Effects of Basal Temperature on the Rooting
of Hardy Hybrid Rhododendrons
D. N. Whalley and K. Loach
Glasshouse Crops Research Institute, Rustington, Littlehampton, Sussex
In: Scientia Horticulturae, 6 (1977):83-89
Reprinted from Ornamentals Northwest, Dr. James Green, editor
"The effects of 3 basal temperatures (15° C, 20° C, and 25° C) on rooting of the hardy hybrid Rhododendron cultivars 'Pink Pearl', 'Mrs. R. S. Holford' and 'Fastuosum Fiore Pleno' have been studied.
"A temperature of 15° C gave least rotting in all 3 cultivars and therefore more cuttings survived to root. Rotting increased and rooting (survival) decreased with increasing temperature.
"In cuttings without evident rotting, 25° C gave better rooting than 15°C, indicating that with improved control of disease, propagation at the higher temperature may be beneficial.
"The ease of rooting in un-rotted cuttings was similar in R. 'Fastuosum Fiore Pleno' and R. 'Pink Pearl' but lower in R. 'Mrs. R. S. Holford'. Rotting was greatest in R. 'Mrs. R. S. Holford' and least in R. 'Fastuosum Fiore Pleno." It's kind of like "Mary sells seashells by the seashore," but it summarizes to the fact that where the least rotting occurred, the greatest rooting occurred at 25° C! Disease played a significant role in determining success of rooting in this experiment; optimal basal temperature is therefore difficult to specify. It will depend upon the degree of infection and the extent to which the sanitary measures employed control the pathogens. In the present experiment, treatment with Captan (1.5 g/l of Orthocide 75% WP drenched over the cuttings immediately after insertion and similarly applied at two-week intervals) did not provide adequate control of the rotting. "THE NEED FOR IMPROVED FUNGICIDAL TREATMENTS, BOTH IN THE PROPAGATION BED AND POSSIBLY ON THE STOCK PLANTS, IS INDICATED."
Another two points of interest from the report are: 1) the rooting medium consisted of equal parts of peat and grit, and 2) the cuttings were not rooted under the traditional misting system, but rather were covered with 150 gauge (38 microns) clear polyethylene sheeting.
In summary, a temperature of 21°C (70° F) is generally used commercially, and it appears that with adequate disease control, this temperature is probably near optimum.