Found this old thread while researching groundcovers and read through it will interest. I am on the other side of the continent but in a similar zone. I've had horrible luck with most of the creeping thymes, not exactly sure why. I love the look, the fragrance, and the flowers...but they seem to do well for a couple seasons and then get woody or lose vigor in the center. One that has been more reliable is
Red Creeping Thyme (Thymus praecox 'Coccineus') Here it is on my rockery steps a few years ago. This year, sadly, it is noticeably thinner.
My latest success is
Smooth Rupturewort (Herniaria glabra) which has nicely filled in this raised bed from three 4" pots planted this spring. I divided each small pot into 2-3 plugs and was really surprised at how quickly they filled in. This is supposedly evergreen and will turn a bronzy color in the winter. Looking forward to that, and hoping it will remain robust.
My current challenge is to find something that will do well between stones at the bottom of my deck stairs - on the north side so shady. I've tried both Irish and Scotch moss, neither was able to battle the bully dandelions (which are too tall for the area, otherwise I'd just go with them).
Another difficult spot are the about 1-2" gaps between slabs of sidewalk. We used to have wood spacers in there, but they rotted out and I tore them out thinking a nice groundcover would soften it. So far, no luck. These gaps are in full sun and have really sharp drainage (the underlying gravel sub-bed plus whatever dirt has drifted in). I've tried small sedums, thymes, pennyroyal - so far nothing has worked.
In other areas, I have found ajuga pretty, but not as able to hold its own against grasses/weeds. Sweet woodruff is really delicate looking, and I let it colonize where it is happy, but it also is easily crowded out by weeds. I like vinca at the foot of shrubs, it will often kind of crawl up them. And (horrors for some) I really like goutweed at the foot of trees - it seems to appreciate the dappled shade and makes for an easily maintained base.
