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The leaves become bitter and the plants begin to bolt as the stem elongates into a tall bloom stalk. Leaf lettuce varieties take about 40 days from seeding to harvest while head types may take 70 days. This is another reason the head types are more of a challenge. To get more from your garden space, consider interplanting your lettuce with slower maturing crops.
Growing Head Lettuce. 1  1. Choose a head lettuce variety for indoor planting. Head lettuces take longer to mature. 2  2. Prepare seed trays. You can start your lettuce seeds in store-bought seed trays or make your own out of an old egg carton, box or newspaper. 3  3. Sow the seeds 4-6 weeks before the last spring frost.