Photosynthesis is the conversion of sunlight into plant food by the plant itself and they have been converting sunlight into food all season now. In the early parts of the season, plants used this energy to grow and reproduce; now this energy is being stored for autumn root growth and the long hibernation of winter. 
Plants are fat and sassy–at their apex in terms of energy and planting them now means they will develop a robust root system just in time for the onslaught of winter. This time frame of planting lasts through at least the end of September and if good weather persists though most of October!
When I was a County Agent, I taught farmers and ranchers to plant their new grass pastures and their alfalfa fields now instead of in the spring and for good reason. They could spend the hottest part of summer preparing their fields and not worrying about whether they were getting enough water to their new crops; all the while saving seventy-five percent on their water and labor costs to get the exact same results as a spring planted field.
If farmers and ranchers can do this for their crops, it stands to reason gardeners can too! If you are thinking that this is not the time to be planting trees, shrubs, perennial flowers and ornamental grasses you would be dead wrong–it is absolutely the best time!