Answer: Lettuce generally germinates best planted about 1/8" deep and gently firmed at a soil temperature of 68 degrees or lower, and will begin germinating at about 40 degrees. If overheated or allowed to dry out it will not germinate, but another possibility is that it was so wet it rotted, since the paper can hold a bit of extra moisture against the seed. Mesclun mixes are generally similar -- while they will not germinate in dry soil, they need only a light and gentle sprinkling to keep the soil moist rather than soggy. Other possibilities are heavy or frequent watering which can cause soil compaction and crusting which will stop the seedlings, and wind which can blow dry soil (and seeds) away in no time.
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