Dwarf and Alpine Rhododendrons
H. Lincoln Foster
Falls Village, Connecticut
From Presentation Given at 1973 Annual Meeting, Pittsburgh, PA
It is not easy to imitate in our lowland gardens the natural conditions of the alpine heath where in various mountain ranges of the world dwarf rhododendrons tangle with other miniature shrubs and alpine plants to form an intricate and beautiful carpet. Yet this very difficulty should present to the rhododendron fancier a stimulating challenge; and the aesthetic rewards, though less flamboyant than those from a planting of the large leaved species and hybrids, are subtle and infectious.
In the heath lands above and near the tree line plants have evolved to endure what we would generally consider rather difficult environmental stresses. Winters are long and usually cold, winds are fairly persistent and often violent, sunlight is intense but frequently modified by fogs and clouds, the growing season is brief. Vast areas of the rocky moors at high elevations rush into bloom and plants set their seeds in the brief period between the melting of the snow cover of the past season and the onset of the next snows. The melt waters of the snow, frequent showers, and cool nights all combine to provide adequate moisture at the root level of the plants, but the water moves rapidly through the open stony soils. When the sun and wind combine on clear days of summer, transpiration and evaporation from the above ground portions of the plants are excessive.
In response to this combination of stresses, plants with particular characteristics have been selected for survival and over time especially adaptable combinations of growth pattern have become genetically established in these inhabitants of the alpine heath lands. The general growth pattern is dense and low to the ground as defense against the fierce winds and to trap blowing snow. Leaves are evergreen to begin promptly the business of photosynthesis during the brief growing season, and the leaves are small in order to reduce transpiration from the leaf surface. In addition many leaves are protected by waxy scales or indumentum as an additional shield against the parching winds and intense light and heat reflected from bare rock.
These are the features, then, that we commonly encounter in dwarf, small-leaved rhododendrons of the alpine regions in various parts of the world. A typical example is Rhododendron lapponicum, the only true alpine species native to North America. Always densely twiggy and dwarf in stature, it may, in that fierce environment on the top of Mount Washington, New Hampshire, become absolutely prostrate. It forms mats in the moist peaty rock pockets, entwined with the minute carpets of that miniature azalea, Loiseleuria procumbens and rock hard hummocks of Diapensia lapponica. This Lapland Rosebay does not take willingly to cultivation at lower elevations. But there are many other more amenable species within this section to which R. lapponicum has lent its name.
Over fifty species have been named in this Lapponicum series of dwarf alpine shrubs with very small scaly leaves. Not all are in cultivation nor are they of uniform hardiness, but among them are to be found what can be considered the backbone of the mixed alpine heath. Among the satisfactory purple and bluish-purple species are R. drumonium, fastigiatum, glomerulatum, impeditum, intricatum, ravum, rupicola, and russatum and scintillans. With white flowers is microleucum and with yellow flowers, chryseum and flavidum.
It must be confessed that these Lapponicum and other dwarfs from other sections which I shall mention are a real challenge to grow in most parts of the United States except for the favored Northwest. They do not tolerate happily drought and heat of summer. Remembering the high mountain conditions in which these plants are found native, we must try to approximate as many of the conditions as possible. We can provide the well-drained peaty soil by combining acid leaf mold, sphagnum peat and sharp sand and stone chips. Artificial watering, unless the site has a natural subsurface supply of moving water, is certainly desirable and in most places necessary. The ideal is a misting system built into the artificial heath. Most of us, however, will have to rely on the awkward hose. if the soil is made properly well-draining there is little danger of over watering and even daily treatment in late afternoon during the heat of summer will work wonders.
If, however, hose outlets are not convenient to the location of your heath, or water is scarce or expensive, then try to find your coolest north facing slope and mix a bit more peat into the soil. Shade to the south or even high open shade overhead will mitigate the torridness of eastern and midland summers. By such devices you may preserve your dwarf rhododendrons from summer-sickness, but they will tend to become lank and loose, and the blossoms will be poor and scanty. For dense growth and abundance of flowers these alpine creatures need plenty of light.
Even in the best of sites the miniature rhododendrons do not need and, indeed, abhor excess of fertility. Because of the short season at high elevations the breakdown of organic matter is slowed and the release of mineral nutrients from rock is scanty and quickly leached away by moving water, to grow the plants of the austere alpine world in our gentler climate and give them a rich diet is to encourage an awkward spread and a lethargic constitution not inclined to blossom. Give them plenty of pure water, quick drainage, and a lean diet and they will take on that bloom of health which marks the sturdy kneed and rosy cheeked mountaineer. There is no rule that says, moreover, that you may not crop them down to size. Prune them by all means to keep them in character, make them work to conform.
Besides the species of the Lapponicum series already mentioned there are many other low-growing rhododendrons of various sections that benefit from this Spartan diet and combine with other alpine plants to form in our man-managed garden a small and gratifying replica of the mountain moorland.
Introduced by those stalwart explorers like Forrest and Farrer, Wilson and Ward, are many wonderful species from the remote and frightful Himalayan and other highlands. Their freight of seeds and specimens came back to civilization at great expense of money, courage and effort. To those who backed the adventurers and those who grew on the flood of numbered seeds we owe an additional gratitude. Not all the introductions have proved worthy or adaptable, but fortunately specimens of many of them still exist in botanical gardens and in specialist gardens in England and Scotland. When interest is aroused in these species, especially among the less spectacular ones of low stature, perhaps seeds will become available so that we can select among the seedlings, not only for blossom size and color, but for adaptability in the less favored portions of the country.
FIG. 30. Dense, low-growing forms of R. keiskei with its
lemon-yellow flowers make excellent additions
to the alpine heath.
Photo by C. Smith
There are fortunately already available a few very good ones like R. keiskei of the Triflorum series, especially in the dense, low-growing forms, and R. racemosum of the Virgatum series, a reliable and charming species with particularly fine dwarf forms. Both of these species have been widely grown and have played an important role in hybridizing.
There are two prostrate species in the Saluenense series, R. keleticum and radicans and one in the Anthopogon series, R. sargentianum that reward every effort expended to make these rather fussy characters willing to persist and flower.
Of more willing disposition are the dwarf alpine forms of R. dauricum, especially charming in the rare white-blossomed alpine edition.
For late bloom are the alpen rose of the European Alps, R. ferrugineum of the acidic rocks, R. hirsutum of the lime stones and the near relative R. kotschyi. In a severe winter the flower buds of these mounded shrubs may be damaged if they do not have a snow cover.
An added accolade may be won for beauty and skill by the inclusion of that strange deciduous sprite of coy temperament, R. camtschaticum with large, flat-faced, plum colored blossoms on the stiff twiggy shrublet.
The foregoing species, and many others in the same series, give us a wide range of appropriate heath-like rhododendrons, small of stature when hard grown, all bearing quite small undistinguished leaves of rather rusty appearance, and comparatively and appropriately small blossoms.
To these may be added some hybrids that tend frequently to produce larger and handsomer flowers. There are, for instance, the good blues of the 'Blue Tit', 'Blue Diamond', 'Blue Bird' tribe in which the more tender handsome R. augustinii is combined with various of the Lapponicums.
R. racemosum and R. keiskei have been brought together in their various forms, and with other species, and back-crossed to each to provide us with a growing collection of splendid and adaptable progeny. The possibilities along these lines, because of the yellow of keiskei and the pink of racemosum, promise even more for the future.
Curiously enough it seems possible that some of the handsome dwarf species with rather tender disposition may produce hybrids of greater cold resistance and heat tolerance. Experimenting with these may require potted material held in a cool greenhouse for winter and plunged in a cool north facing site for the summer. A cross made in an alpine house between R. pemakoense, not hardy in the northeast, and 'Rose Elf', itself a cross of racemosum x pemakoense, also not hardy, has produced large flowered dwarf plants able to tolerate -20° F. The possibilities have certainly not been thoroughly explored.
In order to vary the texture of foliage in these heath plantings and add size of blossom it is not unreasonable to include some of the larger-leaved rhododendrons that have a compact and low profile, such as the repens hybrids or yakushimanum and its dwarf relatives. These do, indeed, make their homes at higher elevations in the mountains and are adapted to the same conditions in which have evolved the small, scaly leaved species. To use the larger-leaved species in the heath, however, poses a problem of design and careful composition to avoid unfavorable diminishment of the small-leaved sorts by comparison. Juxtaposition can be tricky.
For restrained and harmonious combinations there are other worthy companions, many in the same Ericaceae family. The very conditions that suit the heath-like rhododendrons accommodate the true heaths and heathers, Erica and Calluna, among which are an endless variety of species and named clones. And in the same aristocratic family are dwarf Gaultherias, Gaylussacias, Leiophyllums, Ledums, Menziesias, Vacciniums, Epigaea, Cassiopes, Phyllodoces, small Pieris, Loiseleuria, dwarf Kalmias and Kalmiopsis. In fact there is no better location for low-growing members of this wonderful ericaceous family than in the general thicket of the alpine heath. How lost they become if they are grown, as they frequently are, at the feet of the more tree-like and large leaved rhododendrons.
In addition to these family relatives there are other select plants fittingly admitted into this special and august company. From the Diapensiaceae, there are the Shortias and Schizocodons of distinguished foliage and superb flower. The restrained Coptis, low growing Daphnes, partridgeberry and twinflower, and Cornus canadensis, and because they do grow in similar sites in the wild even some of the flamboyant Lewisias.
The temptation is there to use the site for a mixture of showy herbaceous alpine plants but the particular combination of circumstances that gives us the high alpine heath is just enough different from the scree slopes and wet meadows that blaze with such things as Gentians and Primulas, that we are wise, I think, to use restraint when we compose in our gardens the heath for our collection of precious dwarf rhododendrons.