Investigative Study of the Propagation of Certain Rhododendron
Species of the Subgenus Anthodendron
S. L. Solymosy
University of Southwestern Louisiana, Lafayette, Louisiana
The demand for native azaleas increased considerably in the past few years. The market bulletins carried advertisements from the so-called "Native Plant Collectors" offering for sale the wild honeysuckle or as I have heard it called here in Georgia, the wild honey-sucker. The plants obtained through these channels were usually unsatisfactory. Since, due to my position at my university, I am in constant contact with the gardening public and the nurserymen of the area, I had the chance to conduct a survey concerning the "whys and why nots" of our native rhododendrons as landscape plants for homes and public places.
The gardening public and landscape architects were in accord in stating that they would like to use our plants to a greater extent if only the nurserymen could handle them. Gardeners complained also about the complexity and undependability of the propagation.
The nurseryman's reason for not handling more of the azaleas was that they are faced with difficulties concerning the propagation. It is true that seedage is easy and cheap, provided some basic rules are kept in mind, but the seedlings are extremely variable in most species. This means that he has to wait for his seedlings to bloom, then separate them according to their characteristics.
All this takes time and in a nursery operation, maybe more so than in other business, time is money. Plants so obtained could not compete in price with the Southern Indicas, Kurumes, Belgians, etc.
The old rule that places the rooting percentages between 0 and 100 applies to our azaleas to a great extent. Graftage in root, neck or stem is feasible. but here again the market price plays a big role.
A few years ago, at one of the Board of Directors meetings of the Louisiana Society of Horticultural Research - a non-profit organization for furthering plant research in horticulture - I presented the case of our neglected rhododendrons and suggested not only taxonomic investigations, collections and study of the taxon, but also the necessity to study the propagation methods and to find one which is easy and practical enough for the average home gardener and profitable enough so it may be used by commercial nurserymen.
With a grant received from the Louisiana Society of Horticultural Research, co-sponsored by the University of Southwestern Louisiana, I began my work which brought some noteworthy results. In the following, I shall give you a brief report on my research.
The species of the North American members of the subgenus Anthodendron are extremely variable. The variability is so far reaching that even the most liberal taxonomist may arrive at a point of no decision. Some taxonomists - including this investigator - for convenience's sake, accept the collective term of "Alliance". For example: R. viscosum?, R. oblongifolium, R. arborescens and presumably R. coryi belong to the same "Alliance". The taxonomist would be satisfied at this point, not so the horticulturist. The taxonomist does not differentiate between the dark pink R. canescens and the light pink one; the horticulturist does. It is seemingly obvious that the only feasible method of propagation to segregate a clonal variant would be the standard method of cuttage. In this respect, however, the southern azaleas are extremely temperamental; the percentage of rooted stem cuttings is so low as to make this method unpractical for amateur gardener and nurseryman alike.
Some white azaleas (R. viscosum, oblongifolium, serrulatum) are stoloniferous-rhizomatous. This means that they should - and they do - respond to root cuttage.*
*As early as in 1966 this investigator, lead by more of a suspicion than by deductive reasoning, experimented with root cuttings of various R. canescens clones. The results were positive and rewarding. It was decided therefore to continue the investigative work along these lines to determine the feasibility and the practicality of propagation by root cuttage.
After a careful study of the rhododendron exsiccatae (dried specimens) of the herbaria of the United States National Arboretum, Southern Methodist University, University of Southwestern Louisiana, this investigator became convinced that due to the intergradation of southern azaleas, stoloniferous-rhizomatous clones must occur in pink species since pink forms have been observed occasionally in populations of the originally white taxa. In spring of 1967 a stoloniferous clone of Rhododendron canescens was found in Sabine Parish. In this parish. the stoloniferous white R. viscosum, R. viscosum v. glaucum, and the non-stoloniferous R. canescens grow only a short distance apart. Quite frequently, the efflorescence of the early flowering white specimen coincides with that of the late flowering specimens of R. canescens thus making a natural hybridization possible and even probable. In the fall of 1967, a few plants of the stoloniferous clone of R. canescens were collected and brought back to the University. On September 26, 1967, cuttings of various sizes - from 3 to 10 mm. diameter, 4 to 10 cm. long were made. Four flats were prepared containing various media: sphagnum, sphagnum and sand, peat moss, and peat moss and sand respectively. The cuttings were inserted horizontally, and covered with a 1 cm. thick layer of the medium. Finally the flats were placed on inverted 2½" pots to insure proper drainage, moved under Saran lath with a 50% calculated shade and left unattended, subjected entirely to natural climatic conditions. It has to be mentioned at this point that previously, in 1965, root cuttings were also inserted in sand in a propagation bed. The cuttings grew for a short while, then rotted away probably due to too wet conditions.
On February 25, 1968, it was noted that the cuttings sprouted, growing several branchlets. A few of the sprouted cuttings were removed from their respective media for observation. It became apparent at that time that new root formation was taking place, showing as minute, transparent, mycelium-like rootlets. Between February 25 and December 9, 1968, the cuttings were left undisturbed, except for constant observation. During the spring and summer dry seasons the flats dried out for extended periods. This, as a matter of fact, was the duplication of the climatic conditions to which the plants are subjected in the wild. The plantlets grew satisfactorily, the shoots hardened and lignified, the leaves developed well. In December 1968 the formation of the terminal winter buds was completed and the plantlets ceased to grow. At this point, on December 9, 1968, it was decided to remove the plantlets from their respective media and transplant them in 6 inch pots in the medium in which the cuttings' performance was the most satisfactory.
At the present time (April 1969), the transplanted plantlets are in the lath house, have become well branched and will remain there undisturbed to await further development.
While transplanting the cuttings, the following observations were made: Cuttings in sphagnum and in a mixture of sphagnum and sand developed a healthy and very strong root system. The medium was penetrated through and became interwoven by the maze of the roots. Cuttings in peat moss and sand performed poorly. Only one cutting with a very poor root formation survived. It seemed as if the latter mixture did not provide for the necessary air circulation. Rhododendrons are very shallow rooted, covered only by a thin layer of soil and leaf mold in their native habitats.
It is of utmost interest to note that Rhododendron minus Michx., a taxon belonging to the Series Carolinianum, section Lepipherum of the subgenus Eurhodendron, is also a stoloniferous plant. Root cuttings were made and at the present, several cuttings have shoots with 3-4 pairs of leaves. This could open a new horizon in regard to the vegetative propagation of members of the whole subgenus.