Lettuce is a cool-weather crop and a garden favorite, since it is the basis of many delicious salads. There is an ideal temperature range for growing lettuce, and extreme cold or heat will lead to poor growth or bitter leaves.
So, what is the best temperature for growing lettuce? The ideal temperature range for lettuce seed germination is 60 to 86 degrees Fahrenheit (16 to 30 degrees Celsius). After germination, the ideal temperature for growing lettuce is 55 to 65 degrees Fahrenheit (13 to 18 degrees Celsius).
Of course, if the temperature is too high or too low, lettuce seeds will fail to germinate. Lettuce is a cool weather crop, so it will bolt (grow tall, then produce flowers and seeds) in high temperatures.
In this article, we’ll talk about the ideal temperatures for lettuce germination and growth. We’ll also get into ways you can protect lettuce from cold weather and how to prevent bolting in warm weather.
Let’s get going.
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What is the Best Temperature for Growing Lettuce?
The best temperature for growing lettuce will depend on the growth stage. Lettuce seeds can germinate just fine at higher temperatures that would cause established lettuce plants to bolt (produce seeds and become bitter).
Best Temperature for Lettuce Germination
The best soil temperature for lettuce germination is 60 to 86 degrees Fahrenheit (16 to 30 degrees Celsius). In this temperature range, lettuce seed takes only 2 to 4 days to germinate.
The time for lettuce seed germination will vary, depending on soil temperature. For example, lettuce seeds can still germinate in soil close that is close to freezing, but it will take several weeks to do so.
The table below summarizes the temperature ranges for lettuce seed germination, in both Fahrenheit and Celsius temperatures, based on information from the University of California Extension.
| Temp Range | Degrees Fahrenheit (Celsius) | Notes About Germination |
| Minimum | 32 (0) | 49 days to germinate |
| Ideal | 60 to 86 (16 to 30) | 2 to 4 days to germinate |
| Maximum | 95 (35) | Seeds do not germinate over this temperature. |
ranges for lettuce seed germination.
Ideal Temperature for Lettuce to Grow
The ideal temperature for growth of lettuce plants is 55 to 65 degrees Fahrenheit (13 to 18 degrees Celsius). Lettuce is a cool weather crop, so it prefers cooler temperatures for its growth.
When the weather gets warmer, lettuce will start to “bolt” or go to seed. That is, the plant will start to grow tall and produce flowers, which will eventually produce seeds.
This is a normal part of a lettuce plant’s life cycle. However, it also means that the lettuce leaves will become bitter, to the point where most people would not enjoy eating them.
The table below summarizes the temperature ranges for lettuce growth. Note that this can vary depending on the variety of lettuce and any heat protection (shade cloth, etc.) that you can provide.
| Temp Range | Degrees Fahrenheit (Celsius) | Notes About Growing |
| Minimum | 26 to 32 (-3 to 0) | Can survive a hard frost if conditioned. |
| Ideal | 55 to 65 (13 to 18) | Lettuce is a cool weather crop. |
| Maximum | 70 to 80 (21 to 27) | Plant will bolt (grow tall, make seeds, get bitter) |
ranges for lettuce plant growth.
What Temperature is Too Cold for Lettuce?
According to the Michigan State University Extension, lettuce is a plant that is hardy to hard frost. A hard frost is defined as a temperature less than 28 degrees Fahrenheit (-2 degrees Celsius).
Although lettuce can survive a hard frost, this assumes that the plant has proper conditioning. Conditioning means that the plant has had time to grow accustomed to cooler temperatures, instead of being subjected to extreme cold all at once.
For example, if a hard frost comes suddenly after warm weather, then lettuce may not survive. On the other hand, if the weather gets progressively cooler over a long time period, lettuce will be able to survive cold down into the 20’s Fahrenheit (-6 to -2 Celsius).
How to Protect Lettuce from Frost and Cold
When a sudden hard frost threatens, your lettuce might be able to survive. If you want to take extra steps to protect your plants, you have a few options.
Cloche
A cloche is a clear plastic or glass cover used to protect plants from cold, pests, and wind. A cloche acts as a mini greenhouse, trapping heat from sunlight in the air and soil underneath it.
Since a cloche is clear, it still allows sunlight in so that plants can continue to grow. You can make a cloche out of a clear plastic bottle by cutting out the bottom and using the cap on top as a vent.
You can also make a cloche out of wire (such as chicken or rabbit wire), although this will not provide cold protection by itself. You would need to drape a frost blanket or piece of row cover over the cloche to insulate the plant.
You can learn more about cloches in my article here.
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Row Cover
You can protect an entire row of lettuce plants from cold by using a row cover. A row cover is a piece of fabric material that protects plants against cold and pests.
One way to use row covers is to first install supports along a row and then drape the row cover over the supports. For example, you can bend a flexible plastic rod into a half-circle and drive the two ends into the ground.
Space these half-circles a couple of feet apart in a row and you can cover them with a row cover, giving you a miniature “hoop house”.
Row covers can provide a few degrees of cold protection for your lettuce. This can make the difference for their survival when extreme cold is on the horizon.
Just remember that lettuce do need some sunlight, so don’t leave your plants covered all day in heavy shade.
What Temperature is Too Hot for Lettuce?
The lettuce will still grow after it has started to bolt. However, the leaves of lettuce will become bitter after the plant has bolted, to the point where the leaves will be inedible.
As with many crops, there are differences between varieties within the same species. For example, there are many heat-tolerant lettuce varieties that can withstand warmer temperatures than their cousins before they begin to bolt (more on these varieties later).
How Do You Grow Lettuce In Hot Climates?
To grow lettuce in hot climates, you will need a way to keep the lettuce a bit cooler. The following methods will all help with this to some degree (no pun intended!)
You can combine two or more of these methods to keep your lettuce even cooler and extend the season to enjoy a longer harvest.
The first step is to choose lettuce that will work with you to resist bolting in the heat, so we’ll start with heat-tolerant lettuce varieties.
Choose Heat-Tolerant Lettuce Varieties
One of the best ways to grow lettuce for a longer time period in a hot climate is too choose a heat-tolerant lettuce variety. Here are some types of heat-tolerant lettuce from Johnny’s Selected Seeds:
- Adriana – this heat-tolerant butterhead lettuce has dark green leaves. It matures in 48 days and resists bolting. You can learn more about Adriana lettuce from Johnny’s Selected Seeds.
- Cherokee – this heat-tolerant lettuce has red leaves with green veins. It matures in 48 days and resists bolting. You can learn more about Cherokee lettuce from Johnny’s Selected Seeds.
- Coastal Star – this heat-tolerant lettuce has dark green leaves that can be used as heads or romaine leaves. It matures in 57 days. You can learn more about Coastal Star lettuce from Johnny’s Selected Seeds.
- Jericho – this fast-growing, heat-tolerant lettuce has bright green leaves that can be used as baby-leaf or for heads. It matures as baby lettuce in 29 days. You can learn more about Jericho lettuce from Johnny’s Selected Seeds.
- Magenta – this bolt-tolerant lettuce has green and red leaves. It matures in 48 days and can be planted in spring or summer. You can learn more about Magenta lettuce from Johnny’s Selected Seeds.
- Muir – this heat-tolerant lettuce has light green leaves. It bolts very slowly and matures in 50 days. The heads can be harvested small or allowed to grow to full size. You can learn more about Muir lettuce from Johnny’s Selected Seeds.
- Winter Density – this romaine variety tolerates both heat and frost. It bolts slowly, but grows quickly, maturing in only 44 days. You can learn more about Winter Density lettuce from Johnny’s Selected Seeds.
Provide Shade for Your Lettuce
Any lettuce that gets partial shade during the hottest part of the day will take longer to bolt. Luckily, there are a few different ways to provide shade for your lettuce.
Plant Lettuce Near Taller Crops
First, you can plant your lettuce next to taller plants. For example, you can leave extra space between your tomato plants and put a head of lettuce between every pair of tomato plants.
As the tomato plants grow, they will provide some shade for the lettuce. However, this isn’t the only way to do it.
Another option is to plant vining crops, such as cucumbers, at the bottom of a lean-to trellis. Then, plant lettuce below the trellis.
As the cucumbers grow, they will provide shade for the lettuce. You can learn more about lean-to trellises and other designs in my article here.
Use Shade Cloth Over Lettuce
Shade cloth is another method you can use to keep lettuce cooler. Shade cloth is a mesh material that filters out some of the sunlight to keep plants beneath it cooler.
Shade cloth still allows some light to get through, which means that your lettuce will still be able to grow. Depending on the grade of the shade cloth, it can provide up to 10 degrees of heat protection for your lettuce!
This shade cloth from Gardener’s Supply Company can be used with hoops for support. As an added bonus, shade cloth can protect lettuce from birds.
Harvest Lettuce So It Keeps Growing
Finally, keep harvesting your lettuce as soon as the leaves are ready (or even when they are still a bit small!) If the lettuce leaves get large enough, the plant will think it is “done” growing, and will enter the next phase of its life: producing seeds.
Once a lettuce plant starts to produce seeds, the leaves will become bitter. This makes the leaves inedible to anyone with taste buds.
If you harvest your lettuce as it matures, you can extend your growing season by at least a few days.
Conclusion
Now you know the ideal temperature ranges for lettuce germination and growth. You also know how to protect lettuce from temperature extremes to extend your harvest.
You might be interested to read my article on fall planting for cool weather crops.
You might also want to read my article on fast growing lettuce varieties so that you can harvest for salad sooner!
I hope you found this article helpful. If so, please share it with someone who can use the information.
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~Jonathon