LOL, I wonder what it would do to a plant... Doesnt creatine just store fluids in your muscle and help repair faster, thats why it seems like it bulks you up....
I think your thinking of Anabolic steroids. Im no expert on this, but Im pretty sure if the Anabolic steroids that can cause water retention.
Anabolics increase protein synthesis in cells, so if your not working out and your just pounding steroids you get weird muscle growth and water retention.
Creatine is a LOT safer then Anabolic steroids IF, big IF here, Used properly with proper diet and a proper workout program.
Creatine occurs naturally in all vertebrates and it basically increases enregy to muscle and nerv cells.
Like anabolics, if you dont use creatine properly you can get water retention. The idea is to do a steady program with a workout program, then stop taking creatine, and tone down and strengthen all the new muscle growth.
As far as effect on plants....I have no idea, I think its something you would just have to try and see what happens. But I doubt it would do anything beneficial to your plant.