These look like a type of willow; in particular, Black Willow (Salix nigra), a native species that matures into a mid-size tree. It does have great wildlife value, but the stature of the plant (and its rapid growth rate) would make it not very suitable for long-term use in a rain garden. If you wanted to keep one, you could try pruning it back drastically every few years to treat it like a shrub, though even then, it will mature fairly large and might shade-out neighboring perennials. Or, you could transplant them to more suitable locations this autumn, or donate them to another gardener who has a damp sunny area and room for a good-size tree.

Miri