Lettuce

Lettuce is Lactuca sativa, an annual in the daisy family domesticated from a weedy plant of the Mediterranean and western Asia, and the only common vegetable grown for its leaves rather than its fruit or root. The part you eat is a rosette of tender leaves pressed against a short stem. When the plant decides to reproduce, that stem elongates rapidly, the rosette stops expanding, and the leaves turn bitter from latex, which is why timing matters more than variety.

Garden lettuce falls into four loose groups. Crisphead forms the tight, watery iceberg ball and needs the longest season. Romaine, or cos, grows upright into a tight, sweet heart that tolerates a little more heat. Butterhead, including Little Gem types, makes soft loose heads. Looseleaf, the red- and green-leaf salads, never heads at all, which makes it the fastest and the best choice for cut-and-come-again harvesting.

Sow as soon as the ground can be worked in spring, and again from mid-summer for an autumn crop. Lettuce seed germinates in a wide band, roughly 40 to 80°F, and does best near 70°F; above 80°F many varieties go dormant, which is why a July-sown bed of iceberg can simply refuse to sprout while a shaded tray of the same seed succeeds. Cool autumn sowing in a frame, or a winter sowing in mild zones, gives the slowest, most flavorful growth of the year.

Press the seed onto the surface of fine, firm, moist soil and leave it uncovered, or dust it with a quarter inch of mix at most, because light triggers germination in most varieties. Keep the surface from crusting over with a shade cloth or a board until sprouts appear, then thin early. Crowded lettuce grows slowly and bolts sooner, and the seedlings you pull at thinning are already salad.

Lettuce wants six hours of sun in spring and fall, and shade from the afternoon in summer, which is the single most useful trick for carrying a bed into July. Rich, moisture-holding soil with plenty of compost suits it, at pH 6.0 to 7.0, but the crop's real limitation is its roots: they are shallow, fine-branched and concentrated in the top few inches, so lettuce cannot reach deep water the way a tomato can.

That root habit changes how you water. Frequent, light irrigation that keeps the top four inches consistently damp beats a weekly drenching that floods then parches. Steady moisture and a nitrogen supply strong enough to keep leaves expanding without pause are what make lettuce mild; a check in growth concentrates the bitter latex. Mulch between rows as soon as plants are big enough to handle, and feed with a diluted, nitrogen-forward liquid every two weeks on sandy soil.

Because a lettuce plant runs out fast, the harvest plan matters more than the variety list. Sow a short row, roughly eight to twelve feet, or a flat of six plants, every two to three weeks from early spring, and stop sowing in high summer, resuming about ten to twelve weeks before first fall frost. Two or three types at different speeds, a looseleaf, a romaine and a butterhead, smooths the flow further, since they mature on different clocks even when sown the same day.

Design the bed around the crop's size, not your optimism. A 4x8-foot bed gives continuous salad for a household of two or three with staggered sowings, and interrow spaces filled with radishes or spring onions harvest at exactly the right moment to open the lettuce up. Plan the sequence with a spacing and succession tool before you start, because the dates matter more than the seed.

Bolting, the sudden spike of a flower stem, is triggered by a combination of warmth above roughly 75 to 80°F, long days, drought stress and overcrowding, and once a plant starts, nothing reverses it. The leaves do not poison anything; they simply turn bitter and rubbery. Cut bolted plants at the base and compost them, then resow in a cooler, shadier spot rather than hoping the head will fill.

Your defenses are physical and varietal. Thirty to fifty percent shade cloth over hoops in summer keeps leaves sweet for weeks, and day-neutral or bolt-tolerant varieties, often labeled for summer cropping, buy you another two weeks. Water deeply before a heat wave, mulch, and harvest in the morning while leaves are turgid. In autumn, low tunnels and a cold frame extend the same bed well past frost, since many lettuce types survive into the 20s.

For looseleaf, take the outer leaves at four to six inches, no more than a third of the plant at a time, and the center keeps producing for weeks or months. For heads, harvest whole when the rosette feels firm at the base: cut below the growing point with a knife, peel back the outer wrapper leaves and leave the roots, since some types reshoot a modest second head. Pick in the cool of the morning, chill fast in cold water, spin dry and store damp in a bag; wet, warm leaves rot within a day.

Expect slugs and cutworms chewing the low leaves, aphids clustering at the heart, and, in cool wet weather, downy mildew as pale powdery patches on leaf undersides and botrytis rot where leaves touch soil. Wide spacing, morning watering at soil level, residue cleanup and rotation clear most of it. Tipburn, brown necrotic leaf tips on fast-growing romaine, is a calcium shortage caused by erratic water rather than a disease, and it is fixed by steadier moisture.