Boisterous neighbors, nearby roads or outdoor heating and cooling equipment can bring unwanted noise to your home and yard. While you may not be able to eliminate the source of this noise, some well-placed plants and shrubs may be able to reduce sound levels and restore the peace to your property.
A 2010 study published in the Journal of Environmental Health studied the effect of different plant features and plant layout on sound blocking and noise reduction. The study found that planting shrubs or trees in a crossing, or staggered pattern, more effectively reduced noise levels than planting in rows, or abroad. The study also concluded that the density, height, length and width of plants were the most important factors when it came to how much noise levels could be reduced. Plants with wider, heavier leaves, such as arrowwood (Viburnum odoratissimum, U.S. Department of Agriculture plant hardiness zones 8 to 10), are more effective at blocking noise than those with narrow or lightweight leaves, like oleander (Nerium indicum, USDA zones 9 through 11).
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The Georgia Department of Agriculture provides some basic tips for reducing traffic noise using plants. First, position plants as close to the source of the noise as possible, and use multiple rows of plants to block and absorb sound. The row of plants closest to the road should consist of shrubs with branches that grow toward the ground. The second row must be slightly taller, with the back row the tallest of all, and the three rows must contain air space in between to deaden sound. At least one row should consist of evergreens, such as the deodar cedar (Cedrus deodara, USDA zones 7 to 11), to block noise year-round, and plants with large curling or cupping leaves, like the Chinese photinia (Photinia serrulata, USDA zones 7 through 11), are most effective. Each row should be as wide as possible, but anything is better than nothing if you are limited on space. This technique can also be used to block noise from neighbors, nearby businesses or air conditioning and other equipment.
You can also use plants indoors to reduce sound wave reverberation time and cut noise. Indoor plants absorb, diffract and reflect noise in homes and offices, according to the European Federation of Interior Landscape Groups. Using potted plants to reduce noise levels works best in rooms full of hard surfaces, such as stone floors or marble walls, and may not be as effective in rooms equipped with carpet and plenty of cushioned surfaces. Spread plants around the room, focusing on corners and areas near walls for best results.
Vegetated roofs serve as an effective means of keeping noise from outside a home or building from entering through the roof. Extensive green roofs, such as those used on homes, reduce noise transfer through the roof by as much as 40 decibels, according to Green Roofs for Healthy Cities. Intensive green roofs, which consist of deeper soil layers and are commonly found on commercial buildings, reduce sound transfer by 46 to 60 decibels.
The subject of how effectively plants block noise is fairly controversial, and some sources suggest that the effects are largely psychological. The U.S. Federal Highway Administration reveals that when it comes to traffic noise, simply removing the source of the noise from view using plants can reduce a person's annoyance from the noise. Even though the noise is still there, the person feels better about it, and is less bothered by it.