Lactuca sativa aka Lettuce

Lettuce is a cool-season crop that performs best at temperatures between roughly 55-65°F (13-18°C); Wikipedia and NCSU report 16-18°C (61-64°F) as optimal, with tolerance down to about 7°C (45°F) and to light frosts. Above about 95°F (35°C) seed germination fails, and prolonged heat triggers bolting and bitter flavor. In summer regions, providing 3-4 hours of sun followed by shade, or growing in spring and fall, helps prevent bolting.

Lettuce is grown as a cool-season crop, primarily in spring and autumn; in cooler climates spring sowing extends through summer, while in warmer regions like the southern United States it is planted in early spring and again in fall to avoid summer heat. Successional sowings every two to three weeks maintain a continuous harvest, and protecting plants from hot afternoon sun helps delay bolting.

When lettuce bolts in warm weather, it sends up a flowering stalk reaching about 0.9 m tall topped with small yellow composite flowers typical of the daisy family. Flowering signals the end of the harvest window because the leaves turn bitter and unsaleable, so most home and commercial growers harvest before flowers appear.

Lettuce is divided into seven major cultivar groups: Leaf/Looseleaf, Romaine/Cos, Iceberg/Crisphead, Butterhead, Summercrisp (Batavia), Celtuce/Stem, and Oilseed. Leaves vary in color, texture, and shape across these groups, and heat tolerance also varies, with leaf and butterhead types typically tolerating warmer conditions better than crisphead or romaine.

Lettuce is almost always grown outdoors as a cool-season vegetable in beds, rows, or containers, where it matures in roughly 50-80 days depending on the type. Head lettuce needs about 10-12 inches between plants in rows up to 3 feet apart, while leaf lettuce can be spaced 4-10 inches apart in rows 1-2 feet apart. Plants thrive in full sun to partial shade in moist, organically rich, well-drained soil and benefit from afternoon shade in hot weather to delay bolting.

Lettuce is propagated almost exclusively from seed, sown directly in the garden a few weeks before the last spring frost, or started indoors 6-8 weeks earlier and transplanted out. Seeds are sown about a quarter inch deep, and successional sowings every 2-3 weeks from spring through early summer keep a steady supply, with thinning to recommended spacing as seedlings establish.

Common pests of lettuce include aphids, slugs, snails, cutworms, thrips, leafminers, leafhoppers, cabbage looper, corn earworm, and imported cabbage worms, plus damage from birds. Diseases include damping-off in seedlings, downy and powdery mildew, gray mold, Rhizoctonia bottom rot, Sclerotinia drop (lettuce drop), and lettuce mosaic virus. Crowding plants and overhead watering at night both increase disease pressure.

Lettuce leaves are the primary edible part and are eaten raw in salads or lightly cooked in soups and stir-fries, with a mild, slightly sweet flavor and crisp texture. Stems are also edible (notably in the celtuce/stem cultivar group, grown for its enlarged seedstalk), and seeds can be sprouted or pressed for a small amount of edible oil. PFAF rates the species 3/5 for edibility, reflecting its global culinary importance balanced against the limited spread of edible parts beyond the leaves.

Lettuce contains lactucarium, a bitter milky latex with anodyne, antispasmodic, sedative, and mild hypnotic properties; its main active compounds, lactucin and lactucopicrin, have shown analgesic effects comparable to ibuprofen and sedative activity in animal studies. Historically lactucarium was used to treat insomnia, anxiety, irritable cough, and pain, and was standardized in the 1898 United States Pharmacopoeia and 1911 British Pharmaceutical Codex. PFAF rates medicinal value at 3/5 and warns that even normal doses can cause drowsiness while overdoses can be dangerous, so any therapeutic use should be under professional supervision.

Beyond food and medicine, Lactuca sativa has only minor non-food applications. PFAF rates other uses at just 1/5, citing limited use as a parasiticide and traditional reports of hair-growth stimulation, while POWO also notes historical use of the species in traditional poison preparations and as animal feed. Seed oil can be extracted but is not considered commercially feasible at scale.