Black plastic?? Ahhhgggg.....!!
Get rid of that FAST! It will sufficate your plants and sour your soil. I hate that stuff!
Sedum doesn't need roots but it does need rooting space. That black plastic is going to keep it from doing what it does - creep by underground shoots.
You don't say where you are but sedum are one of those plants that do well in bad situations. I planted some last summer from a six-pac from Home Depot. This summer, I am going to have to move/remove about half of them. Each of my 6 little cells have expanded into areas about 2 ft x 2 ft. But they needed to be able to grow out. A slit in black plastic will not do it (in so many ways).
