Is it possible to use plants as a medium to store data? By what data structure?
What you propose sounds like it would be quite error laden, as plants are not necessarily independent, and those grown in the same area of the same species might share a common root system, so what you might consider redundancy is not a guarantee that your data will be preserved.  I would also venture to say that it's rare for a plant to be hardy against cold weather, and especially against something like an herbicide.

What you might consider is the concept of biocomputing and, more specifically, DNA computing, which uses the ordering of the base pairs to store and manipulate information.  The field is still in its infancy, but Turing machine-like behavior has been achieved, and in some cases basic algorithms can be implemented (e.g., the Hamiltonian path problem as cited in the Wikipedia article).