Lettuce is a cool-season crop, and temperature is the most important factor for a successful harvest. Managing the thermal environment ensures the leaves remain tender and sweet. When temperatures are too high, the plant shifts its focus from producing edible leaves to generating seeds, a process that quickly introduces bitterness. Understanding the specific temperature needs at each stage of growth is how growers successfully produce high-quality lettuce.
Optimal Temperature Range for Vegetative Growth
Lettuce plants thrive when the air temperature remains within a relatively narrow, cool range. The ideal temperature for robust growth is consistently between 60°F and 65°F (15°C and 18°C) during the day. Maintaining this range results in the best flavor and texture for the harvested leaves. Growth is minimal when temperatures drop below 40°F (4.5°C).
A difference between daytime and nighttime temperatures supports healthy development. Nighttime temperatures should ideally be slightly cooler, ranging from 50°F to 59°F (10°C to 15°C). This cooler evening period helps the plant recover and reduces the stress that can lead to premature bolting. Exposure to temperatures outside this ideal band reduces growth and negatively impacts the final quality of the leaves.
Temperature Requirements for Seed Germination
The temperature required for a lettuce seed to sprout is distinct from the temperature needed for the mature plant to grow. Lettuce seeds can germinate across a wide soil temperature range, from 35°F to 85°F (2°C to 29°C). However, optimal germination is achieved when the soil temperature is between 60°F and 68°F (16°C and 20°C). Within this range, seeds typically sprout quickly and uniformly.
A major concern for summer planting is thermal dormancy, or thermoinhibition. When soil temperatures rise above 75°F (24°C), the seed’s internal mechanism prevents it from sprouting. The seed enters a protective state, waiting for cooler, more favorable conditions to emerge. This is why direct-sowing lettuce in hot summer soil often results in poor germination.
The Impact of Extreme Temperatures
Lettuce is highly sensitive to temperature extremes. The most common threat is heat stress, which induces “bolting”—the rapid growth of a tall flower stalk. When daytime temperatures consistently climb above 75°F (24°C), the plant perceives the season is ending and redirects its energy to reproduction. This shift causes the leaves to become bitter and tough.
Cold stress presents a different challenge, though lettuce is relatively cold-tolerant compared to other garden vegetables. Young, hardened plants can survive brief dips to temperatures as low as 25°F (-4°C). A light frost (28°F to 32°F) typically causes minimal damage to established plants. However, a hard freeze below 25°F for several hours can cause ice to form within the leaf tissues, resulting in significant injury or total crop loss.
Timing Planting Based on Temperature
Successful lettuce cultivation relies on timing planting to align with cool-season temperatures. In most regions, this means planting the first spring crop as soon as the soil is workable, typically two to four weeks before the last expected frost date. Starting seeds indoors is often safer to ensure good germination rates, allowing the lettuce to mature before the summer heat arrives.
For a fall harvest, planting should be scheduled so the lettuce matures as temperatures begin to cool down. Gardeners often sow seeds in late summer, approximately six to eight weeks before the first expected fall frost. Utilizing succession planting—sowing a small batch of seeds every two to three weeks—ensures a continuous supply of harvestable leaves. This practice extends the harvest window into the late fall when cooler nights enhance the sweetness of the leaves.