I've noticed many people are googling for information on this subject.
I thought I'd mention we DO have a thread on the subject:
Generally plantkin are not considered to be Therians, but rather Otherkin. The Therian experience depends on relating one's mental state with that of an animal that can be observed. Plants have no mental states to speak of, and therefore it is not possible to have a Therian experience with a plant as one's theriotype.
More on that in the thread above, but you'll have to register.