Square-stalked Saint-John's-wort (Hypericum tetrapterum) is prolific this summer, it has lots of flowers. It can grow nearly a meter high and it grows in marshy or swampy, or at least wettish, areas.
It has quadrangular stalks with distinct ridges. The leaves grow in opposite pairs and they are sessile. From most pairs of leaves emerge flowers.
In the leaves you can see tiny transparant points and sometimes also some small black points. They are just visible to the naked eye, a magnifying glass can be of help. The little black points are glands, and they are typical for hypericums. Square-stalked Saint-John's-wort does not have many of them compared to other species of this family.
The flowers are somewhat untidy little stars, five yellow petals and a bunch of stamens that go in all directions.