If you grow Lettuce, and indeed various other vegetables it can sometimes be worth leaving them in the garden even after they are past being useful for eating as they can put on a display just as good as any flowering plant. Tell me if I'm wrong but wouldn't the Lettuce in the picture look good in any Herbaceous border? I haven't tried this but one could probably still pick off individual leaves and add them to a salad.
Incidentally the Leeks in front of the lettuce are a failure. People keep on telling me that keeping rabbits out of the garden is not a priority, I hope they are as understanding when I present them with the Leeks in the picture as the two previous sowing's got eaten by the low priority rabbits last spring.