I would say with your humidity being so high in the south, that a cover would need to be raised at least a foot or two from the top of the plants. You want lots of air flow in your growing conditions.
Also, since it is on a slope, watch the plants, they will tell you if they are getting to much water. From what I can see those semps are looking pretty healthy, and sedum can handle more water than semps.
If it ain't broke, don't fix it.
