|What takes the black growth off my gardenias. It|
is on the leaves and the stems.
That is sooty mold growing on honeydew a substance secreted by various insects such as whiteflies, scale or aphids.
If you wet the bush thoroughly and then spray with an insecticidal soap spray, let sit for 15 minutes then rinse, the sooty mold should flake off as it dries off. A strong blast from a nozzle can also help remove it if you give it a little time to sit wet and soften up a bit.
Thanks for the question. Please stop in again soon!