There is the whole mystical positive chi energy going into the plant concept... Also, you form a stronger emotional connection with the plant and are more likely to care for it better.
Regardless of your belief, you talking to your plant will increase the CO2 around it, and it will have more available to "breath"... if nothing else.
Its proven fact that Plants somehow detect pleasant/unpleasant energies.
i.e. Classical music, domestic fighting
Its crazy to even think HOW it happens.
There is Good Chi
and there is Bad Chi
Which seems highly illogical...
Black, White, and shades of Grey,
Racoon
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There is Truth in Wine and Children
on mythbusters they were doing an experiment on plant talkind the resuls were in this order (worst to best):
6.talking quitly nicely
5.talking quietly with abuse
4.yelling nicely
3.yelling abuse
2.classical music quite loud sorta mf
1!.and top of the list was very loud heavy metal
Did they offer any speculative reasons for the results? I'd think that the pressure wave from the music acted a bit like wind, putting stress on the plants stalk and appendages (what do you call them in plants?)... making it react and grow stronger.