Expanding Mulch Beds

As my ornamental grasses get bigger, the little mulch beds under them look smaller and smaller. I think the beds need to be expanded so that the ornamental grasses don't hang over the lawn. A nice mulch apron out in front looks good, and also helps raise the bed up above the lawn. If the mulch is too low, it collects grass clippings and looks crappy, so I'm going for higher pillowy mulch.

This bed of Northwind switch grass looked nice enough when they were just tiny stumps from divisions, but now that they are getting bigger the slim bed just wasn't sufficient. 

I cut an outline with a spade and rolled up the sod. Even after a year, there was still a clean divide between the sod layer and the clay slab below

I filled in the trench left from the sod removal with some topsoil (a step I didn't do last time)

Then I mulched on top and created an angled lip along the front edge to give the bed a raised fluffy look

In the other corner of the backyard I have my Hardy Pampas Grass and a huge rock (and 3 spare rocks that I guess are just becoming part of that corner permanently unless I can figure out what to do with them). I eventually want to put something else around the bottom of the pampas grass but I'm not sure what. I also wanted to raise the bed up a little because it is a magnet for grass clippings being lower than the actual lawn itself.

Removed some sod and added more topsoil to raise the bed a bit. It's the low spot of the yard and fills with grass clippings whenever I mow near it. I'm also tired of mowing around the rock, so now the rock is a feature instead of a big thing on the edge of the bed. 

What should I put around the base of the pampas grass? Something full sun that can take some damp soil every now and then.