production in Brazil by 2100: https://www.seedworld.com/latam/2025/09/09/heat-threatens-open-field-lettuce-production-in-brazil-by-2100/
[11] Soil temperature conditions for vegetable seed germination (Oregon State): https://extension.oregonstate.edu/gardening/soil-compost/soil-temperature-conditions-vegetable-seed-germination
[12] Frontiers LsMYB15 Regulates Bolting in Leaf Lettuce (Lactuca sativa ...): https://www.frontiersin.org/articles/10.3389/fpls.2022.921021/full

---