Thyme, brass buttons, or other? Ground cover for hard clay
I've got a stretch of very hard clay soil at the foot of a retaining wall. It gets full sun. It's about 60-80' long, and 1-2' wide. It's always filling up with weeds, so I'd like to put an attractive ground cover that will choke out most of the weeds. For this situation, I'd prefer something that will form a nice mat, like thyme does.
It's an isolated strip of dirt, so I'm not worried about invasive plants, unless they can send out roots 3-4' under a rock wall and spread that way. (I understand bamboo does this.)
I've planted thyme and brass buttons, and they both seem to be happy there. I prefer the thyme for appearance.
Will the thyme be able to spread in hard clay soil? (I've already noticed the brass buttons spreading) Is any one type of thyme better suited to this purpose? I'm using Woolly Thyme (Thymus lanuginosus) right now. The slugs ate the Elfin Thyme (Thymus serpyllum 'Elfin') I planted there.
Can anyone suggest other ground covers that might do well? I'd use sedums, some of my all-time favorite plants, but since it's right by a sidewalk I just know they'd get trampled. Ideal plants would be drought-resistant too. (I was surprised that the thyme is doing so well. I haven't watered in for 2 months, in one of the driest summers ever.)