Fantastic! Any organic matter that is heavy enough to keep the cardboard in place and help make it impossible for existing roots or new sprouts from seeds under the cardboard to get into the light can be added, leaves, kitchen scraps, small tree or shrub trimmings. As it all decomposes, it will turn into a layer of compost on the soil surface.
Anywhere I've ever tried to garden without doing that first has been a weedy disappointment, way too much bending, pulling, digging, cussing.
