See the paragraph regarding black/clear plastic.Although intent of post was for SWC,the temps are greater with clear plastic.
http://aggie-horticulture.tamu.edu/g...ummer_heat.htm
"The plastic used to cover the area should be clear, not black. Clear plastic lets light energy pass through and then traps it, much like a greenhouse. Black plastic absorbs most of the sun's heat without letting it pass through to the soil below. 1-to-6 mil plastic will work fine - the thicker the better."