In the very small concentrations we are talking about here, where you only use the ethanol as a solvent for the cinnamon and then dilutes this with water, I don't think it will be dangerous for the rhizomes. But when in doubt, always try it on a very small scale first. I wouldn't personally want to spray directly in strong sun either.
That said I'm still looking for a really good recipe for a spray. I'm concerned that the spray in the French link might have a too low cinnamon content in the prepared solution as compared to the more scientific papers, but on the other hand I use google translate on it, so might be something else that I misunderstand.
However I might have found a few more interesting recipes and in English this time. It seems cinnamon have some popularity among orchid growers:
http://firstrays.com/free-info...
One of those recipes use rubbing alcohol and another uses hot water to make a spray.
I'm sort of inclined to try the rubbing alcohol one, but add some water to the mixture. Thoughts?
Another very interesting idea is the one to mix cinnamon powder with a casein-based glue (Elmer's) or cooking oil to make a thick paste.This would then have some water resistant properties, which certainly would be welcome in rainy weather.
