 Black plastic doesn't work very well. And the idea is that with the water in there and with the heat being built up under the plastic, it should, in theory, get temperatures high enough to actually kill a lot of those weed seeds. Now, you know, how else do I say that? Should, in theory, and a lot of fudging on that. So I tried it with black plastic last year. I don't think it killed a single weed seed.