Auxins are plant hormones that can stimulate or inhibit growth. The most well-known naturally occurring auxin is indol-3-acetic acid (IAA) . Plants contain varying a mount of IAA and those with high concentrations produce roots more quickly and easily than plants with low concentrations of auxin. Although cuttings of some plants will produce roots with just the naturally contained auxins they contain, many will not. In these cases, a synthetic auxin can be used to stimulate root growth.
There are two main synthetic auxin rooting hormones, indolbutyric acid (IBA) and napthale-neacetic acid (NAA). Synthetic auxins come in several forms including powder, liquid and gel. The powdered form is the cheapest and easiest to use as well as being the most widely available. One end of the stem cutting is dipped into the powder (a combination of auxin and talcum powder), tapped off, and put into a hole in the planting medium. The powdered form, however, the least effective because an excess of hormone can inhibit root growth, cause leaf yellowing, and lead to damping off. Liquid forms are a mixture of auxin and alcohol or water. They are easy to use and involve either sticking the stem into the liquid for a few seconds before sticking them in the planting medium or spraying the foliage of a group of cuttings that are already in the planting medium. The former method of application may result in both excessive levels of hormone application as evaporation occurs during the dipping process, and bacterial contamination of the hormone liquid by successive dippings. The spraying method of application minimizes both of these risks. The gel form of synthetic auxin is considered the best and involves painting the gel on the end of each cutting before sticking it in the planting medium.