QBARS - v17n1 Dwarf Rhododendrons

Dwarf Rhododendrons
by Virginia Jefferis
From an illustrated talk delivered at Media, Pa. to the Philadelphia Chapter ARS

When Dr. Franklin West, our Program Chairman, asked for this talk he specified one on lepidotes and really caught me unprepared. I wasn't even too sure just what constituted a lepidote. We first became interested in dwarf rhododendrons because of their desirability for a small shady rock garden, and for the past three years I've been collecting the plants in a kind of un-intellectual way that gave me no idea how they fitted into the lepidote vs. elepidote classification. So the first thing I had to do was to list all our dwarf rhododendrons in their series and then find out whether those series were lepidote or elepidote. ¹

¹ See Cox, E. H. M., and Cox, P.A., Modern Rhododendrons, Thomas Nelson and Sons Ltd., 1956; and Leach, David G., Rhododendrons of the World, Charles Scribner's Sons, 1961

By the time that was done, and it was obvious that our dwarfs were about evenly divided between lepidotes and elepidotes, I began to be curious about the actual difference between them and whether there was any easy way to classify a rhododendron by casual observation. The answer seems to be: only within limitations. There are certain generalities that can be made about lepidotes and elepidotes which make it possible to guess intelligently about the classification of a rhododendron, but the exceptions will confuse you if you rely on casual observation alone, In general lepidotes are small to medium in size and elepidotes range from large to tree-like. However, R. minus , which is a lepidote, grows 20'-30' feet tall, while one of the most attractive elepidotes R. forrestii var. repens creeps along the ground, For the most part lepidotes have small leaves and flowers, and elepidotes have the broader leaves and larger flowers of the average garden hybrids, But this obviously doesn't cover elepidote dwarfs which have relatively small leaves and flowers. In the final analysis there is only one decisive difference between lepidotes and elepidotes: the lepidotes have scales, the elepidotes do not, Both may have papillae, another type of trichome or leaf covering; both may have hairs (in fact elepidotes usually do); but only the lepidotes have scales.
It isn't pertinent here to expand the subject of the rhododendron leaf and its trichomes except to say that all three types of trichomes serve the same purpose of regulating transpiration. They aid the plant in giving off the large quantities of water which it takes in during the rainy season, and prevent it from becoming too dried out during windy and dormant seasons. ²

² Cowan, J. II., The Rhododendron Leaf, Oliver and Boyd, 1950

There is no sure way of determining the presence or absence of scales with the naked eye, but after examining several dwarf rhododendron leaves under a microscope I got so carried away with this phase of rhododendron study that I had some macro-photographs made to show a little of what I found out.
If you were to examine the back of a rhododendron leaf with a hand lens of 10x or more you would discover some of the plate-like discs and know you were looking at a lepidote. This is R. ferrugineum which is so heavily covered with scales that they overlap and make the leaf appear brown on the underside even to the naked eye. Of course the more scales or hairs there are the more the leaf is protected from transpiration, and you can see that R. ferrugineum is well suited to its habitat 4,000 feet up in the Alps. It also seems to adapt well to our area. This is a cross section of a leaf of R. carolinianum , another lepidote, showing how scales are like saucers on a stem, with the stem fitting into a depression in the under surface of the leaf and the saucer providing a protective layer just above the surface. This slide shows a cross section of an elepidote, R. yakushimanum , illustrating the brown furry indumentum made up of two of the several types of hairs and also providing excellent protection for the leaf. This slide is the under surface of a leaf of R. hemitrichotum , a lepidote with all three types of trichomes-scales, hairs and papillae.
While some of the dwarf rhododendrons seem difficult to cultivate in our area, and some would be of interest only to collectors, most of them are reasonably tractable, and there are several which rate among the most beautiful of all flowering evergreens. As a group they extend the season of bloom considerably, since the lepidotes are at their best about a week ahead of the early azaleas and the dwarf elepidotes continue to bloom when most of the Kurumes have faded. They range through all the flower colors common to the other rhododendrons and evergreen azaleas. They also add many more plants - which bloom in shades of blue, blue-violet and purple. These are particularly noticeable in contrast to pinks and whites of dogwoods and azaleas, or to the bright colors of many of the early spring bulbs.
They provide endless foliage variety and interest throughout the year. For example, such dwarfs as R. scintillans and R. fimbriatum have delicate leaves almost as narrow as yew needles; R. leucaspis leaves are oval, two inches long and an inch or more wide. The foliage of R. impeditum is noticeably blue-green in color. R. sanguineum is black-green. R. microleucum is yellow-green. Some leaves, like those of R. degronianum and R. racemosum have an under surface decidedly different in color from the upper surface. Thoughtful placement of the many shapes and colors in dwarf rhododendron foliage can create very pleasing contrasts.
Winter produces a whole new color effect using the very same plants. R. fastigiatum which is a lovely blue green in summer is a no less beautiful maroon in winter. R. 'Blue Diamond' becomes a warm dark red and R. 'Bluebird' turns a bright red reminiscent of autumn. The buds of next spring's bloom are yellow-green against the dark green foliage of R. forrestii var. repens . The buds of R. 'Cilipenense' are red against bright green leaves. R. 'Hockessin's' buds are pink against the dark red color of its winter foliage.
A not so pretty picture is presented in winter by a few dwarfs such as R. uniflorum , R. hippophaeoides and R. scintillans . Their leaves shrivel with the cold till the plants look completely dead. Their revival with the first-warm days of spring is one of the unexpected rewards of growing dwarf rhododendrons.
In the search for plants which will be scaled to today's small properties the dwarf rhododendrons should be in increasing demand. Such plants as R. carolinianum , R. 'Windbeam' and R. makinoi grow large enough to make foundation plantings in proportion to modern ranch houses. Miniatures like R. impeditum , R. nitens , R. pemakoense , R. radicans and R. keleticum will never outgrow even a tiny rock garden. Medium growers, R. 'Blue Diamond,' R. hemitrichotum , R. 'Jock,' R. 'Augfast' and R. 'Wyanoki' are good plants for the foreground of a border. Many which start off as small plants in our rock garden are transferred to the border in a few years; and some which started in the border have been moved to the rock garden in order to provide a setting more in scale with their size or conditions more to their liking.
This brings up the subject of culture. Complete success has certainly not crowned all my efforts, but three or four years of experimenting have disclosed a few general truths which pertain to growing dwarfs well in this area. Almost all of them are plants native to extremely high elevations or are epiphytic (R. moupinense) . In either case sharp drainage and an airy medium are even more vital to their health than is true with the average hybrid rhododendron. This can he achieved in several ways, but in our location, where the soil is heavy, we have found that raising the beds and using large quantities of sand are two essentials. This past year I saved several ailing dwarfs by setting them on top of the level from which they were dug and putting a 50-50 sand and peat moss mixture around the root halls. It was amazing to watch the green color return to a plant of R. 'Sapphire' treated this way. The one exception seems to be R. hippophaeoides which didn't do well until it was planted in a pocket lined with polyethylene and filled with sphagnum moss. Apparently this is a bog loving plant.
Hardly less important to the dwarfs in general is keeping them cool during our hot summers. Shade, of course, is one way; and shade from the strong winter sun is important too. White pines are particularly good with rhododendrons: A pine tree only a few inches high will do a good job of shading a small rhododendron and will keep ahead of it in growth. Another way of keeping plants cool is to provide air circulation and air drainage. Plants which dislike a bed at lawn level will thrive on a slope or an elevation. This same elevation will also keep the plants warmer in winter since the cold air will settle to the lowest part of the garden.
The last suggestion I can make for keeping the dwarfs happy is this: having provided them with sharp drainage and a light medium they will be benefited by an occasional cooling sprinkle during hot dry spells.
Getting plants off to a good start is important. A method which we have found invaluable is to acquire the plants in the fall and keep them over the winter in the "shanty." This shanty is a homemade redwood frame covered tightly on all sides with 4 mill polyethylene and unheated. It is placed on the southeast side of our stone garage; put together with screws; and can be taken out of the ground easily by hand if it ever outlives its usefulness. The frame is 9 feet long, 42 inches wide, 24 inches high in front and 30 inches high in back. The top is made in three removable sections. We have used various planting media with equal success: half peat, half perlite; half peat, half sand; 100% peat.
In each case the mixture goes on top of a generous drainage layer of sand or pebbles; DDT, a fungicide, and plant foods are added, the latter being ammonium sulphate, potassium sulphate and super-phosphate. But these are not necessary as we have had success with and without the addition of pesticides, fungicides and fertilizers. The medium is watered either by one or two heavy rains or by hose. Once the plants are put in the shanty (after frost occurs and fall has set in-late November) and the top fastened in place, the whole setup can be forgotten till spring. The inside temperature never seems to get too high, and sufficient moisture comes through the soil. On an Airguide Humidity Indicator the relative humidity stays consistently over 90% Beads of moisture are present at all times on the inside of the top and sides.
Although plants in the shanty are not off-cycled as they sometimes are in a greenhouse, nevertheless they seem to make considerable root growth under the conditions of still air and high humidity. This enables them to overcome the shock of shipping and to establish readily when they are put into their permanent location in the spring.
I could group the dwarfs we are growing by color, or list them in order of ease of cultivation or preference, but it appears more logical to arrange them in terms of lepidote or elepidote and according to their series, including with each species plant the hybrids of it. The following are plants which are on our property at the present time, with some indication of how long we have had them. Complete descriptions are not included because they can be obtained from many sources, but I hope the list of names will encourage you to try something new in your own gardens.

* These plants were in our garden at least as far back as the hard winter of 1960-61, some of them longer.
** These are especially desirable, even for those who are not necessarily collectors of dwarf rhododendrons. The rest of the plants we have not had long enough to be able to speak about them with any authority.
For people in this area who are interested in making a collection of dwarfs, in addition to those with ** above. I can at present recommend the following: