Evergreen noninvasive groundcover for shade and clay
Just in front of my house I've put in a row of azaleas and rhododendrons. I've planted bulbs and a few odd perennials around them. But I'd like to have a low carpet of a well-behaved evergreen groundcover underneath it all for year-round aesthetics. My preference in flower color is red, white or blue. In the summer it is almost full shade - just a little dappled sun in the afternoon. The soil was heavy clay and slightly acid, but I when I built the bed in Spring of 2006 I tilled in lots of peat and gypsum, not to mention some sulfer, bone and blood meal.
My first choice was wintergreen (Gaultheria procumbens). I planted a lot last fall. Only about half did anything this year, and those that did only showed a few leaves. At this point even those leaves don't look good - many have brown spots. The local extension service thinks it is the clay.
My second choice would be Mitchella repens, but from what I read it won't do well in heavy soils either. Vinca minor and pachysandra do well in the yard, but I assume they will crowd out everything else, maybe even the shrubbery!
What about Galax? The Internet is inconsistent about its invasiveness, and it is not clear to me how it will do in heavy soil. Then there is Mazus reptans. I get a mixed message on soil type and how it handles full shade. Also, what will it look like in NJ in the dead of winter?
Does anyone have any suggestions? Thanks.