Yes, I call this planting in the petticoat zone under the skirts of rhodies and I have done this in my own garden. (You can watch some of my YouTube videos to see the results under shade gardening.) The trick is to add a mulch of compost mixed with fine wood chips and use only shallow rooted groundcovers such as ajuga, saxifrage and lamium directly under the shrubs but then as you get to the dripline or edge of the branch overhand add brunnera, hellebores, pulmonaria and other woodland type perennials that will coexist with the rhododendron roots. If you dig a hole and hit a mass of solid roots stop digging. The rootzone is not very wide for even a large rhododendron and they love the application of compost/bark as a mulch. Just don't pile the mulch up the bark of the trunk. Taper it so that the soil near the trunk is barely covered and gets deeper as you work your way away from the trunk. It will be lovely. Keep growing, Marianne Binetti