I have some bloody cranesbill geraniums growing under some juniper shrubs near the dripline of the shrubs. They have been there for many years, and although they have spread slowly, they seem to like it there. You might try them. You might also try impatiens or little hostas. Once they're started, they might like it there. Seems like small low growing white flowers with low foliage would look nice in such an area. You could try creeping wintergreen.

There's a discussion on another forum mentioning all cedars having a growth retardant. Sort of like the juglone(sp) that leeches into the soil from black walnut trees. Could that be the problem?