Answer: Considering the heat that will be absorbed and reflected by the slate, plus 4-5 hours of direct sunshine, I'd suggest a member of the rock cress family. Groundcovers, by their very nature, are spreading plants. Some grow more quickly than others, and they're the ones considered invasive. If succulent rock cress doesn't appeal, you might want to sow seeds of annual alyssum to brighten the area during spring and summer, knowing the plants will die off at the end of the season.
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