In an old column [Do plants grow better if you talk to them?19-Apr-1985] which was included in the e-mail I got today, there was a lot of talk about music, and various frequencies of sound.
No one however mentioned that people exhale carbon dioxide when they talk, and, if my H.S. Biology memories are correct, plants “inhale” CO2 and require it for growth.
Also, we exhale some water vapor too, I think.
So might it be that talking to plants just provides them with things they need to grow? Whatever frequency soundwaves we produce may be an additional benefit.
But I suspect that the real answer is that people who talk to their plants pay more attention to them, notice bugs or dry soil or other problems, and tend to them sooner and with more care than those who don’t.
Welcome to the Straight Dope Message Boards, Roscoe, we’re glad to have you with us.
When you start a thread, it’s helpful to others if you provide an actual link to the column being discussed. You did cite the column and date, which is very useful, but the link helps keep us all on the same page, saves search time, and is just generally more convenient.
No biggie, I’ve fixed it, and you’ll know for next time. And, as I say, we’re glad to have you with us.
At the end of the column, where Little Ed talks about plants liking dentist music and hating rock ‘n’ roll, he says plants have “feelings but no taste.” They like dentist music? Then it’s fillings, Ed, wo wo wo fillings. 
This was always my understanding, even in childhood.
One would have to assume that, while talking directly near the leaves, which do the bulk of the photosynthsizing, causes the air to have a higher concentration of CO2. No miracle here.
While it’s true that our exhaled breath contains a higher proportion of carbon dioxide, it’s still not very much. I seem to recall that it’s only about a percent of the air. And even if one were to blow directly onto the leaves, I have to think that that enriched air would mix with the rest of the air in the room in fairly short order (much shorter timescale than that for absorbtion by the leaves). I think that Roscoe’s attentiveness hypothesis is the simplest explanation.
Well, not very much isn’t the issue. There’s never very much CO2 in the atmosphere, but the very fact that we have plants at all indicates they have adapted to this scarcity. What resources I can find say that normal atmospheric concentration of CO2 is about 0.04%, whereas the concentration in human exhalant is 4%. We’re talking about a 100-fold increase in concentraton here.
True, the greater concentration and higher temperature of exhalant means it will diffuse fairly quickly. However, even with fairly rapid diffusion, it will take time before the concentration reduces by two powers of ten. We must also remember that a fanatical plant talker may sit in the immediate vicinity of the plant for several minutes, constantly replenishing the CO2-rich air.
Furthermore, since one tends to exhale more slowly while talking than while blowing or even breathing normally, the inhaled air spends more time in the lungs, allowing more blood to flow past, allowing for more gas exchange. I have to hypothesize here that exhalant during talking may even be slightly richer than normal breathing.
So the plant may spend several minutes a day in an atmosphere several orders of magnitude richer in CO2 than it may have evolved to expect. I don’t know much about rates of plant aspiration, but I have to speculate there is some benefit to be had here.
No doubt this plays an important role as well, but I don’t think the High-CO2 scenario can be ruled out as easily as your post would suggest.
I’m surprised I’m the first to bring this up in this thread, but the Mythbusters tackled this very subject this week. They set up a bunch of identical greenhouse tents on a building’s roof and filled them with identical pots with identical seeds and identical automatic waterers. They then played various kinds of sounds to different tents, ruling out the carbon monoxide from the breath theory by keeping the people out of the tents and playing recorded sounds. One tent got a person talking nicely, one got a person talking abusively, one got classical music, one got heavy metal, and there was a control tent that got no sounds.
The Mythbusters conclusion amazingly echoed Cecil’s: The plants that got the heavy metal music were substantially taller and healthier than any of the others. Who woulda thought?