A close relative of the mahogany, the neem tree (Azadirachta indica) is a native of the evergreen lowland forests of southeastern Asia. Tolerant of tropical heat and seasonal drought, and immune to insect pests, the neem tree grows quickly and also can live where ocean salt spray fills the air. It is prized as a shade tree across the tropical world where water or fertile soils may be lacking, such as on the margins of deserts, according to Margaret Barwick, author of "Tropical and Subtropical Trees."
An evergreen, the neem tree grows 50 to 70 feet tall. Its thick, singular trunk with smooth, gray-tan bark supports a widely spreading branch canopy that is 60 to 75 feet wide. If shaded, when closely spaced in a grove or forest, the trees become as tall as 90 to 100 feet but significantly narrower in width.
Neem tree leaves are compound and feather-like, each measuring up to 18 inches long. The leaf comprises 10 to 12 pointy leaflets with jagged edges. The tips of the leaflets often curve slightly. In spring, airy sprays of tiny, white and fragrant flowers appear on branch tips. Honeybees flock to the flowers for their nectar. The fruits that form are botanically called a "drupe" and are inflated, fleshy and bitter-tasting. They protect a very hard seed in the center.
All parts of the need tree contain a compound called azadirachtin. This naturally occurring chemical causes sterility in insects. In fact, up to 200 different insect species can be killed by ingestion of the compound according to "Tropical and Subtropical Trees." Since azadirachtin is non-toxic to humans, neem oil is widely used as a biological pesticide.
Plant neem trees in subtropical or tropical regions where only light winter frosts might occur. This corresponds to U.S. Department of Agriculture hardiness zones 9 and warmer. Sow seeds or plant saplings in any well-draining soil, but a moist soil will result in even faster growth and larger mature size.
In some regions, the neem tree is considered noxious or an invasive biological species since it produces many seeds that are eaten and spread by birds and bats. The germinating seeds survive in so many different soil and moisture regimens that they readily out-compete local native plants. While the chemical azadirachtin is not toxic to humans, Peter Garnham wrote in a 2010 "Horticulture" magazine article that pregnant, nursing women and young children should avoid contact with the plant altogether. Moreover, neem oil as a pesticide is non-selective, meaning that it will kill all insects (bad ones as well as beneficial ones, like honeybees) that eat plant parts treated with neem (including pollen and nectar).