When to start lettuce seeds

How many weeks before the last frost?

University of New Hampshire Extension gives lettuce 4 to 5 weeks from seed to transplant and says it is safe to set out 3 to 4 weeks before the last frost. That puts indoor sowing 7 to 9 weeks before your last frost. With an average last frost of May 10, you would sow indoors March 8 to March 15 and set plants out April 12 to April 19.

Starting indoors is optional. It helps if you want neatly spaced heads or an earlier harvest, and University of Minnesota Extension points out it makes the tiny seed easier to handle. Many gardeners just direct sow.

Germination: cool is fine, hot is not

Lettuce germinates in cold soil. Harrington's data (published by Oregon State Extension) list a 35°F minimum, with seedlings appearing in about 15 days at 41°F, 7 days at 50°F, 4 days at 59°F and 3 days at 68°F. At 95°F, germination was little or none. That is why midsummer sowings often fail: the soil is simply too hot.

Iowa State Extension lists 60–70°F for germination, 7 to 14 days to emergence, and notes that lettuce seed needs light to germinate. Indoors, press seed onto the mix and cover it only very lightly. Outdoors, UMN plants a quarter to half inch deep; keep the bed evenly moist until seedlings appear.

Direct sowing

UMN advises sowing as soon as the soil is workable in spring, in a well-prepared, crumbly seedbed, because a cloddy bed reduces emergence. The seed is small and easy to sow too thickly or too deep. Pelleted seed (coated in clay) is easier to space every 2 to 4 inches and cuts down on thinning. Rows should be at least 18 inches apart.

Final spacing depends on what you want. UMN's guide gives about 5 inches for baby-leaf lettuce, 10 to 12 inches for full-size leaf or head lettuce, and 6 to 8 inches for mini heads. When thinning, snip extras with scissors so you don't disturb the plants you keep.

Transplanting and hardening off

For transplants, UMN suggests sowing indoors about four weeks before transplanting and hardening seedlings for two to three days by reducing water and temperature. Lettuce is cool-tolerant, so the hardening period is shorter than for tomatoes or peppers.

Bolting and fall crops

Lettuce does best in cool weather. UMN gives ideal conditions as night lows around 50°F and daytime highs up to 68°F. Long days, strong light and heat trigger bolting, when the plant sends up a flower stalk and turns bitter. In much of Minnesota, UMN says, June through August is too hot.

For fall, UMN suggests sowing about three months before the average first fall frost, or starting seeds indoors in July for August transplanting. Leaf lettuce often performs better in fall because it is less likely to bolt.

Common mistakes

• Burying seed too deep. Lettuce needs some light to germinate.
• Sowing into hot soil. In Harrington's trials, germination failed at 95°F.
• Sowing too thickly. Thin early, or use pelleted seed.
• One big planting. Sow small batches every couple of weeks for a steady supply.