If you have kept your Dogwood well watered and maintained Andrew it seems rather strange that in short period of time the leaves on the tree have shrivelled and there is die back on the lower branches. It is difficult to pinpoint exactly what the cause could be but it could be the spray drift from someone using a weed-killer however this would have also affected plants in the near vicinity. It may be a soil borne pest which is causing the problem but again, I feel this would have caused wilting of the leaves rather than instant shrivelling. I feel it would be worthwhile to explain the situation to the Garden Centre/Nursery where your purchased the tree from and as you have kept your Dogwood well watered and maintained the majority of reputable Garden Centres should replace your tree.
Stephanandri Incisa Jacqueline which Dr Hessayon has recommended is a shrub which grows to approximately six foot in height with a similar spread and will produce tiny white flowers in the summer time but I am just wondering whether this shrub will be too large for your needs and if you just require plants to cover your bare soil the perennial Violas would be fine and also ground cover plants such as Saxifrages, Sedums and the Silenes (Campions). Also, there is the Flowering Thymes which are ideal as ground cover plants. If you require plants which are slightly larger you could try perennials such as Lupins, Geums and Heucheras of which there are numerous varieties and all these plants can be easily obtained for Garden Nurseries and Garden Centres.
Rock water garden expert by dr d g hessayon - essay-farmall.