Is this better or worse than the one where they are cooking corn husks to make oil to squirt into the ground?
Excess manufactured methane could also be injected underground, presumably.
Do you have any uses for the plant, or does it create interesting byproducts for you? Photosynthesis is not very efficient, but it is great at making complex organic molecules like sugar or cellulose.
But a plant needs more than power. It needs nutrients, usually in the form of fertilizer.
They also don't generally make hydrogen, not directly at least.