The Garden Club option is only available on the phone during the daytime.
Call them and have a Garden Club member come over.
Greet them when they arrive and then choose Buy Potion>Plantrophic- C.
The potion can then be found in the sim's inventory just like the ReNuU Portachug.
1 & 2, fine. But there's no "Buy Potion" option when I do it.