The height of a sheet from the ground is one factor for sheeting treatment effectiveness. Regardless of sheet type (black, white, or clear), it holds air heated by sunlight in an enclosed volume around the covered kudzu. This thermal treatment is more effective as temperature increases. Consider the following scenarios.
Regardless of sheet height, kudzu foliage in contact with the sheet is either killed or seriously stressed. On the other hand, there is an increasing potential for killing kudzu root crowns in this sequence of scenarios, thereby eliminating the plant: 0% kill at 3 foot or higher, to approaching 90% kill at 1 inch or lower.
The treatment effect is apparently based on the amount of air trapped under the sheet. The higher the sheet, the greater the volume of trapped air, the more the thermal energy supplied by the Sun is diluted (because it must heat more air), and the lower the temperature treatment.
The first photograph shows a 5 foot fence covered with a clear sheet. While foliage on the fence is killed to make the kudzu easier to remove, the ground below the fence is never heated sufficiently to kill root crowns. This is true even when the sheet drops to ground level, and is weighted down to "seal" the air inside the sheet. The second photograph shows a 3 foot brush pile, which was overrun with kudzu, covered with a black sheet. The pile was mostly shaded by overhead trees, so the kudzu foliage killed underneath was mostly due to lack of light. One expects kudzu crowns to "hibernate" rather than die. Note the covered fence in the background. Click for larger images. Both photographs June 2005.
The first photograph shows an example of a 3 foot brush pile that was uncovered (right in photo) after a relatively short treatment. June 2005. Note that some of the foliage has wilted or died back. The second photograph shows that longer exposer leads to dead foliage. September 2004. Any attempt to kill the root crowns using sheeting requires multiple treatments of this kind, along with the removal of dead plant material so that sheets move closer to the ground with successive treatments. The preferred method of elimination is surgical removal of root crowns, after sheeting treatments during the growing season make the crowns accessible. An alternative is to wait until frost kills the foliage, followed by root crown removal.
When sheets are close to the ground, then foliage is killed more quickly, and their effectiveness for killing root crowns increases. September 2004.