Exogenously applied rooting hormones can increase rooting percentage and uniformity of difficult or slow-to-root genera.
by Roberto G. Lopez - email@example.com
As shipments of unrooted cuttings (URCs) will be arriving soon, it is important to review the list of genera that can sometimes be problematic for propagators. This can include those that are difficult or slow-to-root, are prone to lower and upper leaf yellowing and senescence, and have high losses. Prioritizing sticking of these problematic crops is one step to prevent losses (For more information on avoiding cutting losses view e-GRO Alert 3:14; http://e-gro.org/pdf/2018_712.pdf and table 1). Lower leaf yellowing is most often a sign of ethylene or temperature stress and/or low carbohydrate levels during shipping and storage (For more information on how to reduce (geranium lower-leaf yellowing view e-GRO Alert 3:14; http://e-gro.org/pdf/3-14.pdf) and upper leaf yellowing is a symptom of micronutrient deficiencies during propagation. While you have no control of what may have occurred during shipping, you can increase uniformity and speed of rooting with the use of rooting hormones.
Figure 1. Dahlia a first priority genera for sticking and benefits from rooting hormones. Lower leaf yellowing and poor rooting can be an indication of ethylene & extreme temperature stress during shipping. This is an example of extreme dahlia URC shrink.
The application of synthetic auxins [indolebutyric acid (IBA) and napthaleneacetic acid (NAA)] rooting hormones are generally not required for the majority of URCs. Although difficult- or slow-to-root genera and cultivars are often treated with basal (stem base quick dips) or foliar rooting hormones to improve the uniformity and speed of root initiation or to improve the rooting percentage. Their use can also improve rooting when environmental conditions and cultural practices are not ideal. Examples can include suboptimal temperatures or light levels or uneven mist.
Figure 2. Thunbergia is a first priority species and should be stuck immediately. It is slow to root, requires a higher mist frequency and rooting hormone.
The recommended concentration for stem and foliar application will vary by species. Dipping the stem of several URCs at a depth of ¼ to ¾ inches into liquid or powder formulation of IBA for 1 to 4 seconds has been the most common approach. Care must be taken to avoid over application or application to the leaves as leaf yellowing, curl or distortions can occur. Powder based IBA products are generally less toxic, reduce the likelihood of spreading disease and are quicker and easier to apply than liquid IBA. Basal application rates of IBA range from 500 to 1,500 ppm.
Today, growers are using a duster (powder) or hand held spray bottle (liquid) to apply rooting hormones to the basal end of the stem.
Figure 3. Rooting of osteospermum cuttings 2 weeks after stick. No rooting hormone (top), URCs were sprayed with 200 ppm of IBA 24 hours after stick (bottom).
A coarse foliar spray of IBA that allows some of the solution to run down the stem of newly stuck cuttings can also promote rooting, while reducing the time and labor required for dipping cuttings and most importantly the potential spread of diseases. Foliar application rates of IBA are generally made 24 to 48 hours after cuttings are stuck and range from 50 to 600 ppm. The potassium-salt formulation of IBA is often used for foliar applications as it is water soluble, and therefore causes less foliar damage compared to alcohol-soluble formulations. Figure 3 shows the results of studies conducted at Michigan State University (MSU) showing how a 200 ppm IBA foliar spray application promoted rooting of osteospermum. Figure shows the results of MSU studies comparing foliar applications of Hortusto a new K-IBA product that will be released by Fine Americas in 2020 to a basal application of Dip n’ Grow.
Figure 4. Root dry mass of scaevola 14 days after stick. Spray treatments were made 1 day after cuttings were stuck and dip treatments occurred at stick.
Table 1 was compiled by Mike Gooder, Plantpeddler and Bob Dickman, Dickman Farms, Harvey Lang, Syngenta, Jason Twaddell, Ball Horticultural Co., Roberto Lopez and Garrett Owen, Michigan State University for a session at Cultivate in 2018. The table categorizes URCs based on their sticking priority, rooting time, dibble size, mist and rooting hormone requirements.