Answer: If the site is sunny, wooly thyme is a good choice, as is Blue Star Creeper. The foliage of both will contrast with the slate and soften the edges of the stones. If the site is shady to partly sunny, Soleirolia (Baby's Tears) will produces a carpet of mossy-looking green with its tiny cup-shaped leaves. All can take some foot traffic, but if it's a high traffic area, Sagina subulata (Scotch or Irish Moss) will grow in full sun or partial shade and stand up to daily foot traffic.
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