Rooting hormones are substances designed to stimulate the growth of new roots on plant cuttings. These substances, which can be either synthetic or natural, mimic the plant’s own growth regulators known as auxins. Applying a rooting hormone to the base of a cutting increases the success rate of propagation. Selecting a good rooting hormone depends on understanding its chemical components, physical form, and the specific needs of the plant material being used.
Primary Ingredients and Commercial Forms
The effectiveness of a commercial rooting hormone is primarily due to two synthetic auxins: Indole-3-butyric acid (IBA) and Naphthaleneacetic acid (NAA). Both chemicals mimic the function of natural auxins, promoting cell division and differentiation at the cut site to form root primordia. IBA is widely used and is known for its reliability in inducing root initiation, particularly in semi-hardwood cuttings. NAA is generally considered a more potent and persistent auxin, meaning its effects can be longer-lasting, which may be advantageous for species that are typically difficult to root. Many commercial products successfully utilize a blend of IBA and NAA to achieve a synergistic effect, enhancing the speed and quality of root development across various plant types.
Commercial rooting hormones are available in three main physical forms: powder, liquid concentrate, and gel. Powdered hormones, often mixed with talc, are the most common and offer an excellent shelf life and ease of use for hobbyists. Their dry nature reduces the risk of disease transmission compared to liquid options, but they may offer less uniform coating and lower absorption into the cutting tissue. Liquid concentrates provide the advantage of customizability, allowing the user to dilute the solution to the precise strength needed for a specific plant type. However, if mixed incorrectly, a liquid can cause root burn, and the mixed solution has a shorter useful life.
Rooting gels offer a middle ground, combining the ease of application of a powder with the coating consistency of a liquid. The gel adheres well to the cutting and is less messy than powder, which can be prone to creating airborne dust. A potential drawback of gels is that a thick coating may sometimes impede oxygen access to the stem, which is necessary for root initiation, and they are generally more expensive than powder or liquid forms. Gel formulations also tend to have a shorter shelf life than powders.
Matching Hormone Strength to Plant Type
Rooting hormone strength is measured in parts per million (ppm), and selecting the correct concentration is the most important factor for successful propagation. Too weak a concentration will have little effect, while a concentration that is too high can inhibit bud growth, cause leaf yellowing, or even kill the cutting. The ideal ppm level is directly correlated with the maturity and woodiness of the plant material being propagated.
Softwood and herbaceous cuttings, which are young and flexible, require the lowest concentration, typically ranging from 500 to 1,500 ppm. This mild treatment is sufficient to encourage rooting in easily propagated plants without causing damage to their sensitive tissue. Semi-hardwood cuttings, taken from slightly matured growth that is firm but still somewhat flexible, respond well to medium concentrations. These levels often fall between 1,000 and 3,000 ppm.
Hardwood and difficult-to-root cuttings, which are mature, woody, and firm, demand the highest concentrations to successfully induce root formation. For these types, treatments often range from 3,000 ppm up to 10,000 ppm.
Step-by-Step Application Methods
Proper application of the rooting hormone begins with preparing the plant cutting to maximize its absorption potential. A healthy, non-flowering stem should be selected and cut cleanly just below a node, which is a point on the stem where a leaf or bud is attached and where auxins naturally concentrate. All leaves on the lower half of the cutting should be removed to prevent them from rotting once planted, and any large remaining leaves can be cut in half to reduce moisture loss.
For all hormone forms, pour a small amount of the product into a separate, sterile container before use. This prevents contamination of the main supply by avoiding the introduction of bacteria or fungal pathogens from the cutting.
When using powder, the base of the cutting should first be moistened, often by dipping it in water, so the powder adheres correctly. The base is then dipped into the powder, and the excess is gently tapped off, aiming for a thin, even coating.
If using a gel, the cutting’s base is simply dipped into the separate container of gel, ensuring only the bottom quarter-inch is coated before immediately planting. Liquid concentrates require the cutting to be dipped or soaked for a few seconds up to several minutes, depending on the concentration and the plant material. Following application of any hormone, the cutting should be immediately inserted into a suitable, moist rooting medium, such as perlite or a soilless mix. The cutting should be kept in a warm, humid environment and out of direct sunlight until new root growth is established.