1. Why do lettuce seeds sprout badly in the dark and in the heat?
Lettuce seeds are fussy. In a Brazilian study of a lettuce variety called 'Dark Green Boston', seeds soaked in the dark sprouted well only below about 22.5°C (72.5°F). Light raised that limit to about 27.5°C (81.5°F), but even with light the seeds did not sprout well above that [1].
Heat can shut sprouting off almost completely. At a steady 20°C (68°F), 97% of the seeds sprouted. At 35°C (95°F), none did [1]. Scientists call this "thermoinhibition," which means heat holds the seed back. The same study found a way to improve sprouting: seeds that were first soaked for 16 hours at 20°C and then moved to 35°C sprouted 97% of the time, so the first few hours of soaking turned out to matter a great deal [1].
These numbers come from one variety in one set of experiments, and the authors note that many kinds of lettuce sprout unevenly or not at all in heat. Other varieties will have different limits. The pattern is what matters: lettuce seeds want light and coolness, so in this study light helped germination, and deep sowing or a hot spot may make sprouting harder (results could vary by variety). The next question is what happens to the plant after it sprouts.
2. What happens when a lettuce plant gets too hot?
A young lettuce plant grows a loose rosette of leaves, and many kinds then curl those leaves into a head. Heat can make the plant flower sooner. It stops making as many leaves, and a tall flower stalk shoots up from the middle. Growers call this "bolting." A research team describes the result bluntly: once it bolts, leafy lettuce "becomes bitter and unsaleable" [2]. Because lettuce is grown for its leaves, a plant that has put its energy into flowers is worth little at the market.
How early a plant bolts depends strongly on the variety. In the study by Chen and colleagues, nine lettuce varieties grown under mild conditions (25°C by day and 15°C by night, or 77°F and 59°F) bolted anywhere from 38 days to 75 days after planting [2]. So "lettuce bolts in the heat" is true, but some lettuce is much quicker to do it than others. That difference is one reason plant breeders look for slow-bolting varieties for hot summers [2].
The paper does not explain how a head forms in the first place, and neither do the other sources used here. Head-forming is a good question to chase on your own.
3. What is going on inside a lettuce plant that decides to bolt?
When the researchers moved 28-day-old plants from mild weather to hot weather (35°C by day and 25°C by night, or 95°F and 77°F), a gene called LsFT became more active within 48 hours in all nine varieties. In the slowest variety its activity rose about 2.5 times. In the quickest variety it rose about 13 times [2]. Genes like this are part of the plant's flowering switch, so a stronger signal means the plant gets closer to flowering.
The team then tested the idea directly. In plants engineered to make less LsFT, bolting came later (at 45, 55 and 75 days in three of the lines, compared with 38 days for the unaltered plants), and these plants no longer reacted to the heat the way ordinary plants did [2]. This is good evidence that LsFT helps heat trigger bolting. It does not show that it is the only part.
A 2009 Japanese study, which the Chen paper cites, reports that heat also raises the amount of a growth hormone, gibberellin, that helps the stem stretch [2]. We read that study only through this citation, so we describe it as a pointer, not as something we checked.
4. How can lettuce be sold all year in the United States if it struggles in heat?
If lettuce needs cool weather, farms need cool fields, and in the United States the cool fields change with the seasons. A University of California, Davis summary says that most US lettuce comes from California (about 70%) and Arizona (about 30%). The Salinas Valley in California supplies lettuce from May to October. Then growers move to the desert areas around Yuma, Arizona, and the Imperial Valley in California, where the harvest runs from December to March [3]. A University of Arizona extension document, which we saw only in a search summary, says the Arizona harvest runs from about November to April and supplies most of the iceberg lettuce shipped across the country in those weeks [4].
The sources do not say outright that growers choose these places because of temperature. We think the pattern fits the science above, since the harvest moves between regions as the seasons change, but that is our reading, not a claim made by these sources, and we did not check the climates of these places. The result is like a relay race: as the seasons change, one region's harvest gives way to another's.
5. How can you check the lettuce story yourself?
Next time you shop with an adult, look at the label on a head of lettuce or a bag of salad. Many packages say where it was grown. Write down the place and the month, and repeat in a different season. If you are in the United States, do the places change from summer to winter the way the Salinas-and-Yuma story predicts?
If you have a garden or a windowsill, you can also try an experiment with an adult's help. Sprinkle lettuce seeds on damp soil in two cups, covering them with only a very thin dusting of soil. Put one in a cool room and one in a warmer room, away from direct sunlight (ask the adult to help choose safe places and check that the cups do not dry out), and write down how many sprout and when. Then compare your results with the seed numbers above.
To read further, the Chen paper is free online [2], and so is the abstract of the seed study [1].