Sub-mist Is an Effective Alternative to Overhead Mist for the Propagation of Coleus By Stem Cuttings

Overhead mist facilitates the propagation of tender cuttings from a variety of taxa by preventing transpirational water loss. Despite its success, drawbacks to overhead mist include the application of large volumes of water, potentially unsanitary conditions, irregular misting coverage, and leaching of foliar nutrients. We explored the feasibility of three alternatives to overhead mist that might avoid these problems by applying moisture exclusively from below. These included 1) a sub-mist aeroponic system configured to provide intermittent mist only to the rooting zone, 2) a sub-irrigation system that provided water via capillary action through perlite from a reservoir maintained below the base of each cutting, and 3) a sub-fog aeroponic system (Nutramist) that was configured to provide constant fog only to the rooting zone. To initiate each system, we wetted perlite or filled reservoirs using either water or quarter-strength Hoagland solution. Each combination of system and fertilizer was replicated three times. A total of 240 herbaceous stem cuttings of coleus (Solenostemon scutellarioides ‘Wizard Mix’) were inserted into the systems for 21 days with 50% shading. Cuttings in the sub-mist system produced measures of rooting superior to those in the other systems. They produced more than three times as many roots as cuttings in the overhead mist system, with roots more than six times the length. Root dry weights averaged 28 mg for cuttings in the sub-mist system, compared with only 3.5 mg among cuttings receiving overhead mist. The sub-fog and sub-irrigation systems produced results broadly comparable to the overhead mist. Fertilizer did not consistently improve rooting measures across the systems. Although we observed few root hairs on cuttings rooted using sub-mist, they transplanted well into a soilless substrate and quickly produced new root growth.  The sub-mist system used less than one-fifth the water used by the sub-irrigation system, and less than one-fiftieth the water used by the sub-fog system. In comparison, a single overhead mist nozzle operating for 10 seconds released approximately one-third of the total water lost through transpiration from each sub-mist system over the entire experiment. Our results show that sub-mist systems merit further evaluation for propagation of plants by leafy stem cuttings. Potential advantages of propagation by sub-mist include low water usage, high sanitation, low potential for nutrient leaching, fast rooting, and rapid and non-invasive evaluation of rooting. The reason for improved rooting in the sub-mist system is unclear, and warrants further investigation.