When seeds are unavailable or germination is difficult, new plants can usually be grown from cuttings of mature, healthy plants. This method of starting new plants, called vegetative propagation, guarantees that the new plant is an exact copy of the parent, so the fruit or flower is exactly what you expect. To successfully propagate plants from cuttings, a rooting hormone is often required to encourage new roots to grow.
Plants naturally produce their own rooting hormones called auxins. Auxins allow plant cells to elongate, forming new parts or enlarging existing parts. Auxins have different effects at different stages of growth, but when applied to cuttings they encourage new roots to form where the plant cells were disrupted or cut. The dominant naturally produced auxin is indole-3-acetic acid, or IAA. As early as 1932, it was known that two synthetic auxins, indole-3-butyric acid, or IBA, and napthaleneacetic acid, NAA, encouraged more root growth than the natural IAA.
IBA and NAA are the most commonly available synthetic auxins for promoting root growth. Most commercially available powder root hormone products are made from one of these two synthetic auxins diluted in a base of talc to ensure that too much hormone is not put on the plant in a single application. These powders can cause minor irritation to the eyes and skin, so always wear gloves when handling the powder to stay safe and store the product out of the reach of children. There is some disagreement over how long these products are effective.
There are three main causes for breakdown of rooting hormones: microbial destruction, ultraviolet wavelengths in sunlight and enzyme destruction. Most of the known sensitivities of auxins have only been measured with the natural auxin, IAA, which is quickly broken down by microbes and UV wavelengths. However, IBA and NAA both appear to be light stable, and IBA is not broken down by the same microbe known to affect IAA. Similarly, the enzyme that breaks down IAA has no apparent effect on IBA or NAA. Some manufacturers recommend refrigeration of IBA to prolong its shelf life, but a 1988 article in the Journal of Horticulture showed that, at least for six months, temperature did not affect IBA.
In spite of these apparent inconsistencies, the U.S. Forest Service has published that most auxins have a shelf-life of 18 to 24 months. The listed shelf life on commercial products varies from two years to five years. Synthetic auxins have been observed to be stable up to two years, but after that time their effectiveness is unknown. The best way to find out if your rooting hormone has expired is to buy a new batch and compare it to the old batch in a head-to-head test of effectiveness.