It was about this time last year that I started preparing a bed that by early September was ready for fall planting.
It was a
garden bed already but the soil was not too good.
I started with horse stall cleanings a layer about 4 inches thick. Then a layer of grass clippings, hay, and leaves, then another layer of stall cleanings, and a thin layer of grass clippings on top. I gave this area a good soaking about once a week and that was it. I had very little problems with weeds till this spring, but they were easy to get rid of.
I agree with Paul about the glue in the cardboard, plus any other chemicals that may be in there.