Perhaps some small flowering bushes? Or something that has winter appeal?

Thanks BeesNeeds... I had originally thought ornamental tall grass. But with the tree...I don't feel it would get enough sun. I will look Into some winter drama shrubs to go behind it.

If you are worried about sun, you might want to try shade tolerant succulents. Some of those are pretty hardy, can tolerate shade, and might look nice among the rocks.

There are a whole handful of winter drama plants that are lovely year round. Hollies can be lovely too.