Lettuce is a cool-season crop, meaning success depends on timing the planting to avoid temperature extremes. Improper sowing can lead to poor germination, stunted growth, or premature bolting, which makes the leaves bitter and inedible. Successful cultivation hinges on understanding the plant’s temperature preferences for both sprouting and mature growth, allowing you to harvest crisp, sweet leaves over an extended season.
Optimal Timing Based on Temperature
Lettuce seeds can germinate in a wide range of soil temperatures, but the ideal conditions are surprisingly specific. Germination can occur in soil as cool as 40°F, but the process is much faster and more reliable when the soil temperature is between 60°F and 72°F. Once the soil temperature rises above 75°F, many lettuce varieties experience thermal dormancy, causing the seeds to fail to sprout altogether.
The best air temperature for vegetative growth (leaf production) generally ranges from 59°F to 68°F. Temperatures consistently exceeding 75°F cause the plant to bolt, signaling the end of the harvest window. Sowing too early in spring risks slow, stunted growth if prolonged temperatures drop below 45°F. Therefore, the main spring planting should begin as soon as the soil is workable and has warmed slightly, typically 2 to 4 weeks before the last expected spring frost.
Starting Seeds Indoors vs. Direct Sowing
The planting method chosen affects the starting date. Direct sowing outdoors is generally recommended for lettuce, as it establishes a robust root system quickly when conditions are right. This method can start when the soil temperature reaches at least 40°F and the danger of hard frost has passed.
Starting seeds indoors allows for a head start on the growing season, which is advantageous in regions with short spring windows before summer heat begins. Sow seeds indoors approximately 4 to 6 weeks before the average last spring frost date. Seedlings can be moved outdoors 3 to 4 weeks before the final frost, as they are slightly more tolerant of cold than newly sown seeds. Gradually acclimate the young plants to outdoor conditions, a process called hardening off, for about a week before permanent transplanting.
Planning for Continuous Harvest
Gardeners use succession planting to avoid a single, overwhelming harvest. This involves staggering the sowing of seeds over time to ensure a continuous supply of fresh leaves. The frequency of resowing is adjusted based on the plant’s maturity rate and the season’s temperature.
For most lettuce types, sowing a small batch of seeds every 10 to 14 days maintains a steady flow of fresh leaves, distributing the harvest window throughout the spring and early summer. As the weather warms, the interval between plantings may need to be shortened because plants mature more quickly under heat. This practice must cease when intense summer heat makes successful germination and growth unlikely.
Sowing for Fall and Winter Crops
The second major planting window begins in late summer, timing the harvest to mature as temperatures cool down. Fall planting aims to avoid the late summer heat that can prevent germination and cause bolting. To determine the correct sowing time, count backward from your average first hard frost date.
For a full-sized fall harvest, sow seeds approximately 6 to 8 weeks before the anticipated date of the first hard frost. This allows enough time for the plants to reach a usable size before deep cold slows their growth significantly. Cold-tolerant varieties can be sown later or protected for overwintering, extending the harvest into the cooler months. The cool autumn air and shorter days often produce lettuce with a sweeter flavor.