Lettuce

Sow seed a quarter to a half inch deep once soil reaches 40 degrees F, about two to three weeks before your last frost, and sow a short new row every two to three weeks to keep the harvest coming. Thin to 8-12 inches between plants with 12-18 inches between rows. Lettuce germinates best at 55-65 degrees F and emerges in 7-10 days, but germination drops off above 80 degrees F, so the seed has to go down while the soil is still cold. Succession sowing matters because a single planting all matures at once and then bolts.

Start seed indoors about three to four weeks before you want to plant, and set transplants out in spring as soon as the soil can be worked. Space leaf, cos and butterhead types 4-10 inches apart in rows 12-24 inches apart; space crisphead 12-15 inches apart in rows 20-30 inches apart. Setting out transplants in succession buys an earlier harvest and skips thinning entirely. Transplants get a head start on the heat. Increasing day length and high summer temperatures cause seedstalk formation and bitter flavor, so weeks bought at the front of the season are weeks of sweet leaves before the crop bolts.

Give lettuce 1-2 inches of water per week and keep it steady rather than letting the bed swing from dry to soaked. If you water overhead, always do it in the morning so the plants have time to dry off. Moisture fluctuation makes the leaves tough, slows head development and causes off-flavors, and wet-then-dry swings favor tipburn, a browning of the edges of the inner leaves on this shallow-rooted crop. Watering overhead in the morning leaves time for the foliage to dry before evening.

Four weeks after transplanting, or at thinning for direct-sown rows, apply a quarter cup of a nitrogen fertilizer such as 21-0-0 per 10 feet of row. Place it beside the plants rather than on them and irrigate it into the soil. Lettuce is harvested as leaves, so the whole crop is vegetative growth, and it has to be fast: a plant that grows slowly sits in the bed long enough to meet the heat and bolt before it sizes up.

Look under the outer leaves and down into the heart of the plant, not just at the top growth. Wash light infestations off with a strong stream of water, hand-wipe small localized colonies, and control any ants farming the honeydew. If you must treat, insecticidal soap (potassium salts of fatty acids) or azadirachtin are the products listed for home use. Aphids feed in colonies on the leaves and prefer the newer growth, so the youngest leaves in the center of the plant are where an infestation establishes before it is obvious from the outside. Keeping nitrogen moderate matters too: high foliar nitrogen encourages aphid reproduction.

Clear the weeds, debris, rocks and boards that give slugs daytime shelter first, then scatter an iron phosphate bait such as Sluggo or Escar-Go around the bed in the evening, when slugs come out to feed. Hand-pick any you find. Look for the silvery dried slime trail to confirm slugs rather than insect chewing. Slugs feed at night in cool, moist weather and go for leafy greens first. They will strip a lettuce seedling to nothing before you have a crop to defend, and holes in mature leaves are the same animal at a later stage.

Sow leaf and butterhead types in late summer, and transplant crisphead and romaine for fall. Carry the planting later into autumn with floating row covers or a cold frame. For an overwintered crop, set healthy three-week-old seedlings out around mid-October using a cultivar bred for it, such as Winter Density or Black Seeded Simpson. Fall is the better half of lettuce's year in most of the country: the crop matures into falling temperatures instead of racing rising ones, so it sizes up without bolting and the leaves stay sweet. Row covers also blunt slug feeding while they promote growth.

Site long-season lettuce where a taller crop such as sweet corn, staked tomatoes or pole beans shades it through the hottest part of the day, or run shade cloth over the bed. Switch to slow-bolting or heat-resistant varieties such as the oak-leaf types for anything you are asking to stand through summer. Increasing day length and high summer temperatures together cause seedstalk formation and bitter flavor, and lettuce grown above 80 degrees F turns bitter. Day length is not something you can change, so shade is the one lever a grower actually has over bolting.

Take leaf lettuce any time after the leaves form but before the seed stalk appears, either by pulling the older outer leaves or, for a thickly sown bed, cutting the whole plant just above ground level with sharp scissors at 6-10 inches so it regrows for a second cut in 2-3 weeks. Cut crisphead when the head is firm, and butterhead and romaine at the early heading stage. Once the seedstalk starts, the plant moves its sugars into flowering and the leaves turn bitter, and no amount of watering brings them back. Harvesting on schedule rather than pushing for size is the whole game with lettuce.