Lettuce is a highly valued cool-season annual, but its success depends entirely on planting at the right moment. Temperature fluctuations dictate the quality of the final harvest. Careful timing prevents bolting, which is a premature shift into the reproductive cycle. Planting too late or during a heat wave causes the plant to rapidly send up a seed stalk, leaving the leaves bitter and tough. While lettuce tolerates light cold, a hard freeze can damage or kill young plants, making the window for optimal growth narrow.
Understanding Lettuce’s Temperature Needs
The growth and quality of lettuce are tied directly to environmental temperature. Optimal air temperatures for vigorous vegetative growth, which produces tender leaves, range between 60 and 65 degrees Fahrenheit. The ideal soil temperature for consistent and rapid sprouting falls between 60°F and 72°F. Seeds can germinate in soils as cold as 40°F, but the process will be significantly slower.
Temperatures above the ideal range trigger the plant’s survival mechanism, leading to bolting. Once daytime temperatures consistently climb above 75°F, especially combined with nighttime temperatures above 60°F, the plant is stressed into reproductive mode. Avoiding this temperature threshold is the most important factor in timing your planting throughout the year.
Timing Your Primary Spring Planting
The first major planting should begin as soon as the soil is workable, using the average last spring frost date as the primary reference. Gardeners starting seeds indoors should sow approximately six to eight weeks before this date. Young seedlings are ready for transplanting about three to four weeks before the last expected frost, as they tolerate a light freeze once hardened off.
Direct sowing is best done two to four weeks before the last frost. For direct seeding, soil temperature is a more reliable indicator than air temperature, requiring a minimum of 45°F for germination. Transplants are generally more cold-hardy than newly sown seeds, allowing them to be placed out earlier. This early spring window allows the lettuce to mature before summer heat initiates bolting.
Strategy for Continuous Succession Planting
Succession planting maintains a steady supply of fresh lettuce rather than a single large harvest. This strategy involves sowing small batches of seeds at regular intervals to ensure plants mature sequentially. A good rule of thumb is to sow a new row of seeds every 10 to 14 days during the cool seasons of spring and fall.
This staggered approach prevents a glut of mature heads and guarantees an extended harvest. However, the strategy must be paused when summer heat becomes dominant. Once average daytime temperatures exceed 75°F, new seeds struggle to germinate, and maturing plants quickly bolt. Gardeners should stop sowing in early summer and wait for temperatures to cool before resuming the cycle.
Planting for Fall and Winter Harvests
The second major planting window extends the harvest into cooler months and often produces the sweetest leaves. To calculate the correct timing for a fall crop, count backward six to eight weeks from your average first expected hard frost date. This allows the plant time to reach a harvestable size before the cold arrives.
Planting during late summer is challenging due to high soil temperatures, which inhibit germination. Gardeners can counter this thermal dormancy by cooling the soil with consistent watering or utilizing shade cloth until the seeds sprout. As the season progresses, choose cold-hardy varieties to withstand lower temperatures. Simple season extension techniques, such as row covers or cold frames, can protect mature plants for continued harvesting into late fall or mild winter weather.