I think since our earth is moist, and we do have a lot of underground water, that the clear just makes a hot, hot greenhouse. The moisture gathers under it so that you cant see through it and the weeds do live in that hot moist atmosphere. When I take care of pesky hard to manage weeds I use boiling water and sometimes that just kills the tops and they come back from the roots. I guess our weeds like it hot. The black plastic makes it a lot hotter but it has to be on for a long period of time before it kills the weeds.
