Lettuce is a cool-season crop, prized for its tender leaves and quick growth cycle. Lettuce thrives in moderate temperatures and struggles when conditions become too warm. Precise timing is necessary to prevent “bolting,” the plant’s natural response to heat stress. When temperatures climb or daylight hours become too long, the plant shifts its energy from producing foliage to developing a tall stalk for flowering and setting seed. This transition causes the leaves to become tough and bitter. Mastering the timing of sowing ensures a continuous harvest of sweet, crisp leaves.
Timing Indoor Starts
Starting lettuce seeds indoors allows gardeners to get a jump on the spring season and produce robust seedlings ready for the garden. The primary timing marker involves counting backward from the last expected frost date in your region. Seeds should be sown approximately four to six weeks before the date you plan to move the young plants outdoors. This window allows the seedlings to reach the ideal size of three to four true leaves before planting.
Lettuce seeds require light to germinate, so they should be sown very shallowly, barely covered with a thin layer of fine soil mix or vermiculite. The optimal temperature range for the young plants is cool, ideally maintaining daytime temperatures between 65°F and 70°F and not dipping below 55°F at night. This cooler environment prevents premature heat stress.
Seedlings need a significant amount of light to grow stocky and strong, typically requiring 12 to 16 hours per day from a full-spectrum grow light. Placing the light fixture about 8 to 12 inches above the plants ensures they receive adequate intensity. Before transplanting, “hardening off” is necessary, which involves gradually acclimating the young plants to outdoor conditions, including direct sun and wind, over a period of one to two weeks. This preparation minimizes transplant shock.
Direct Sowing Outdoors
Planting lettuce seeds directly into the garden soil is a viable method once the soil is workable and the threat of cold has passed. The most reliable indicator for direct sowing is the soil temperature, rather than air temperature. Lettuce seeds germinate best when the soil temperature consistently falls between 40°F and 65°F. This range is considerably cooler than required for many other common garden vegetables, meaning lettuce can be one of the first crops planted.
Sowing the seeds when the soil is too warm, above 72°F, can inhibit germination and cause the seeds to enter a state of heat-induced dormancy. To maximize success, seeds should be sown shallowly, about a quarter-inch deep, or simply pressed into the surface and lightly covered. This shallow planting satisfies the light requirement for germination while ensuring good seed-to-soil contact.
Once the seedlings emerge, they must be thinned to the proper spacing to allow for good air circulation and prevent overcrowding. Looseleaf varieties generally require 8 to 10 inches between plants, while heading types, such as Romaine or Butterhead, need approximately 12 inches of space. Proper spacing ensures each plant develops fully and reduces the risk of fungal disease.
The Strategy of Succession Planting
To achieve a continuous supply of fresh greens, gardeners must employ the strategic method of succession planting. This technique involves sowing small batches of seeds at regular intervals throughout the season. For fast-growing leafy greens like lettuce, sowing a new, short row of seeds every two to three weeks is the recommended practice. This staggered schedule ensures that as one set of plants reaches maturity, another batch of younger plants is ready for harvest.
Maintaining a harvest during the warmer summer months requires two main adjustments. First, gardeners should select heat-tolerant and slow-bolting varieties specifically bred to withstand higher temperatures and longer daylight hours. Second, planting sites must be chosen to provide afternoon shade, or shade cloth can be erected to filter intense sunlight and keep the soil cooler. Keeping the soil consistently moist is also important, as water stress is another trigger for premature bolting.
Planning for a fall harvest involves reversing the spring timing process by counting backward from the first expected frost date. For a full-size head of lettuce, seeds should be sown approximately eight weeks before that date to allow time for maturation. Baby leaf varieties, which mature more quickly, can be sown as late as four weeks before the first frost. These later plantings benefit from the naturally shorter days and cooler nights, providing a final, sweet flush of lettuce late into the autumn.