Lettuce is a classic cool-season crop, making planting timing in Wisconsin a careful balancing act. The state’s climate (USDA Hardiness Zones 3-5) features short, unpredictable springs that quickly transition into summer heat. This narrow window requires precise timing to maximize the harvest before the lettuce bolts, or prematurely goes to seed, causing the leaves to become bitter. Success relies on understanding frost dates, which are the primary guide for a full-season yield.
Spring Planting: Timing for Early Harvest
The initial spring planting window is determined by the Last Expected Frost Date, generally falling between late April (southern zones) and mid-to-late May (northern regions). Lettuce seedlings are relatively cold-tolerant and can survive a light frost once hardened, but the soil must be ready. Direct sowing can begin as soon as the soil is workable and reaches a minimum temperature of 40°F for germination.
For the earliest harvest, starting seeds indoors is the most reliable method, typically done four to six weeks before the last frost date. These established seedlings should be transplanted outside two to three weeks before the last frost. Lettuce thrives best when air temperature is consistently between 60°F and 65°F, allowing plants to mature during this optimal period. Planting too late risks exposing developing plants to the sudden arrival of summer heat. Temperatures consistently above 75°F trigger bolting, halting leaf production and ruining the flavor and texture.
Extending the Harvest: Succession and Warm Weather Tactics
Once the initial spring crop is established, maintaining a continuous supply is achieved through succession planting. This technique involves staggering small sowings of seed every 10 to 14 days, rather than planting the entire crop at once. These smaller plantings ensure new, young plants are always developing to replace older ones nearing the end of their harvest window.
During warmer Wisconsin summer months, heat mitigation strategies are necessary to prevent bolting and bitterness. This involves creating a cooler microclimate, as lettuce is highly susceptible to heat stress when daytime temperatures exceed 75°F. An effective tactic is using a 30% to 50% density shade cloth draped over the plants during the hottest part of the day, which reduces air temperature and limits sun intensity.
Choosing the right varieties is a strong defense against heat, as some types are naturally slower to bolt. Romaine and butterhead varieties, such as ‘Buttercrunch’ or ‘Little Gem,’ are generally more heat-tolerant than most loose-leaf types, making them better candidates for summer production. Gardeners can also employ a cut-and-come-again harvesting method on loose-leaf varieties. This involves snipping only the outer leaves and leaving the central growth point intact, extending the life of the plant and maximizing yield.
Fall Planting: Timing for a Second Season
To secure a productive second season, calculate the planting date by counting backward from the First Expected Frost Date (late September to mid-October). This reverse calculation must account for the specific lettuce variety’s “Days to Maturity” (DTM). An additional two weeks must be added to the DTM to compensate for slower growth as days shorten and temperatures cool. For example, a 50-day maturity variety should be sown 64 days before the first anticipated frost to ensure a full-sized harvest.
Fall crops benefit from a planting location that receives partial sun, as intense late summer heat can still trigger germination failure or bolting. To avoid this, pre-chill seeds or ensure the soil is kept consistently moist and cool during the initial germination phase. As the season progresses, simple protection can extend the harvest well into late fall. Covering the plants with a row cover or a cold frame on cold nights shields the lettuce from damaging temperatures. This allows the leaves to continue maturing, providing fresh greens long after most other garden vegetables have finished producing.