Lettuce is a popular cool-weather crop that thrives in mild temperatures. Successful planting in Missouri requires careful timing due to the state’s variable springs and intensely hot, humid summers. The goal is to maximize the harvest window before heat causes the plant to transition prematurely into seed production. Understanding temperature triggers allows gardeners to strategically place crops for two distinct harvest seasons.
Spring Planting: The Early Window
The primary window for planting lettuce opens as soon as the soil becomes workable in late winter or early spring. This initial planting should be timed around the average last spring frost, which typically falls between early and late April across the state. Since lettuce is frost-tolerant, it can be planted before the last frost date, unlike warm-season vegetables.
For the earliest harvest, starting seeds indoors is the most reliable method. Seeds should be sown four to six weeks before the expected last frost, generally targeting early to mid-March for transplanting. These small plants can be hardened off and set into the garden in late March or early April, benefiting from the cool, moist soil conditions.
Direct sowing can begin as soon as the soil is loose enough to till, usually in late March. Lettuce seeds can germinate in soil as cold as 40°F, but a range between 60°F and 65°F is ideal for rapid growth. Sowing seeds directly into the garden every two weeks through April ensures a staggered and continuous supply of fresh leaves.
Managing the Transition to Summer Heat
Lettuce plants are highly sensitive to rising temperatures, and the spring season has a firm deadline set by the impending Missouri summer heat. Once daytime air temperatures consistently climb above 75°F to 80°F, the plant is triggered to “bolt” (send up a flower stalk to produce seeds). This transition makes the leaves bitter and tough, rendering the harvest unpalatable.
To delay this process, gardeners can use techniques to keep the root zone and plant canopy cool. Succession planting involves sowing small batches of seeds every 10 to 14 days, ensuring only young plants are present when the heat arrives. The final spring planting should occur no later than mid-May, as temperatures become too high by late May or early June.
Afternoon shade is an effective tool to extend the harvest into early summer. Planting lettuce on the east side of taller crops or using a shade cloth can reduce the intensity of the sun during the hottest part of the day. Keeping the soil consistently moist also helps regulate root temperature, which delays the bolting response.
Fall Planting: Timing the Second Season
The fall season provides a second, often more reliable, opportunity for a lettuce harvest, as temperatures cool gradually. Timing this planting requires counting backward from the average first fall frost, which generally occurs between mid-to-late October. Seeds should be sown approximately eight to ten weeks prior to this date, placing the ideal planting window in mid-to-late August.
The main obstacle for the fall crop is the August heat, which can inhibit germination. Lettuce seeds often fail to sprout when soil temperatures exceed 80°F, a phenomenon known as thermal dormancy. Gardeners must actively cool the seedbed during the germination phase to overcome this.
One method involves soaking the planting area with cold water, then covering it with a board or mulch to reduce sun exposure and lower the soil temperature. Another technique is to start the seeds indoors, often in a cool basement or pre-germinating them in a refrigerator. Once seedlings emerge, they can be transplanted into the garden in early September after hardening off.