Lettuce prefers cool conditions and can be planted as soon as the soil can be worked in spring. Best growth occurs at 16-18 degrees Celsius. Sow seed .3 cm deep, firm soil and cover lightly as some light is required for germination. Sow indoors 3-4 weeks before your transplant date. Outdoors space plants 21 cm apart in rows 30-45 cm apart. Continue to sow every 3 weeks throughout the season for a continuous supply of fresh greens. Many lettuce varieties will not germinate at temperatures over 24 degrees Celsius. Sow seed in a shaded location mid-summer. Some varieties do well in the unheated winter greenhouse.