Bats are insect eaters with large appetites, and when they take up residence in a garden, they can help reduce the garden's population of mosquitoes and other insect pests. Bats feed at night, consuming 60 to 1,000 insects per evening, depending on the size of their prey. They roost during the day, sleeping and raising their young in secluded cracks and crevices. To attract them, you'll need to provide them with food, water and shelter. If you do that, the bats will do the rest.
Bats feed on insects that are active at night, so a garden designed to encourage nighttime insect activity is more likely to attract them. Choose plants for the garden that bloom at night or that have a pronounced nighttime fragrance. One such plant is evening primrose (Oenothera macrocarpa), which is hardy in U.S. Department of Agriculture plant hardiness zones 3 to 7. Another is moonflower (Ipomoea alba), a night-blooming vine that produces 6-inch-wide white flowers. It is hardy in USDA zones 10 to 12.
Like all living creatures, bats need access to a water supply. If you live within one-quarter mile of a natural water body, you need not add water to your garden. If you live farther than that, however, you'll need to add a water feature to attract bats. A birdbath or small fountain will both do the trick. Keep the water fresh, both for the health of the bats and to avoid mosquitoes, which breed in stagnant water. A fountain works best, because it keeps the water moving, but a birdbath will do if it's cleaned and refilled often.
Bat houses are small boxes that provide shelter to roosting bats during the day. Bats roost most readily in houses with tall but narrow cavities, with openings in the bottom through which they enter. You can purchase or build a bat house. Ideal designs are 12 inches wide, 3 inches deep and 24 inches high. The house's joints should be caulked to protect the interior from the weather, and it should have a vent above the entrance to allow air circulation.
Place bat houses up high, at least 15 feet from the ground, on a pole or the side of a building where bats can fly to it easily. To maintain the optimal temperature for bats (85 to 100 degrees Fahrenheit), houses are best placed in south-facing locations that get six to eight hours of sun exposure during the day. Building walls absorb heat during the day and radiate it to the bat house at night, so bat houses mounted on walls tend to stay warmer than those mounted on poles. Painting the exterior black can also help your bat house absorb and retain heat during the day.