https://aggie-horticulture.tamu.edu/earthkind/landscape/air-layering/

Texas A&M Extension

Air layering is a useful method of producing roots on the stem of indoor landscape plants that have become “leggy” through the loss of their lower foliage.

This method, believed to have been developed centuries ago by the Chinese, has been used successfully as a mean of propagating some of the more difficult-to-root plants. Because it required excessive care and patience, air layering was used only by the highly trained plantsman.

The procedure was to wound the stem or branch of a plant and enclose the wounded stem with moist sphagnum moss or similar rooting medium until roots develop from the wounded area. Success was dependent upon the ability of the propagator to keep the rooting medium moist until the roots were formed and large enough to support the new plant. Only since the development of polyethylene film has air layering become a practical method of propagation for the home gardener and amateur horticulturist.

Air layering seldom is used on plants that root easily by other less complicated methods, but it is useful for rooting ornamental plants such as ornamental figs, dieffenbachia, croton and others of a herbaceous nature. Woody plants frequently propagated in this manner include magnolia, holly, camelia, azalea and many of the fruit and nut bearing plants such as citrus, apple, pears and pecans.

For optimum rooting make air layers in the spring on shoots produced during the previous season or in mid-summer on mature shoots from the current season’s growth. On woody plants, stems of pencil size or larger are best. The stem may be much thicker on the more herbaceous plants.