A Little Known Cultural Technique
Collingwood Ingram, F. L. S., England
For the 1974 issue of the International Dendrology Society's Year Book I wrote a paper entitled "Can a Half-hardy Rhododendron be made Artificially More Hardy?" In that paper I expressed a belief that there did in fact exist a cultural technique which made that apparent impossibility possible. If I am right in that assumption (and the technique proves to be really effective) it will enable rhododendron enthusiasts in America to grow semi-tender members of that genus in a great many more areas.
Let me briefly give my reasons for coming to that conclusion. Many years ago the French firm of Vilmorin-Andrieux et Cie used to send me periodically a most exciting list of plants entitled ''Plantes rares ou particulierement interessants". From one of those lists dated March 1933 I ordered, among other plants, two rhododendrons; one a seedling of R. macrophyllum and the other a seedling of a hybrid between R. decorum and R. discolor which they said had been bred or raised in their own nurseries - presumably those situated at Les Barres which is in the department of Loiret and is more or less located in the center of France.
Both of those rhododendrons arrived stripped of all their leaves. As I did not think this had been done to comply with any export or import regulation I could not help wondering why they had been defoliated in that way. As it turned out, apart from probably retarding their development to some small extent, neither rhododendron seemed to suffer any ill effects from their drastic treatment and the following spring both resumed their full quota of foliage.
On making the discovery that a rhododendron is virtually unharmed by the removal of its foliage on the few occasions I have found the parasite Stephanitis rhododendri on one of my plants, in order to make quite sure of getting rid of the pest I have now never had the slightest hesitation about removing all its leaves.
Apparently it is excessive transpiration during periods of low temperature that is the commonest cause of a half-hardy evergreen shrub's death during a spell of rigorous winter weather. We are told that the main reason for its demise is the inability of its roots to replace in a cold, and especially in a frozen soil, the moisture which has been automatically dissipated through the stomata of its leaves.
In a deciduous species that danger is effectively countered by the shedding of its leaves in the autumn - its foliage being, of course, the organs through which it mainly transpires. To further protect a deciduous plant from undesirable transpiration during a spell of cold weather, its dormant leaf - and flower-buds are encased in tunics of impervious overlapping scales.
One cannot help wondering if in those long-ago-days it was a routine practice at the Les Barres nurseries to defoliate in the autumn all their doubtfully hardy rhododendrons in order to render them more resistant to cold by the simple expedient of turning them into pseudo-deciduous shrubs! An interesting thought.
But there are still several more points in that very unorthodox technique which call for further research. For instance one would like to know whether or not the defoliation of an evergreen rhododendron, if carried out annually for a number of successive years, would have a debilitating effect on the plant's constitution. One would also like to know if the defoliation of a rhododendron will induce it to form flower-buds during the summer following the operation, or if defoliation will have the opposite effect and inhibit it from doing so. And, lastly, further research is badly needed to ascertain the amount of frost the defoliated stem of a tender rhododendron can stand before being fatally injured by the lowness of the temperature.