Leaf lettuce is a snap to grow, but raising a picture-perfect head lettuce is definitely tricky. Pests like aphids, slugs and earwigs all feast on the leaves, and growing conditions need to be spot on. Head lettuce needs uniform moisture and a long, cool growing window. Once summer days start lengthening, lettuces go through a process known as bolting, where they start to flower. Bolting destroys the head and makes leaves taste bitter.