Plants as Filters

A gardener asked me about what shrubs to plant as a screen against a fence for privacy, to filter noise and provide a barrier against car exhaust coming into her yard.

Can plants remove air pollutants? The answer is yes. Leaves remove gaseous pollutants from the air by absorbing them through their pores. Leaves, stems and twigs also trap air pollution particles on their surfaces. Some particles are absorbed into the plant, while others are washed off by rain or overhead irrigation or dropped to the ground via leaf and twig drop. Trees are good absorbers of sulfur dioxide. A hedge may remove as much as 50 percent of sulfur dioxide blowing through it or reduce airborne lead by 40 to 60 percent. Not only do the plants absorb pollutants through their foliage, the soil surface around them is another effective pollution sink for carbon monoxide, sulfur dioxide, ozone, nitrogen oxides and hydrocarbons.

Can plants also reduce noise pollution? The answer is, not much when there are just a few plants, but in dense buffers, they can reduce noise somewhat. You get five to eight decibels (dB) of sound reduction per 100 feet of buffer width. A power mower three feet away puts out 107 dB; bird calls are 44 dB. In order for plant buffers to work at all, the plants have to be close together, have dense foliage (leaves) and the buffer must be the correct length and height. Foliage of the plants should grow from the ground up. A combination of shrubs and trees usually works best. The buffer must be planted close to the noise source rather than close to the area to be protected. On a road with speeds less than 40 mph, a 20- to 50-foot-wide buffer planted within 20 feet of the center of the nearest traffic lane can reduce road noise. On a road with speeds greater than 40 mph, a buffer would need to be 65 to 100 feet in width and within 50 to 80 feet of the center of the nearest traffic lane to be effective (USDA National Agroforestry Center). What works better to reduce noise is a solid fence, wall, soil berm or other solid material.