Soup is something I don't make much when the weather is warm but watercress soup is an exception as it is quite a light summer soup. I often use American land cress which I have grown on the allotment instead of buying watercress. It is similar to watercress but has a stronger flavour being hotter and more peppery.
I sowed some land cress many years ago and let it go to seed and now I never need to sow it as there are always enough self sown plants around. Each year I make sure to leave a few clumps to go to seed so that I know I will have plants for the following year. In fact, it seems to grow better if left to seed by itself than when I have planted it!
The soup I make is very simple. I saute chopped onions in olive oil. When they are translucent but not browned I add garlic, cubed potatoes and chicken stock (or vegetable stock) and simmer until the potatoes are almost soft. The land cress (a couple of handfuls/litre) is added about five minutes before the end of cooking. Once the potatoes are soft and the land cress is wilted I use a blender to make a smooth soup. I like to have the cress chopped but still visible as green flecks in the finished soup.