Rooting Hormones or substances that promote the development of roots are used to increase the percentage of rooting of cuttings.
by Ray Ong
Rooting hormones are called “auxins.” Auxins applied to the cut ends of plant tissues stimulate the formation of calluses, which is where the roots grow from. Most plants do need the use of rooting hormones; however, these plants will root faster and more uniformly when treated with auxins. Some auxins that are commonly used in horticultural industries are indolyl acetic acid (IAA), indolyl butryric acid (IBA), alpha-naphthalene acetic acid (aNAA), and alpha naphthalene acetamide (aNAAmide).
Different types of rooting hormones are made with varying concentrations of auxins as active ingredients. The weaker concentrations are used on easy-to-root cuttings, while higher concentrations are used on more hard-to-root mature tissues.
Rooting hormones are usually manufactured in liquid or powder form. The liquid formulations are diluted and cuttings are soaked for a specified time depending on the materials. This technique is useful for homogeneous materials but may cause cross contamination or infestation if materials used are not sanitized. The advantage of using liquid solutions of auxins is that fungicides may be added to the soaking solution. Leftover auxin solution may not be used the next day and must be discarded.
Auxins in talc powder carriers are easier to use. There is no soaking time. The cut ends of the cuttings may be lightly dusted with the rooting powder and excess powder shaken off. Care should be taken that the powdered cut end be placed in prepared holes. The medium is then compacted around the cuttings.
Never try to push cuttings into solid media if rooting powders are used. This will drive the rooting powder to the media level, leaving the cut end stripped of the needed auxin. Rotting usually commences if this happens, or the roots will sprout on the central portion of cuttings instead of the bottom.
Healthy cuttings derived from healthy mother stocks should root easily. If the mother plant shows any symptoms of chlorosis (yellowing of leaves), this should first be corrected before cuttings are harvested.
Since the major cause of the failure of cuttings to strike root is infection, fungicides are usually incorporated into the rooting hormone formulations.
One of the most common problems in the regular extraction of cuttings from mother stocks is that the boron present may drop to critical levels. Continuous extraction of cuttings from the same mother stocks results in boron depletion.
Boron is dubbed the “inorganic rooting hormone.” It is responsible for callus development. A deficiency in boron may affect the efficacy of the rooting hormones. Some rooting hormone manufacturing companies incorporate small amounts of boron into their hormone powder products for this reason.
EXTRAORDINARY USES OF ROOTING HORMONES
Auxins may be applied to healthy plants to induce rooting. But regularly spraying rooting hormones onto healthy plants will result in disaster. The auxins will activate the food reserve of the plants (starches) and convert them into free sugars. High levels of free sugars in the cytoplasm will increase the turgor pressure, posing the threat of cells bursting. The excess sugar is then condensed by the plant to form a purple pigment called anthocyanin to counter the hyperglycemic (high sugar) state.
The auxin will also affect the healthy growing roots. Ironically, while it is a rooting hormone that induces root formation, auxins will stop the roots from growing by heading off the root meristem and making them split or branch to outright root tip destruction.
Plants regularly sprayed with hormones will have massive branching roots and pigmented leaves. The leaves grow consecutively smaller and shorter over time, giving the plant a cretin (dwarf) look. It is not advisable to give plants a regular spray of rooting hormones. Aside from being counterproductive, it increases production costs.
2.4D is a very strong auxin used by farmers to kill unwanted plants or weeds.
Rooting hormones benefit farmers through the production of uniformly rooted cuttings. A single application is usually what is needed, whether it is dipped, sprayed, or dusted.
ROOTING POWDER IN THE MARKET
There are several brands of rooting powders in the horticultural industry. Most brands use indole butyric acid at different levels: 0.1% IBA for soft cuttings like begonia, huperzia, saintpaulia, and gloxinia; 0.3% IBA for semi-hard cuttings like hydrangea, hibiscus, and roses; and 0.8% for woody cuttings like salix, mellaleuca, and hibiscus stems. Most rooting powder formulations include a very small amount of boric acid and a general fungicide (usually for water molds and other damp-off organisms).
This story appeared in Agriculture Monthly’s March 2017 issue.