I'm indebted to Dr. Bruce K. Kirchoff, Professor of Biology, UNC Greensboro who alerted me to a twenty years study on peppering presented by Hartmut Speiss in 1999.
The bad news, in Dr Kirchoff's own words:"... ... 10+ years of research on the plant peppers in Germany? It was all negative. They tried every conceivable burning time and preparation type you can think of. All negative."
Because my German is laughable, Google translate gives this gobbledegoogle from which you are invited to pick out the bones; "Subtracting out after nearly 20 years of experimental design for weed control with a summary of the ashing method, could in no case the scientific claims of sufficient plot trial Be demonstrated that the use of drugs to ashes R. Steiner's recommendation to a complete reduction of the investigated Wild plants resulted. In isolated cases tendencies, but also significant Differences between treatments on. On the other hand, the analyzed weeds, respectively. Wild plants in the test period to a level reduced, that their stocks are no longer regarded as problematic si nd These facts can be drawn out some conclusions even if some hypothetical, and are open to more questions lead-ben can be given as answers."
You can get the whole google translate wodge here. If you are happy to translate it properly please do let me know.