We have an 18 inch deep tree well at the front of our house. The well exists because they raised the grade on an existing tree, so they built this retaining well to keep the tree at the original grade.

I'm looking for suggestions on plantings to place in the well. We had started simply by adding mulch, but now it's starting to get taken over by weeds.

I'm looking suggestions of perennials or shrubs that would grow big enough to be visible from the street. I'm imagining tropics fern leaves, huge palm branches coming up. But I know that I'm in Chicago so that's not going to work. :)

Any thoughts?