Breeding of frosthardy rhododendrons

Abstract

In 1973 a Rhododendron breeding program was started at the Department of Plant Breeding, University of Helsinki. The program was based on the extremely hardy rhododendron material that had been naturally selected at Arboretum Mustila. The aim was to create new frosthardy cultivars that could tolerate minimum temperatures below —35°C in mid-winter. Rhododendron brachycarpum subsp. tigerstedtii Nitz. was used as the maternal parent in the majority of crosses. Also R. smirnowii and R. catawbiense were used as sources for winter hardiness. Totally 148 different combinations were done between species, species and hybrids and between hybrids. The total number of matings was 496. Over 20 000 seedlings were obtained and a total of 13 752 plants were planted in field trials on eight different test sites between 1975—1979. After two extremely cold winters, 1984/85 and 86/87, about 60 % of the plants died or were severely damaged. R. brachycarpum subsp. tigerstedtii crosses with either R. smirnowii or R. calawbiense gave the best genetic material for better climatic adaptation. The progenies of these crosses survived without any severe damage when minimum temperature was as low as 37°C. Tissue culture methods were developed and used in vegetative propagation of selected ortets. By the end of 1987 six clones have been released as new cultivars for commercial propagation.