Humic Acid Does Not Promote Rooting of Vegetative Cuttings of Rhododendron
Michael R. Evans
Department of Horticulture
Iowa State University, Ames, Iowa

Following is a summary of research done by Michael R. Evans and William R. Graves at Iowa State University.

One of the most common responses of plants to humic acids is an increase in root growth. Increased root growth has been reported in vegetative cuttings of Pelargonium x hortorum , Lagerstromia indica and Euphorbia pulcherrima when cuttings were rooted in the presence of humic acids. The objective of this research was to determine whether humic acid treatments could improve rooting of vegetative cuttings of Rhododendron .

In May 1998, 7.5 cm softwood vegetative cuttings of Rhododendron PJM Group and the Northern Lights azalea hybrids 'White Lights' and 'Rosy Lights'* were stuck into subirrigation trays filled with perlite. The reservoirs of the subirrigation trays were filled with deionized water, or solutions containing 2500 mg.L ^-1 humic acid, 5000 mg.L ^-1 humic acid, 2500 mg.L ^-1 nutrient control solution or a 5000 mg.L ^-1 nutrient control solution. The nutrient control solutions were required because commercial humic acid contains mineral element residues. The pH of all solutions was adjusted to 6.5. After sticking, cuttings were placed under shade. No overhead mist was used. After 3 months the experiment was terminated.

Regardless of the solution used to fill the subirrigation reservoirs, none of the cuttings formed roots. Many of the cuttings had become necrotic and died by the termination of the experiment. Mortality rates ranged from 30% to 40% in the deionized water. Mortality rates ranged from 40% to 60% in humic acid solutions and 80% to 100% in the nutrient control solutions.

On June 20, 1999, 7.5 cm cuttings of Rhododendron 'Rosy Lights' and PJM Group were taken, and the leaves on the lower inch of the cuttings were removed. The cuttings were subjected to one of eight rooting treatments. The treatments consisted of a combination of root medium and cutting treatments. The root medium was either unamended perlite or perlite amended with humic acid at a rate of 10% by weight. Cuttings were either stuck directly with no treatment (control), or the lower inch was dipped into humic acid powder, Hormodin or a 50:50 mixture of humic acid powder and Hormodin. Cuttings were placed under intermittent mist. After 4 months, the experiment was terminated.

For 'Rosy Lights', 30% to 70% of the cuttings formed callus. The only significant difference among the treatments was for cuttings rooted in a medium of perlite amended with humic acid and dipped in humic acid. Only 30% of these cuttings formed callus. None of the 'Rosy Lights' cuttings formed roots.

For PJM Group, 0% to 90% of the cuttings formed callus. None of the cuttings placed in the rooting medium amended with humic acid formed callus. By contrast, 90% of the cuttings placed in unamended perlite and dipped in Hormodin formed callus. Only those cuttings placed in unamended perlite and dipped in Hormodin formed roots. For this treatment, 70% of the cuttings formed roots and the average fresh root weight per cutting was 62 mg. Cuttings dipped in Hormodin and placed in a medium amended with humic acid or dipped in a combination of humic acid and Hormodin failed to form roots.

* Name is unregistered.

The current address of Michael R. Evans is: Department of Horticulture, University of Arkansas, Fayetteville, AR.