Hydrangea mycrophylla in early spring.
Do I believe that music make plants grow better? No. The plant that is sitting in the family room where music is played all day is no better than the one that is in the bathroom where no music is played except for an intermittent humming by my husband.
Do I believe that a gardener singing in the garden would help the plants grow better? Maybe. Let me explain. When a gardener is singing in his garden he is close to the plants. He observes the things that are going on and he can respond to such observations accordingly. On the other hand, when a gardener is singing in the garden and is focused on his music book for the right note, then that singing does not have any effect on plant growth. In other words, it is the action-initiating proximity, and not the reverberating sound waves, that make the difference. That is my practical explanation of the matter.
The Royal Philharmonic Orchestra was supposed to have been commissioned by QVC (a shopping channel) to perform to the plants [*]. With that I guess that the effort was not so scientific in nature as much as it is commercial. The plants that were present at the concert are now for sale along with the CDs that contain all the classical music played at that performance. By the way, what type of music do your plants listen to? I wonder if the type of music makes the difference. That would be another study. :)
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[*] Royal Philharmonic Orchestra Performs to Plants
Music in the garden is good for the gardener but not good enough to make his plants grow better. Plants need the gardener's attention more than music.