Neem (Azadirachta indica) is a tropical tree of many traditional uses and the source of neem oil, a natural pest repellent and ingredient in cosmetics. It can be grown in U.S. Department of Agriculture plant hardiness zones 10 through 12. Neem trees spread prolifically by seed, making it very easy to propagate, though it is considered invasive in some areas for this reason.
To grow neem by seed, first remove the seeds from the flesh of the fruit and wash them clean. Seeds can be dried in the shade for up to a week before planting. Plant neem seeds 1/2-inch deep in pots or directly in the ground. The seeds germinate readily in one to two weeks in warm soil. Outside of tropical climates, plant neem seeds in pots inside a greenhouse.
Neem seed does not keep well and generally must be planted fresh. If there is no seed available, stem cuttings are another option to propagate the plant. Neem cuttings root easily if they they are maintained in a warm, humid environment with filtered sunlight. Cuttings should be about 4 to 6 inches long and the diameter of a pencil. Remove all but the top two leaves from each cutting -- and cut these two leaves in half to minimize moisture loss while the cutting is forming roots. Stick the bottom half of the cutting into a sterilized growing medium such as perlite, and allow several weeks for the roots to begin to form.
Air layering stimulates the growth of roots from a stem while it is still attached to the parent plant. Choose a vigorous shoot between 3/8- and 5/8-inch thick. Cut through the outer bark layer around the entire circumference of the stem in two places about 1 inch apart. Make a vertical cut that connects the first two cuts and remove the bark from this area. Pack moist sphagnum moss around the exposed stem and secure with a plastic bag to seal in the moisture. Once roots have formed, cut the stem just below the roots and transplant. At the same time, cut the top of the stem back to where there are just a few leaves on the part with the roots.
Keep seedlings and rooted cuttings in partial shade until they are one or two months old. Slowly expose to full sun and wait to plant in permanent locations until the trees are at least 3 feet tall, usually after one year of nursery growth. Maintain the planting area weed-free, as neem saplings are vulnerable to grass competition. Young neem trees typically begin to flower and produce fruit after four or five years of growth.