Here is an observation made at Spartanburg Humane Society. To extend the life span of polyethylene sheets, we roll up each sheet at the end of the growing season until the next growing season. During October 2007, while "bringing in the sheets", we noticed something interesting. There was no green foliage under the clear sheet on the south side of the small ridge (berm), shown in the first photograph. But when we looked at the north side, a big difference was apparent. Looking from the north toward the south we see abundant foliage alive and well on the north slope, shown in the right photograph.
Here is a photograph taken from the top of the ridge, and includes its north side. The sloping north face, crossing the center of the photograph, still has green kudzu foliage. The right side of the image is where the slope has stopped and the ground levels out. In this level area the heat under the sheet has had its effect and killed kudzu leaves, so we see bare ground. This suggests that the heat under the sheet on the north slope was not sufficient for defoliation, while the treated south slope and level areas became hot enough for leaf kill. Therefore, slope direction affects how well polyethylene sheets kill kudzu foliage. It is well known that north slopes receive less sunlight, and our observations are consistent with this fact.
We will post more information as we further investigate this interesting effect!