Plants clean and purify existing air as well as manufacture oxygen. Each person requires 30 to 60 pounds of clean air to breathe each day. Ornamental plants provide clean air in four ways: First, their "manufactured" air created through photosynthesis dilutes polluted air and increases the ratio of "clean to dirty" air.

Second, moisture that plants give off traps airborne particles and acts as a filter for some pollutants. A single, mature apple tree gives off 300 gallons of water per day.

Third, hairy leaves and stems of many plants trap airborne particles and hold them until they are washed to the ground by rain. In one large city, the dust count on the down-wind side of a heavily planted area measured 75 percent lower than a similar count on the up-wind side. Fourth, trees can slow the air movement sufficiently for heavier dust particles and pollutants to settle. Additionally, Fumes and odors can be masked by plants with particularly fragrant blooms or foliage.

By modifying the intensity of sunlight and by wind and humidity control, plants can have a significant impact on the temperature around the home. The shady area under trees is cooler because of both an increase in moisture and a decrease in direct solar radiation.

Trees not only give off moisture, but the leaf canopy tends to prevent moisture-laden air from moving away. This moist air is more difficult to heat than dry air and can help reduce the temperature near the tree.

Trees are also effective at shielding something from the sun's radiation. Leaves, twigs and branches absorb part of the radiation, transmit a smaller portion, and reflect the rest.

There are several more ways plants can modify the climate. One important way is to use trees that are deciduous (those which lose their leaves in the winter) in landscaping, to reduce both heating and cooling bills. Trees planted so that they reflect the sun away from the house and shade it during the summer help keep the house cool. In the winter, when they have lost their leaves, the sun is able to reach the house and warm it.

Noise can be reduced with properly placed ornamental plantings. Trees, shrubs, vines and turf absorb noises. Plant parts break up sound waves, changing their direction and reducing their intensity. Densely growing plants are best for sound control but the width of the planting is critical. To effectively screen highway noises, plantings would have to be 25 to 35 feet thick.

Plant sounds, such as rustling leaves, help conceal or disguise offensive noises.

Glare from car headlights can be annoying. Control of such glare can be achieved by placing buffer plantings between the light source and the room. Place them close to the house to be most effective.

A secondary source of glare, such as reflected sunlight from pavements, can be reduced by plants too. Plants, even in tubs or containers, can be used to visually "break up" or soften a reflective surface. The light is reflected to the plant, which softens it by diffraction before it reaches the viewers' eyes.

Finally, erosion control is an important role for plants. Trees, shrubs, vines, ground covers and grasses help prevent serious erosion by intercepting and breaking the impact of raindrops and by holding the soil in place with their roots.

(Prepared by Diane Relf, Extension Specialist, Environmental Horticulture, Virginia Tech, Blacksburg, VA 24061-0327.)