Root developent from different application rates.
Photo: Christoper Currey

The ability to propagate plants by rooting cuttings is essential for greenhouse crop production. There are several environmental and cultural factors affecting the efficiency of cutting propagation. However, it is fair to say one of the most important tools for propagating cuttings is rooting hormone. Rooting hormone is a colloquial term for the plant hormone auxin. When applied to cuttings, auxin promotes adventitious root formation.

The most common method of applying auxin-based rooting hormones to cuttings is through dipping cuttings. The most common method of dipping cutting is to place the cut bottom, or basal, end of cutting into the rooting hormone. Formulations for rooting hormones used in dips will be either a powder, with the auxin suspended in talcum powder, or a liquid, with the auxins suspended in alcohol. Regardless of whether rooting hormones are in powder or liquid, the auxin(s) used are most commonly indole-butyric acid (IBA) and/or naphthalene-acetic acid (NAA). Cuttings can be individually dipped into the rooting hormone; however, to save time and improve efficiency, cuttings can be bundled in bunches so numerous cuttings may be treated within a single dip. After dipping the cuttings, they are placed or “stuck” into substrate and placed into the propagation environment.

While dipping cuttings into rooting hormones has been the long-standing approach to applying auxin to promote root formation, new auxin formulations allow for different applications. As previously mentioned, suspending IBA and/or NAA in alcohol is one of the common auxin formulations used for dipping cuttings. However, the development of IBA products intended for mixing liquid solutions that can be applied as foliar sprays to cuttings is making it easier to promote adventitious root formation.

There are currently two different rooting hormone products on the market intended to be applied as a foliar spray to cuttings. The first is IBA Water Soluble Salts by Hortus. This is a granular product formulated with potassium salt of IBA. It is simply mixed with water to the desired concentration, then can be used on cuttings. The second, and newest, rooting hormone product is Advocate by Fine Americas. Advocate also contains 20% IBA but, unlike the Hortus product, it is a liquid formulation. Why would you spray auxin on the leaves if you want to promote rooting at the base of the cutting? In plants, auxin is formed in young leaves and transported downward- or basipetally- to the base of the cutting, where it promotes adventitious root formation. But aside from the fact that spraying these auxin formulations onto leaves is effective in promoting roots, it can be a major labor-saver.

When applying foliar sprays of plant growth regulators, the most common application volume is two quarts of solution applied over 100 square feet of growing area; the same 2 qt./100 ft2 is also a useful volume for applying solutions with these auxins to cuttings. While applying some products, such as BA+GA4+7 (Fascination or Fresco) to suppress lower-leaf yellowing in geranium, it is important to avoid runoff reaching the base of the cutting to avoid adverse effects. There is no similar cause for concern with solution runoff with these foliar auxin formulations used in propagation. In fact, applying enough auxin solution to cause runoff down the stem can further promote rooting, as it delivers more hormone directly to the base of the cutting; increasing solution volume can be used to further promote rooting.

An important consideration for using Advocate of IBA Water Soluble Salts is the concentration. When using foliar auxin sprays, be sure to adjust concentrations to suit this application method. The concentrations used for dip applications are generally higher and not appropriate for foliar sprays. For herbaceous annuals and easier perennials, 80 to 400 ppm is an effective range, while woody plants may require up to 1,500 ppm. Foliar sprays are a newer application method, so in-house trials will be essential for determining the concentrations required for the crops you are propagating.

Give these auxin formulations labeled for spraying a try. They can promote rooting, while saving labor compared to traditional rooting hormone application techniques. With some trialing including different species, concentrations, and application volumes, foliar auxin sprays may become another useful tool in your toolbox.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu