Lettuce plants grow in cooler temperatures, best when it is between 15 and 20 degrees during the day. Lettuce seeds can even withstand light frost in the soil. As is done in greenhouses, lettuce can be grown indoors in a window box, keeping the seeds cool and moist to thrive.

 

First fill the flower box with special sowing soil and moisten the soil well.

Place three to four seeds together and cover with one centimetre of soil. Lettuce should be sown about six weeks before you plan to transplant it into the garden.

Place the trays in a cool and damp room, such as a cellar or garage. Make sure that the soil is always kept moist, but not too wet.

When the lettuce seedlings are big enough to be transplanted after six weeks, the outside temperature should be at least 8 degrees. The seedlings can be transplanted when they are 7 to 10 centimetres high.