As the more serious cold kicks in, it is better to grow only varieties of lettuce suitable for winter, and even so they will need some winter protection. Looking at the images above, there are two types of lettuce. In the background, the tall lettuce at the far end of the cloche is the summer sown lettuce and in the foreground is the newer varieties of winter lettuce.
Even seed/small plus sold as winter lettuce, if there is a spell of low temperatures, frost and snow, winter lettuce requires cloche protection. Winter lettuces are tough, but not indestructible. Varieties sold as winter lettuce illustrated here are Winter Purslane, Corn Salad, Land Cress and Winter lettuces ' Density' and ' Winter King'
Once established, take the cloche off in milder days to allow air circulation, to prevent the soil from getting mould, and to allow modest watering.
At the back of the image is a light red leaved lettuce, which is the same lettuce as illustrated above centre pictured in late summer as a large seedling. I planted it out in the veg plot in the late summer and cropped for several months, with protection, providing a continuous supply of lettuce.
From this it is clear that the demarkation line between winter and summer lettuce is a bit blurred. The scope to prolong the growing period for summer lettuces may well depend on the aspect of your garden and how much protection is afforded to the crop.
Given that lettuce is expensive in the shops, especially over winter, this makes it an easy to grow economical crop. We are also being asked more and more to consider the potential greenhouse effect when selecting food, and no air miles is always good.