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Cannabis produced through non-feminized seed is dioecious, which means the male and female reproductive systems are found on separate plants. This poses a risk for growers who are producing cannabis for flower or oil production because they only use non-fertilized females. The presence of males requires extensive monitoring and roguing to ensure the females are not pollinated.

One way to overcome this challenge is to produce cannabis transplants from vegetative cuttings (clones). To ensure a uniform rooting and consistency of these cuttings, cultivators need to consider certain environmental and production factors. This includes the use of rooting hormones to speed up the rooting process and ensure the clones are uniformly rooted. The rooting hormone indole-3-butyric acid (IBA) is one such hormone that helps growers produce a more uniform product in less time (Fig. 1).

IBA at a Glance

IBA is available in multiple forms, from a talc powder or gel applied to the basal end of the cuttings as a dip, to overhead spray applications of potassium-based K-IBA. In general, the most commonly observed products used by cannabis growers are the basally applied materials (talc and gels), which are typically 0.3% strength.

Throughout our research work at North Carolina State University and observations of grower operations in North Carolina and surrounding states, we have observed situations where IBA use has resulted in leaf burn or a proliferation of undifferentiated root cells without roots. A closer look at the symptoms of IBA phytotoxicity will aid growers in their ability to identify a problem if it occurs in their operations and serve as a guide to help them pinpoint the optimal rate and product for their needs.

Figure 1. Rooting and plant growth are enhanced with the use of the rooting hormone indole-3-butyric acid (IBA) (right), versus the untreated control (left). Figure 2. Higher concentrations of foliar-applied IBA results in twisting and curling (epinasty) of the growing tips. Figure 3. Excessive concentrations of IBA applied to the root system result in a proliferation of roots along the stem base. Figure 4. Extreme concentrations of IBA applied to the root system result in a proliferation of roots cells along the stem base that fail to develop into functional roots. Figure 5. Below-ground view of the same plant as in Figure 4, in which the development of fully functional roots have stalled. This delay in root development will result in cutting failure. Figure 6. Excessive moisture in the rooting substrate will also result in the lack of root development. These cuttings were being rooted in a float tobacco bed system in which the moisture level was excessive due to the wicking action of the substrate. This mimic can be misdiagnosed as excessive IBA applications.

IBA Toxicity Symptoms

The application of foliar IBA spray can result in the twisting and curling (epinasty) of the growing tip within 48 hours of application (Fig. 2). Cannabis appears to be more sensitive to IBA foliar applications compared to other floriculture species, such as petunias, calibrachoa and fuchsia, which have improved rooting with the application of 200 ppm IBA. Therefore, growers should conduct their own small-scale trial with rates below 100 ppm IBA.

Basal-applied IBA improves root quantity and quality. Commercially formulated products generally contain 0.3% (3,000 ppm) IBA and provide excellent results. Using higher rates than 3,000 ppm does not improve rooting. Cannabis plants respond to higher rates of IBA by developing aerial roots at the plug base and up to the base of the stem (Fig. 3). If the clone is transplanted deep enough in the final container, these roots will continue to develop as the plant grows.

Excessively high IBA rates will have a negative impact on rooting and will produce (undifferentiated) cells that are programmed to become roots but will not fully develop into functional roots (Fig. 4). This overapplication of the rooting hormone will cause the rooting process to be delayed by perpetuating too many immature and undeveloped rooting structures. The result of this overapplication is a mass of undifferentiated cells developing on the stem. Under extreme overdose situations, an inspection of the below-ground portion of the plant may reveal the lack of roots developing (Fig. 5).

Growers should also note that excessively wet rooting substrate (Fig. 6) will also result in a mass of root nodules and incomplete root development. When diagnosing why plants fail to root, cultivators need to be aware of other possible disorders and environmental factors.

Rooting of cannabis is a fairly straightforward procedure for success. The use of IBA helps promote efficient rooting and improve the overall quality of cuttings. If used properly, IBA provides an insurance policy for success.

A little IBA goes a long way, and excessive rates should be avoided for rooting success.