Dr. Gustav Theodor Fechner and Cleve Backster
In 1848, German professor Gustav Theodor Fechner published a book titled "Nanna." The book suggested that plants are capable of feeling human-like emotions and that proper attention and care could result in healthy plant growth. Cleve Backster, a polygraph expert, later tested this idea of plant perception with a polygraph test. Backster attached a leaf to the polygraph equipment and noted electrical resistance when he harmed or threatened to harm the plant. This led him to the theory of primary perception, arguing that plants are sensitive to human thoughts. This theory led many to believe that treating a plant kindly would offer it comfort and aid in healthy growth. The team at "Mythbusters" tested this theory in a controlled environment and concluded that it is false.
It has been proposed that vibrations measuring 70 decibels, about the level of a normal conversation, may turn on two plant genes -- rbcS and Ald -- involved in its response to light. This finding was discussed in a 2007 paper from scientists at South Korea's National Institute of Agricultural Biotechnology. Plants use light in photosynthesis, creating food for the plant. These vibrations may aid in plant growth by stimulating the two hormones. However, plants in windy conditions with excessive vibrations release the ethylene hormone, which stunts growth.
Plants absorb carbon dioxide, during photosynthesis, to produce carbohydrate energy. This fact goes along with the idea that talking to a plant provides extra carbon dioxide released in human breath. Plants do respond to carbon dioxide in human breath, but a person would need to talk to the plant for several hours daily to notice any kind of visible growth response.
An experiment was conducted at RHS Garden Wisley in Surrey to determine if voices had any effect on plant growth. A variety of male and female voices were chosen to play through headphones to 10 tomato plants over the course of a month. The results concluded that plants responded to female voices. The plants stimulated by female voices grew an inch more than those stimulated by male voices which, in some cases, grew less than a plant left completely alone.
Healthy Plant Growth
Plants need proper care to experience adequate plant growth, including a regular watering and fertilizing regimen. Although some studies show it is possible that plants may grow when exposed to talking, there is not enough conclusive evidence to prove that this is a reliable method to encourage plant growth.