Will Privacy Trees Prevent Road Noise?

Effective Noise Reduction

• Trees can be most effective for noise reduction when they are closer to the noise's source, such as a road, than to the location where noise is heard, such as a house. Trees used for soundproofing should be planted close together, in the fashion of a hedge, to form a solid wall. Choose varieties that have dense foliage from near ground level to their top. That form eliminates the need to augment the soundproof plantings with an understory of shorter shrubs. An example of such a tree variety is Leyland cypress (x Cuprocyparis leylandii), which is hardy in U.S. Department of Agriculture (USDA) plant hardiness zones 6 through 10. It grows 60 to 70 feet tall and 10 to 15 feet wide.

Optimal Distances

• In order to have optimal reduction of moderate road noise -- up to 5 to 10 decibels, or 50 percent to human ears, plant tree borders so they are 20 to 50 feet from the middle of the nearest traffic lane. Soundproofing trees should be a minimum of 15 to 20 feet tall and fronted by shrubs a minimum of 6 to 8 feet tall. The line of trees should be twice as long as the length of the distance from the road to the recipient of the road noise. The line should run an equal distance in both directions from the recipient of the road noise.

Evergreen or Deciduous

• Barrier trees can be either evergreen or deciduous, but evergreen species are best for maximum, four-season noise reduction. Evergreens create a solid wall of sound-absorbing material year-round. Choices include vigorous "Green Giant" arborvitae (Thuja "Green Giant"), an evergreen that is hardy in USDA zones 5 through 8 and grows 40 to 60 feet tall and 12 to 18 feet wide. Evergreen hollies such as "Nellie R. Stevens" (Ilex "Nellie R. Stevens"), hardy in USDA zones 6 through 9, also provide good soundproofing, plus bright berries for winter interest. "Nellie R. Stevens grows 15 to 25 feet tall and 8 to 12 feet wide.

Sound Considerations

• If trees already exist in places where noise barriers are planned, leave them in position and incorporate them into the planting scheme. If they are deciduous or do not have leaves down to ground level, augment them with tall evergreen shrubs. Sometimes, for ecological or biodiversity reasons, a mixed-tree border is desirable. Several varieties of conifers and other evergreens can be mixed together, possibly with a second row of deciduous trees or shrubs, space permitting, to provide soundproofing as well as color, nectar sources for pollinators and additional fruits for birds and other small animals.