A: At a certain stage of development, most children are likely to put everything into their mouths! This pattern would appear to be something that is wired into their brains. Luckily, this tendency will pass in a few weeks or months as the little one's nervous system matures and moves on to another behavioral pattern. Naturally, the parent doesn't feel enthusiastic about this behavior--so that the fact that you (of course!) don't applaud and look pleased every time your daughter does this, will help phase out this behavior. You don't need to show disapproval--but you can say kindly, "Oh no no, we don't eat plants" as you rescue the plant from being mouthed; giving this information can't hurt. A child in this situation is really too little to make use of instructions in a reliable way, however, so trying to explain a "rule" is not really going to help. Making this an issue or a power-struggle wouldn't work either. She will just have to outgrow this on her own.
It might would be a good idea for your daughter use something harmless that she can keep handy while she is in the midst of this phase. Is there some snack food or favorite toy or beloved rag that she might like to carry around, so that she has something to put in her mouth when she likes? And, of course, it is important to clear the area where she plays so that you minimize the chances of her putting something into her mouth that might hurt her. Safe spaces and watchful supervision are all you can do for the moment. This will pass.