Lettuce is a cool-season annual that thrives within a narrow temperature range, making precise planting time the most important factor for a successful harvest. This leafy green is highly sensitive to heat and long daylight hours, which trigger bolting. Bolting causes the plant to rapidly send up a flower stalk, making the leaves bitter and tough as the plant shifts energy from producing tender foliage to creating seeds. Seedlings must also be protected from deep freezes, making the start and end of the growing season the most productive windows for growth.
Timing Based on Climate and Frost Dates
Planting lettuce is determined by local climate conditions, primarily the last and first expected frost dates, rather than a specific calendar month. Lettuce grows best when the ambient temperature is between 60°F and 65°F. Once established, young, hardened plants can tolerate cooler conditions and survive temperatures as low as 20°F, but prolonged subfreezing exposure can cause injury or crop loss.
The seed itself is sensitive to heat, which can induce thermal dormancy and prevent germination. While lettuce seed can germinate in soil temperatures as low as 40°F, optimal germination occurs between 68°F and 80°F. If the soil temperature rises above 80°F, seeds often fail to sprout, which affects both spring and fall planting. Gardeners must use local frost dates as a guide to ensure the crop matures before summer heat causes bolting.
The Spring Planting Strategy
The spring planting strategy focuses on allowing lettuce enough time to mature during the cool period between the last expected frost and the arrival of summer heat. To get a head start, seeds can be started indoors approximately six to eight weeks before the last expected spring frost date. Starting seeds indoors helps ensure successful germination, especially if outdoor soil temperatures are still fluctuating.
Once seedlings have developed four to six true leaves and a well-developed root system, they must be hardened off for outdoor conditions. This process involves gradually exposing the young plants to wind, sun, and cooler temperatures over about a week. Hardened transplants can then be safely set out into the garden three to four weeks before the last spring frost date.
For direct sowing, plant seeds two to four weeks before the last expected frost date, as soon as the soil is workable. Seeds should be sown one-quarter to one-half inch deep and thinned to proper spacing once seedlings have their first true leaves. Planting small batches of seeds every two to three weeks ensures a steady supply of fresh lettuce rather than a single large harvest.
Extending the Harvest (Summer and Fall Planting)
Extending the lettuce harvest through warmer months requires succession planting, where small amounts of seeds are sown at regular intervals. This technique ensures a continuous supply as new plants mature while older ones are harvested. For baby leaf varieties, sowing can occur every seven days, while full-sized head lettuce may be sown every ten days.
The primary obstacle during summer is heat, as sustained daytime temperatures above 75°F or 80°F trigger bolting and turn leaves bitter. To mitigate this, mid-summer plantings should be placed in partial shade, such as under a shade cloth or on the east side of a taller crop. Gardeners can also use cool water and mulch to keep the soil temperature down, which is essential since lettuce seeds struggle to germinate in hot soil.
For a fall harvest, planting should be timed backward from the first expected fall frost date, aiming for maturity during cooling temperatures. Fall crops often taste sweeter because cooler nights enhance the flavor. Seeds should generally be sown about 60 days (eight to twelve weeks) before the first expected fall frost, aiming for maturity one to two weeks prior to that date. Starting fall seeds indoors during late summer and transplanting them out is a reliable way to ensure successful germination.
Specific Planting Methods by Lettuce Type
Planting methods and timing require slight adjustments based on the lettuce type, as varieties possess different levels of heat and cold tolerance.
Leaf and Looseleaf Varieties
Leaf lettuce and looseleaf varieties are the fastest growing and most forgiving, making them the best choice for succession planting. These types are relatively heat-tolerant and are less likely to bolt than heading varieties, maintaining quality longer into the summer.
Butterhead Varieties
Butterhead varieties, including Bibb and Boston types, offer moderate heat tolerance and maintain a sweet flavor in slightly warmer conditions. They are well-suited for the initial spring planting and the transition into early summer.
Crisphead and Romaine Varieties
Crisphead varieties, such as Iceberg, and Romaine (Cos) lettuce require the longest cool period to form dense heads. These types are generally more sensitive to heat and are best reserved for very early spring or late fall planting. Some Romaine and Summercrisp varieties have been bred for better heat resistance, making them an exception for summer planting when provided with afternoon shade. Selecting a variety labeled “slow-bolting” or “heat-tolerant” significantly improves success outside of the typical spring and fall windows.