Classified botanically as Lactuca sativa. A short browse through a seed catalog, seed display rack, or selection of nursery seedlings will reveal enough variety to keep your salad bowl crisp and colorful throughout the growing season. There are four principal types of lettuce: crisphead, butterhead or Boston, loose-leaf, and romaine.
Crisphead is most exasperating for home gardeners to produce. Heads form best with monthly average temperatures of 5560F. Best selections include 'Great Lakes', 'Nevada', and 'Summertime'. 'Rosy' is a small crisphead with reddish burgundy leaves.
Butterhead or Boston type has a loose head with green, smooth outer leaves and yellow inner leaves. Good choices include 'Bibb' ('Limestone'), 'Buttercrunch', and 'Tom Thumb'. 'Mignonette' ('Manoa') stands heat without bolting to seed. 'Tennis Ball' is an heirloom Boston type with small, loose heads. 'Key Lime' is another heirloom that is larger than most butterheads and resists bolting.
Loose-leaf lettuce makes a rosette rather than a head. It stands heat better than other types. Choice selections are 'Black-seeded Simpson', 'Green Ice', and 'Oak Leaf' (all with green leaves); 'Salad Bowl' (with deeply cut green leaves); and 'Prizehead' and 'Ruby' (red-tinged leaves). 'Slo-Bolt' has ruffled green leaves and resists bolting.
Romaine lettuce has an erect, cylindrical head of smooth leaves; outer leaves are green, inner ones yellowish. Stands heat moderately well. Try heat-resistant 'Jericho', 'Medallion', 'Olga', or 'Parris Island'.
Lettuces with bronzy to pinkish red leaves add color to a salad. 'Freckles', 'Merveille des Quatre Saisons', and 'Perella Red' are butterheads; 'Lollo Rosso', 'Red Oak Leaf', 'Red Sails', and 'Ruby' are loose-leaf selections; 'Rouge d'Hiver' and 'Sierra' are romaines.
Various loose-leaf and romaine lettuces are typically included in mesclun mixes mixtures of fast-growing, tender salad greens (usually some mild and some tangy) that may include mustards, arugula, cress, chicory, radicchio, and/or mizuna.
All lettuces need loose, well-drained soil. Sow in open ground; barely cover seeds. Loose-leaf lettuce can be grown as close as 4 in. apart; thin all other types to 1 ft. apart. Grow mesclun in blocks 4 in. wide, and don't thin.
For prolonged harvest, sow at 2-week intervals. In Upper South, begin sowing seed for all types after frost, as soon as soil is workable. In the Coastal and Tropical South, plant in fall or winter for harvest in winter and early spring.
Feed plants lightly and frequently. Control snails, slugs, earwigs. Harvest when heads or leaves are of good size; once lettuce reaches maturity, it rapidly goes to seed, becoming quite bitter. Snip off young leaves of mesclun mix for salads.