IANAL, but I think marijuana is specifically mentioned in the U.S. Code and it would take more than an executive order to change the law. While it might be removed from "Schedule I", that would just make Schedule I incomplete.
(21 USC 841)
(21 USC 841)
Opium poppies are also listed by name, but while you can grow them in your flower garden (but putting one in a vase on your dining table is a crime!), you can't even grow cannabis.