I've heard a few conflicting things about this and would like a clear answer.
Answer - Chief Medical Officer of HelloMD
Laws vary state to state for medicinal cannabis. In California Under Prop. 215, patients are entitled to whatever amount of marijuana is necessary for their personal medical use. However, patients can be arrested if they exceed reasonable amounts and they can be cited or fined for exceeding local laws. Under MMRSA, which took effect Jan. 1, 2016, qualified patients can cultivate up to 100 square feet for personal medical use, and primary caregivers with five or fewer patients are allowed up to 500 square feet. Exemption under this section does not prevent a local government from further restricting or banning the cultivation of medical cannabis.
If you have cannabis in a National Park, which is under federal laws, you can be arrested since under federal law it is still illegal to posses cannabis even if you have a physician recommendation.