The two prominent rhododendron species range so widely up and down the Appalachians, through the Allegheny plateau and on into the fringes of Canada, Rhododendron catawbiense and Rhododendron maximum, have a counterpart in the Northwestern United States.
Here, in the low and middle altitudes west of the Cascade Range, extending from northern California almost to British Columbia grows Rhododendron macrophyllum, a species which, in its grosser characters, is nearly an identical twin of Rhododendron catawbiense.
Flower color differs a bit, to be sure, and the Eastern type is generally and correctly regarded as being distinctly hardier, but, by and large, the similarities outweigh the differences.
A Northwestern Species
Home gardener may find little interest in such a fact, and we can and will leave the technical considerations to the plant geographers and species-splitters. There is, however, one rather significant observation bearing upon home-garden use of these magnificent rhododendrons, which is pertinent.
The natural range of Rhododendron macrophyllum, Nortli, and South, for a distance of approximately 600 miles along the Northwest coast, mark out one of the most promising sections of the United States for those increasing numbers which grow these beautiful flowering shrubs.
Rhododendron Country
The natural home of rhododendrons is not in the Eastern United States nor the Northwest. There are but thirty-five or so distinct species throughout our country, including the azalea scries.
There are perhaps a half-dozen more in Europe, coursing southeastward into the Caucasus Mountains. One must go much farther to the East before the center, and heart of rhododendron land is reached.
Overall extensive land area beginning in Siberia, stretching and broadening southward through China, creeping laboriously up and across the heights of the Himalayas, crawling down into the rain forests of Indo-China and Malaya.
Jumping the Straits of Malacca and the South China Sea, and hopping from island to island ever southward through the East Indies to and including New Guinea, grows the main mass and bulk of the rhododendron army — more than 1000 species strong. There lies the real rhododendron country.
A Versatile Assortment
On the practical side, we who garden here in the temperate United States will place our reliance upon those Asiatic rhododendrons from the northern half of this extended range — from Siberia, China, and the Himalayas.
That is undoubtedly not restricting us markedly, however; within the many, many hardy species from this colder northern half of the vast region, we find a wonderfully versatile assortment from which to choose.
We can, for example, select those which will mature at heights of 6” or 8” inches; those which get to 12” inches; or others in the 2’ foot, 3’ foot, and 4’ foot classes; besides the ones which range from the 6’ foot to 30’ foot sizes.
Extensive Blooming Season
Then there is the matter of blooming periods. We often think of May as “rhododendron month.”
Actually, among the Oriental species and the hybrids which have been developed from them, we are offered types that begin flowering in the earliest Spring, often February and March here in the Northwest.
They are followed in regular science by others which reach their peaks in April, with many more coming along in May and early June. Even in late June and July, are species and varieties will be in magnificent color.
Anyway, one looks at them from the standpoint of final height, a wide range of flowering time, or the differences in texture — large, middle-sized, and small leaves, blooms, and bloom trusses—the rhododendron group is superbly versatile, able to fulfill almost every garden requirement.
Hybrids Of Tomorrow
No single section of our country can lay claim to being the most ideal for any group of plants like the rhododendrons, at least not on any long-time, permanent basis.
True, the 600-mile stretch of our Northwest coast has certain natural, climatic advantages making for a more accessible culture of the presently available species and varieties, especially those which carry the blood of the Oriental types.
But the time will undoubtedly come when the gorgeous beauty of these Orientals will be combined with the hardihood of Rhododendron catawbiense and Rhododendron maximum to give us races, strains, and particular kinds planted in widespread fashion throughout the Eastern United States, along the coast, and inland.
The remainder of our discussion will be devoted to considerations of:
- What seems to be the broad, general requirements of rhododendrons.
- Their intimate, detailed cultural needs.
- A listing of some varieties that can be regarded as worthy, backbone types —not too fancy.
Soil Requirements
The type of soil, the Winter temperature, the Summer-time temperature, and atmospheric humidity seem to be the four major factors that determine the year-after-year success one will have with rhododendrons.
As to soil, the fundamental needs are:
- good drainage (no water should stand around the roots at any time);
- a physical texture that is light and open (consistency of sandy loam);
- a high content of organic matter (the better to hold ample moisture during seasons of low rainfall);
- a fairly low soil pH (a moderately acid to highly acid condition);
- good fertility.
Temperature And Moisture
The best Winter-time conditions for the fine Asiatic or Asiatic-derived rhododendrons seem to prevail in near-salt-water locations.
In other words, we like to favor these fine varieties with warm-as-possible Winter weather.
However, they are not quite as “panty-waist” as such a remark implies. For example, during the severe winter of 1919-1950, practically all of the suitable forms survived 12° degrees Fahrenheit below zero, benefiting a snow mantle, at Brooks, Oregon, 50 miles inland from the coast.
One has some leeway in the matter of Winter temperatures, therefore, but it does not pay to try to stretch it too far.
Asiatic rhododendrons and their fine hybrids enjoy the comparative coolness characteristic of near-the-sea sections. Daytime temperatures up to 100° degrees should be offset by coolness at night as a minimum requirement.
Of course, atmospheric humidity goes hand in hand with temperature. If the weather is hot and dry, rhododendrons can be made comfortable through fine, mist-type sprinklers mounted in such a way as to cast moisture all around the plants and over the leaves.
Gardeners in California, on the East coast, and inland will find such installations most effective in countering high temperatures and a dry atmosphere.
Once established, the rhododendrons arc is not difficult to handle. The year-by-year program is relatively simple.
We strongly recommend an acid fertilizer, applied during March or April (never much later than mid-May) at the rate of 1 ½ handfuls for each foot of height growth, scattered well over the rooting zone. Follow this each year with a bit of extra phosphate and a mulch of peat moss or leaf mold, an inch or two of either one.
Throughout the Northwest, where Strawberry root weevils abound, an lntc-February or early-March dusting or drenching with any of the good DDT, lindane, or chlordane compounds should be made.
Most rhododendron varieties will stand full sun in the Northwest, though many arcs are used in shaded locations. The minimum requirement for light should be a complete, open sky above and to the North.
In the shade, the growth spindles out, trusses are more minor, fewer-flowered, the blooms are not as large as they should be, and the color is often paler than usual. Plants in shaded situations should be given half again as much fertilizer each Spring to help them grow and flower better.
One of the most typical garden problems is the difficulty of getting rhododendrons established in south-side-of-the-house sites where light and heat are extreme.
Under such circumstances, the frequent use of a good hormone-vitamin, the root-stimulating mixture, is called for, particularly during the first two most critical years following transplanting.
A green-foliaged vine placed against the house behind the rhododendrons will help reduce reflected heat, and a heavy mulch all over and beyond the rooting area is a must.
A few varieties like “Britannia” will show yellowed, unhealthy-looking leaves whenever planted in full sun, no matter what you do to counteract. Avoid such types for bright, hot situations.
Selected Varieties
The list of rhododendron varieties found satisfactory in the Northwest is long.
The following represent some of the better types among the primary standby forms:
- White — Dr. Stocker (April)
- Lodcr’s White (April-early May)
- Beauty of Littleworth (May)
- Blush and Pink — Rosa Mundi (February)
- Faggetter’s Favorite (April)
- Corona (May)
- Pink Pearl (Late May)
- Rose — Cynthia (May)
- Reds — Unknown Warrior (April)
- Earl of Athlone (April-May)
- Britannia (May-June)
- Yellow and Apricot — Slocock’s Unique (April-May)
- Cunningham’s Sulphur (April-May)
- Butterfly (May)
- Mrs. W. C. Slocock (May)
49001 by Dr. John H. Hanley