For Beginners Only
Edward S. Rothman, Ph.D.
Dr. Rothman spent twenty-five years at the USDA in Wyndmoor, Pennsylvania, as a research scientist isolating, structurally identifying and transforming into cortisone and hormone analogs many steroidal materials isolated from wild Central American and Mexican plants. Dr. Rothman has a healthy respect for toxic plant materials occurring even in rhododendrons.
It is hard for the beginner to tackle the problems of growing rhododendrons if he does not live near the active centers of rhododendron culture or if he cannot tap the experience of older hands at the game. There are many factors involved which just cannot be obtained from even the most diligent study of the available literature.
How do you explain to the beginner that the gorgeous specimen on the show table with the blue ribbon on it is from the same clone as the "weed" in the beginners garden? I tell him that when Ricardo Muti and the Philadelphia Orchestra play Berlioz' symphony the magic sound comes from the same written score as he heard played last week by his high school orchestra. He sometimes gets my point.
Different Cultural Procedures
To explain, different cultural procedures may be necessary, not only for different rhododendron hybrids or species, but it may be prudent to treat identical clones obtained from different sources in different manners. This may seem obvious enough to the fellow who wouldn't think of potting up his vireya in the same mix he uses for his yaku or ledum, but sometimes the obvious is as good as hidden. It is quite possible for good rules closely followed, but wrongly grasped, to give poor results.
Mr. Austin Kennell writing in this Journal (ARS Journal Vol. 38:1, Winter 1984) has concluded that Pacific Northwest grown rhododendrons do not thrive in Atlantic regions, while another Easterner obtaining plants from the same source may have excellent results. This is to imply not that Mr. Kennell is right or wrong, but, as they say in slang, "It ain't what you do, not how you do it."
Consider first that the physical-structural nature of a given clone can vary widely as plantlets adjust to different environments. Even ivy roots started in water look different from those growing in soil and again different from those climbing up tree trunks.
A rhododendron grown in pure Oregon or Washington Douglas Fir bark grows massive mats of solid block root systems which are sliced out of their beds for shipment by means of sharp knives. I jokingly refer to these chunks as "fibroid tumors". These are not likely to be happy when shipped east to be buried in pure earth culture. For that matter, neither are friable sand/vermiculite/peat mixtures compatible with clayey loam earthen gardens - and these might be only a few feet, not the width of the continent away.
This is why it is wiser to "import" quite small plants which suffer far less cultural shock than do larger specimens, and why tissue-cultured mini-plantlets adjust to new environments best of all. These plants haven't had enough time to form bad habits and get set in fixed ways. The novice gardener is advised to study the root ball of his newly arrived mail-order plant and to adjust the preparation of the bed designed to receive it so that the two match as closely as possible. In short, plant like in like.
Sometimes by necessity, but this is not recommended in general, these changes can be made as later corrections. Once I damned a "tender" 'Purple Splendour' for routinely winter burning its leaves and thus always hurting the flower display the following spring. But tightly packing the excessively porous soil around its roots with extra rich, even soggy well-rotted pine needles provided the extra winter water the plant needed and the well flowered plant is now considered "hardy". The same treatment could rot and kill another plant under other conditions. This is why my dauricum. listed at -35°F, froze while the hybrid 'Dopey' a few feet away, listed at +5°F, laughs at zero degree winter temperatures.
The Lime Myth
By the way, not all the time-tested rules are good. Following Millais' 1913 book, the myth that rhododendrons cannot tolerate lime is widely, and wrongly, believed to this day. James Mason narrating the excellent 1982 TV show, "The Exbury Rhododendrons", spoke of "the lime-free soil that rhododendrons require". But more and more modern recipes recommend incorporation of dolomitic limestone into highly organic mixes. The more timid use gypsum instead which is really not as useful.
Check any ash analysis of rhododendron parts and it will tell you that calcium is a major and essential part of rhododendron nutrition. Most importantly, in dolomite-liming excessively acid, highly organic media, the calcium ions released by the partial neutralization of the humic acids exchange with the otherwise too firmly bound nutrient ions and allow the plant to utilize these formerly unavailable materials. More on this is treated below under "Old Sphagnum - pH Testing" discussion.
Amount of Sunlight Needed
It takes a great deal of experimentation to discover which clones need maximum light to flower at all, and in a given garden, which are happy with or even demand the bare minimum of light. For me, The Hon. Jean Marie de Montague', 'Mrs. Furnival', Tony' and 'Grace Seabrook' set buds reliably in deep forest shade. This does not meant that they would not do far better if given more light. I am just happy to be able to see their flowers in the darker places along with those of the native deciduous azaleas and maximum. Otherwise flowerless 'Loder's White' and 'Chionoides' respond to the call to flower only in the sunniest areas of my garden and even an occasional Dexter like 'Betty Hume' gives more colorful flowers in more light.
The transfer of a shade grown plant to sudden light can be disastrous though. I think of my greenhouse grown tomato plantlets which bleach white and burn nearly to the ground when suddenly set out in full sun. After a new set of leaves form these are dark blue-green, "hardened" and do not need suntan lotion for blistered tissues ever again.
One gradually learns that a rhododendron usually will not bloom reliably until it has reached a certain age. Why does a sunny field-grown rhododendron from Oregon bought in full flower and planted in my shady garden now require two or three years before it finally hits a repeating flower cycle? Many a rhododendron has been sent to the funeral pyre for failing to flower whereas time would have solved the problem. The catalogs describe this under the heading, "slow to flower, but worth the wait".
On the other hand it is surprising how very early in life some rhododendrons come to flower. These are the darlings of the commercial grower. Yakushimanum, species and hybrids, shine in this respect even though the actual plant size increases like molasses in January. I have a baby 'Tanana', a yakushimanum x decorum hybrid only 10" tall but it has eight ridiculously fat flower buds on its five little leads, paired flower buds are not unusual at all. Smirnowii is a real "slow poke", but catawba types like 'Roseum Elegans' turn into monsters dripping with flowers at the twinkling of an eye.
Growing the Unusual
It seems that most gardeners want to grow the unusual or the difficult. I have tried, unsuccessfully so far, to grow a really saturated yellow big-leafed elepidote here. It is not that yellows in general dislike our winters; but rather that they are exceptionally susceptible to mold-attack diseases in our steam bath summers. Exceptionally, 'Golden Star' (Hardgrove cross - fortunei x wardii var. wardii) thrives here. As yet un-flowered here but growing vigorously are some complex Gable yellow brachycarpum crosses with 'Golden Star' and 'Hawk' / 'ldealist' / 'Chlorops' mixes that may or may not give even pale yellows. But they do thrive.
As a rule of thumb in a Pennsylvania garden, I think that any cross with a heavy proportion of either fortunei, yakushimanum or the white 'Catalgla' types will thrive. I am having fun trying crosses of yakushimanum with tender and not-so-tender species such as elliottii, strigillosum, fortunei, metternichii, bureavii and pseudochrysanthum just to see what happens. Mine is a modest garden so there is room for only a few examples of this and that, but I have never seen a yakushimanum x smirnowii cross that I didn't like so these get extra play. The cultivar 'Schamenek's Glow' is especially fine among the taller forms. One day it is dressed in bright pink and a week later it is a perfect pure virgin white. It is competing with my beloved yakushimanum 'Ken Janeck' for my affections. I have ordered a yakushimanum x falconeri to try and will scan the ARS seed lists for yakushimanum x macabeanum, red arboreum and even sinogrande crosses, if such can be successfully made. But if azaleodendrons exist, any cross is possible! Part of the fun is attempting the impossible; it is even more fun to succeed at it.
By way of another tip, everyone now knows that vireya seed has a short shelf life, but often, more common forms of hardy hybrid seed just do not germinate even when held in dry packets over the winter in the refrigerator. I have learned to stratify seed on damp sphagnum in the storage refrigerator in imitation of natural conditions and rarely find sterile seed using this method.
Old Sphagnum - pH Testing
I might mention too, that it is good practice to get young sprouted plants out of sphagnum seed flats fairly soon. Old sphagnum and sour peaty matter by oxidation gets to be much more acid than a ph meter will reveal. This effect is explained by the fact that acidic (positive) protons are closely held "on short leashes" to the negative charged peat and are not free-swimming ions that can trigger reaction on the part of the meter. This can be experimentally verified by dripping strong alkali into a suspension of old rotting sphagnum in water in a beaker and watching the failure of the meter to notice the effect of lye additions. This titration clearly reveals the powerful buffer action of spent peat.
This effect ties in with the binding of nutrients (chelation) by spent sphagnum so that the plantlets literally starve in the midst of plenty. This process is utilized in commercial and domestic water softeners (permutite) and in the ion exchangers used with the household steam iron to subtract "lime" and other hard-water salts from the well water. Spent sphagnum in just this manner can subtract nutrients even from the plant tissues!
The Years Ahead
To look ahead in time, one can envy the novice the advantages he will be having in the years ahead. Plant explorers working for him will use helicopters in the height of the flowering season in the Himalayas. They will collect, not seeds, but selected flower buds from outstanding clones. They will know exactly what is collected, unlike their predecessors who collected seed pods of random mixture although they could identify the species, or else name it de novo on the spot.
Just as we ordered meristem propagations of virus free orchids in the 1960's - these were "tissue cultures" too - the future rhododendron collector will order flasks of tissue cultured materials even from foreign growers. He will not have to clear imports with the Department of Agriculture nor have plants fumigated. The nearly extinct and hard to propagate clones will have a rebirth, although he might lose in terms of genetic diversity. Can you see a garden of hundreds of a single monoclonal rhododendron monotonously covering a whole hillside?
What will happen when the genetic engineers start to insert chains of gene continua inside rhododendron chromosomes? Will the cobalt-blue elepidote really be an honest rhododendron or will it reflect its delphinium "inserts". It is not impossible. Today one harvests "human insulin" from bacteria as a genuine reality. Rhododendron blossoms the size of dinner plates - each floret that is - are no longer outside the realm of possibility. Come back in a few more decades and see for yourself!