This video was produced by a private company that found a way to generate electricity from plants without harming them. From what I gathered in the description, they accomplish this by placing electrodes in the soil in locations that have differing voltage potentials. This electrical gradient can be attributed to the the breakdown of photosynthetic products released from the plants' roots. Much like a hydrogen fuel cell, the product of this electrochemical reaction is water. I do not know how its efficiency compares with that of photovoltaics, but when you take into account the environmental impact of manufacturing photovoltaics, and that advantage of having more green spaces, this may be a viable alternative.