The entire plant food publicity is indeed a cover-up. Mephedrone is of absolutely no use to plants. The reason it's been marketed in this way is to get around the restrictions of the Medicines Act, which make it illegal to advertise or sell mephedrone for human consumption.
It follows that it might just as well have been advertised as goldfish food for example or even cat food given one of it's alternative names, meow-meow!
The pharmacology of mephedrone is mostly unknown but a number of studies have begun this month to try to address this issue including some at my own university.