Rhododendron Breeding In The Netherlands
Instituut voor de Veredeling van Tuinbouwgewassen, Wageningen
Reprinted, with permission, from Proceedings of the XVIIth International Horticultural Congress,
College Park, Maryland
Breeding work on the genus Rhododendron was started in England round about 1800. It was not until 1860 that Dutch breeders showed active interest in this matter. The first Dutch hybrid was shown by Koster, in 1880, under the name of 'Standard van Boskoop'.
Not until the beginning of this century did breeding work develop further and it was breeders like Koster, Van Nes, Endtz, Den Ouden, Frets, Felix and Kluis, who left us a stock of new hybrids which are still currently grown in and outside our country. Among the many that are grown I may mention 'Queen Wilhelmina', 'Britannia' and 'Earl of Athlone', raised by Van Ness; 'Dr. H. C. Dresselhuys' raised by Den Ouden; 'Madame de Bruin', 'Mrs. Lindsay Smith' and 'America', raised by Koster; 'Dr. Arnold W. Endtz', raised by Endtz; 'Kluis Sensation', raised by Kluis, etc.
Breeding work on a crop like rhododendrons seldom brings the breeder direct profit nor did the Boskoop breeder aim at making profit in the first place. He did this work as a hobby and it was therefore popularly indicated by the nursery-men as "the hobby". After the Second World War this hobby became ever more expensive, as it occupied much valuable soil and the cost of the labor involved became higher and higher.
Moreover, the non-scientifically trained breeders experienced increasing difficulties in solving the problems of breeding which was a serious handicap in obtaining striking results. These are probably the main reasons why the majority of private nurserymen have discontinued their breeding work. This decline prompted the Experimental Station for Arboriculture at Boskoop - actually an association of nurserymen - to pursue the breeding work, in conjunction with the Institute of Horticultural Plant Breeding at Wageningen.
In principle, breeding at the Experimental Station concerns all woody crops. Because of its acid peat soils. however, the Boskoop area lends itself admirably to the growing of rhododendrons, and consequently this genus was also given an important place in breeding.
The main objective in rhododendron breeding is the creation of dwarf and semi-dwarf hybrids with red, yellow or orange flowers and combining these non-blue flower colors with an appreciably greater winter hardiness than normally possessed by these colour types. It has already been found that the linkage between blue flower colour and great winter hardiness can he broken in longer-term and consistently executed breeding programs.
To achieve this, preferably hardy garden hybrids were crossed with species such as R. dichroanthum, R. forrestii repens, R. griersonianum and R. williamsianum. Also, many crosses with hybrids of these species were made. These hybrids were mainly obtained from England and Germany, where much crossing with the above species was already done before World War II. In England, however, the choice of the crossing parent was generally determined by the desire to retain as much as possible the characters of the basic species. The English R. williamsianum hybrids closely resemble R. williamsianum, unfortunately also with respect to the less desirable characters, such as low floriferousness in the juvenile phase and decreased cold resistance. With the Germans (probably on an average more business and less botanically minded) it was just the other way round; selection there was done for multi-floriferous, quick growing and hardy hybrids.
Hence in the German williamsianum hybrids we do see an abundance of flowers and hardiness, but no longer the nice low globular shape and the round little leaf of R. williamsianum. Our breeding program is aimed at obtaining a hybrid that is intermediate between these two extremes. The result of this work is illustrated by the williamsianum hybrids 'Karin' and 'Andy' with their dwarf growth and abundant flowers.
Another important objective is the development of the groups of viscosum hybrids. Attempts are being made to combine the pleasant fragrance of R. viscosum with the varying flower colour and larger flowers of Mollis azaleas and Knaphill hybrids by crossing. From the hybrids of R. viscosum and Mollis azalea 'Koster's Brilliant Red' some attractively flowered and very fragrant cultivars have already been selected.
Attempts to incorporate the yellow flower colour into the group of Japanese azaleas have so far failed, due to the occurrence of apomixes in all crosses of yellow-flowered mollis or pontica azaleas with white-flowered Japanese azaleas, and that in the form of induced apomixes.
Incompatibility, too, is an obstacle which occurs in many places in the genus Rhododendron. It appears that limitations by incompatibility phenomena frequently coincide with the borderlines between the various subgenera. Research into the nature of the incompatibility between the elepidote species. R. williamsianum and the lepidote R. impeditum is in progress. It seems possible to counteract the effect of incompatibility to a large extent by applying embryologic breeding methods of the germ. Where breeding is not hindered by incompatibility phenomena, failure may often he caused by sterility of the F1. This is known to occur in intercrossing rhododendrons of the subgenera Hymenanthes and Pentanthera, resulting in the so-called azaleodendrons. The Boskoop nurseryman Hardijzer obtained a valuable F1 by crossing a Japanese azalea with R. racemosum. The resulting hybrids, however, are sterile and further improvement by backcrossing or continued crossing was not possible.
The question how to get round this sterility forms part of the research program of Doorenbos. Obviously, breaking these incompatibility barriers is highly desirable and opens up prospects of obtaining entirely new combinations of important characters. For this purpose a collection was made of some 10 tropical species, most of which had recently been gathered in N. Guinea. Nearly all these species belong to a distinct group of the subgenus Rhododendron and consequently are lepidote. The flower colors are mostly pure red, orange, yellow or white; the flowers are small to very large and generally differ greatly in shape from what we have known so far.
To get a better insight into the role played by the yellow flower colour in the genus Rhododendron, De Bruin carried out a number of pigment analyses. These analyses have shown that the glaring yellow colour of azaleas and rhododendrons of the subgenus Rhododendron comes from carotenoids, while the dull yellow colour of the rhododendrons of the subgenus Hymenanthes is caused by flavonols. For this reason, if for no other, it might be useful to incorporate a tropical species, like R. laetum for instance, in the existing stock of cream-yellow garden hybrids.
A serious difficulty in the breeding of Rhododendrons is its long life cycle. In most crosses at least five years elapse between the time of making the cross and flowering. However, by improving our growing methods we were able to gain a full year. Therefore we had to harvest the seeds as early as September, they are then still green and wet. Drying is done in a normal room, sowing at the end of October. Germination of these early-harvested seeds is better than when applying the normal method.
Before Christmas the seedlings are pricked out in wooden boxes and are given additional light from fluorescent tubes, 50 Watt/m2 timed to simulate a day length of 18 hours. In the spring they are transferred to the frames and a year later transplanted to the garden in a sheltered aspect. Thus we have gained a whole year compared with the classical method of harvesting in November and growing the seedlings on in cold frames without extra light.