Q: I need trees and shrubs to plant in a mostly shady area. I need them to offer some relief from road noise. Do you have any suggestions?
A: The basic laws of physics show us that sound is best diminished by placing something with sufficient mass between the sound source and the recipient. While planting densely leaved trees and shrubs will psychologically diminish the traffic noise, they won’t do much to lessen it physically.
As highway designers know, the best way to cut traffic noise is to use a solid wall or a berm of earth beside the road. Your “mostly shady” situation limits what can be grown there. I suggest a mixed planting of ‘Bright n Tight’ Carolina cherrylaurel, Canadian hemlock, American holly, mahonia and Florida leucothoe. All are evergreen and somewhat shade tolerant. All will grow best if you plant in October and amend the soil thoroughly beforehand.