If you are trying to propagate a plant through stem cuttings or repairing damage to a plant's root ball, applying rooting hormone can be helpful. Rooting hormones are chemicals that stimulate a plant into developing new roots. Some of these chemicals are found in nature, others can be made in your own home.
Rooting hormone is most commonly used for transplants taken from cuttings. It works equally well with hardwood and softwood cuttings, and you can also use it with hydroponic plants to develop new roots. Gardeners who propagate hard-to-root plants, such as roses, find that rooting hormone stimulates new roots in the woody rose canes. Horticulturalists who transplant root-pruned trees also find rooting hormone helpful for root development.
Rooting hormone is available commercially in most garden centers and nurseries. Commercially sold rooting hormones consist of synthetic plant hormones called auxins. The most common of these is indolebutyric acid (IBA) or naphthaleneacetic acid (NAA). Do-it-yourself gardeners also make a rooting hormone by soaking willow bark in water.
Commercially sold rooting powder is available in various formulations. The weaker formulations are ideal for easy-to-root plants such as hydrangea, while stronger formulations are better for plants such as roses. The powder may also contain a fungicide that prevents damaging fungus or mold from growing on plant cuttings that you are propagating.
To use rooting hormone, tip a little bit of the powder into the lid. Then dip the cut end of the plant into the powder. Alternately, you can dust the powder over the cut surface of your plant. For plants that are already planted in soil, such as trees, simply mix the powder with water and pour it over the root system of your plant.
A gardener should never dip the cut end of their plant directly into the whole container of rooting hormone, as applying too much hormone can cause the tip of the plant to burn and will prevent it from developing roots. Plants may also contain diseases or contaminants. By dipping the plant directly into the hormone container, you run the risk of transmitting contaminants to other plants.