Commenting on the bit about metal. I've read studies that Mozart helped plants grow faster, due to the constant pummeling of notes (soundwaves). Maybe if the plant was given *good* metal...the results would have been different. Say, Cynic, or Death, or Exivious, or Obscura. Something where the musicians are accomplished and have a barrage of intelligently placed notes. Instead of random screaming and electronic effects.
Now: Commenting on "though it is unlikely that the plants themselves respond to the music, their caretakers do! Maybe the plants which are exposed to music receive better care than those which do not. Might be worth looking into!"
I know more people who listen to spoken word, or are into the Harry Potter thing, who are much more careless than many who listen to, a-hem, good metal.
Maybe, it's just the quality of the individual - and one should not base results on the "type" of person who would listen to a certain type of music.
On that note, if this study were performed in a scientific manner, shouldn't the caretaker have been the same? Or have strict constants in place, to ensure the care was the same across the board, the only difference being the type of music being played? Now, that's certainly something that "might be worth looking into!"