I studied as an undergraduate in Ecology, and am going to do my postgraduate work in ethnobotany, as there were no universities in my country that had any ethnobotanical undergraduate options.
I don't know what country you are in, but you can look on the net to see what undergraduate courses the universities in your area offer. As long as they are vaguely plant-related, you can turn it into an ethnobotanically-based postgrad degree, but if you can get any undergrad courses in botany or ethnobotany that would be better.
I would seriously recommend doing volunteer work with any botanically-focused organisation whilst you study, especially try to get into any summer research programs, as these will give you an advantage over any graduates that didn't.
Good luck! Plants rule!