Quick growth makes for good flavor, 

so assure water and fertility. (Lime acid soils).

Lettuce seeds will not sprout once temperatures reach 85 F degrees. 
For hot-weathering sowings:  refrigerator seeds 4-5 days before planting will aid germination. In summer heat, sow in the evening, water well, and give some shade. 

Within each type of lettuce, some are better for planting in each season--

Spring lettuces can be planted as early as you can work the soil. Seeds will germinate when soil reaches 45 degress, and transplants will grow in even colder soil as long as days are above freezing. Brief night frost down to 25 F is not a problem. 

Summer varieties are best planted in succession starting in mid-spring. They are bred to be bolt-resistant under heat stress, though shadecloth will improve quality. 

Autumn varieties can be planted in pots or flats in a shady place in August, and transplanted into the garden under shadecloth or taller crops. 

Late fall crops of baby greens and a few ultra-hardy varieties can be planted September/October to stand through late fall, or winter in mild climates. 

Lettuce types: 

Looseleafs  fan out so that the center leaves are visible. They are tolerant of marginal conditions, and can be cut leaf-by-leaf or all at once. 
Butterheads have a soft, silky texture and small, rose-like heads. 
Romaine (or Cos) makes a tall head with crunchy midrib. 
Summercrisp (Batavian) types are super bolt-resistant, have a tall, ruffled head, with a crisp, somewhat blanched interior. Bred for summer heat, but also cold-hardy.

Lactuca sativa
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Starting Lettuce in Hot Weather

1) Choose bolt-resistant varieties. To simplify that, we have a Summer Salad Collection and a Bolt-Resistant Mix 2)Put the seed in a dry, closed jar in the refrigerator for a few days. Most lettuce varieties do not like to sprout if  the seeds hav...