Aeroponic methods are commonly used in vegetative propagation (a.k.a. cloning) by hobbyist and professional gardeners alike.
Vegetative propagation is an asexual form of plant reproduction. A section of stem that includes a growth tip is separated from a donor "mother plant" and induced to produce roots. This effectively allows parts of the same plant to be grown in different locations. Another way of looking at it is that the cutting will effectively be a genetic copy of an original plant, which is why they are sometimes referred to as clones. Some plants have specialized in exploiting this advantage of this naturally. For example, the spider plant (Chlorophytum comosum) is a common houseplant that sends out inflorescences (flower clusters on a stem) that include little plantlets (clones) that can take root once in contact with surrounding soil, eventually becoming independent plants.
Roots can be induced to form along stems due to meristem cells. Meristem cells start as undifferentiated cells that can further develop into a variety of specialized cells depending on local need. This is important to note as all meristem cells start out the same no matter where they are located in the plant, a fact which is exploited in rooting cuttings. Growth tips and root tips contain such high concentrations of these cells that they are both referred to as apical meristems. Secondary meristem cells are also found along the stem, trunk, and branches, where while they generally develop to increase girth. Under proper conditions these secondary meristem cells can be induced to instead mature into root cells, forming new root tips.
Fortunately, for many plants this can be as simple as keeping the end of that stem opposite the growth tip moist, in room temperature conditions, and under moderate lighting. There are many ways to go about rooting cuttings. Some of the simpler methods are by placing cuttings directly into a moist growing medium or in a glass of water set on a windowsill.
As an improvement to the simple glass of water method, an air-stone can be connected to an air pump and added to the water. This adds air to the water, helping it stay oxygenated and preventing it from going stale. While any portion submerged would be better described as a deep water culture (DWC) the above water stem that is exposed to the water droplets propelled by the popping of the air bubbles benefits from an aeroponic environment. An aerated system such as this tends to perform better and have a higher success rate than a simple glass of water under the same conditions.
To take this same method a step further, an air-lift driven sprayer can be used to concentrate and raise the popping bubble effect, allowing the ends of plant stems to stay at a height above the waterline. Since at this point as far as the plant is concerned, it has stopped being air pumped into the water and become water pumped into the air, it can be considered an aeroponic system. At least until the roots grow to the waterline. Almost all aeroponic systems eventually become hybrid aeroponic-DWC systems at some point, as the roots will tend to grow down far enough to reach the nutrient solution.
Air lifts function because air bubbles will rise in pipe, and water trapped between air bubbles will rise as well. This principle is commonly illustrated in undergravel aquarium filters. Air is released at the bottom of a riser tube. As the air bubbles rise, they carry some of the water along to be returned near the top of the tank. Water from the tank passes through the gravel bottom to replace the water lifted out of the tube by the air bubbles.
Air lift sprayers use a similar method. Air is pumped through an airline into the sprayer riser near the bottom. Nutrient solution enters through a hole in the tube and is lifted along with the rising air bubbles. At the top end of the tube, the bubbles burst and spray the accompanying nutrient solution. Plant stem ends are suspended within range of the spray, allowing them to stay moist.
To prevent the spray from making a mess of the surrounding area, a lid with holes cut for plants covers the reservoir and sprayer. The bottom end of the stem is suspended below the lid (but above the waterline) by a collar or net pot, with the rest of the cutting above where it can receive moderate light. Cool (but not cold) room temperatures are commonly considered ideal.
A small scale propagator can be made from a small bucket, a matching lid with cloning collar sized holes cut in it, collars or net pots, an airpump, airline, and a sprayer. This style of cloner can be undertaken as a DIY project, or purchased in parts, or as a complete unit.
This type of aeroponic cloning has several advantages. As with most aeroponic systems it is practically impossible to over water the cuttings. Since it uses an external air pump, the motor and most of the accompanying waste heat can be kept away from the nutrient solution. Air driven sprayers can have much larger apertures than forced water, and as such are much less prone to clogging. Since the sprayer itself has no moving parts, as long as there is an air supply there is little in the way of failure potential unless it is somehow physically moved, broken or the openings become completely engulfed with roots.
Aeroponic cloning is one of the easier and more foolproof ways to root cuttings. For the impatient or concerned, progress and health of the roots can be monitored simply by lifting the cloner lid for a look. In smaller systems water changes can be as simple as dumping out and refilling a small bucket. It isn"t uncommon for users of aeroponic cloning techniques to report root development a few to several days faster than with older traditional methods. The same concepts can be applied to both small and large scale applications.