Rooting cuttings

I have a wire shelf wrapped in plastic like a temporary greenhouse, and it seems no matter how wet or dry i keep it, the first sets of leaves on new cuttings bud out and then blacken after being vibrant and green for up to a week prior. What the heck!? I have a large chicken coop that id like to winterize and put plants in, but I don't even know where to begin to make it warm enough for small plants.