Trees to Associate with Rhododendrons
G. D. Muirhead, Vancouver B.C.
A lecture presented to the A.R.S., April 19, 1951
Any discussion of plants suitable for association with rhododendrons must be based on the answer to the question, "What are ideal conditions for the rhododendron?" It is well known that the soil should be lime-free, light, and well supplied with humus, and that it should also have adequate moisture and yet be well-drained. However other factors are equally important, though less often considered. Most species are native to a monsoon climate, where the atmosphere is nearly saturated during the growing season. These species, and the hybrids produced from them, will only do well where we can maintain a high humidity by shielding the plants from cutting winds, rapid temperature changes, and prolonged sunlight.
The influence of trees upon Rhododendrons is more significant in this respect than most enthusiasts realize. The climate below a tree canopy is vastly different from the climate in open ground in the same vicinity. The trees affect the plants below them in many ways. They exclude part of the sunlight; they greatly reduce temperature fluctuation; and they retard air movement. These factors produce a much higher relative humidity, so that transpiration and evaporation are greatly decreased. The soil surface remains naturally in a cool, moist, loose condition, and a high - humus content is maintained by the leaf-cast of the trees. This mulched condition is essential for the well-being of a shallow-rooted plant such as the Rhododendron. On the debit side, the competition of the trees for soil moisture and nutrients is a point which must riot be forgotten. The following brief discussion of some of these factors may illustrate their importance.
Rhododendron leaves, since they are evergreen, - transpire throughout the entire year. This moisture loss is reduced during the winter, but it does not cease entirely. If the soil water becomes unavailable due to freezing, and the plant is exposed to wind and sun, the leaves will lose water faster than it can be replaced. Prolonged drying out will result in foliage injury or even premature leaf-abscission. Large trees partially shading the shrubs will eliminate this possibility except in very severe weather. We are all probably familiar with the fact that ground frosts occur frequently in open fields, but seldom in woodlands. The explanation of this is quite simple. The earth is constantly losing heat by long-wave radiation; on a clear, still night this heat loss is very rapid, causing the ground temperature to drop considerably below the air temperature. If the air temperature is near freezing, a ground frost will occur. If this radiation is prevented by anything opaque such as clouds or trees, this heat loss will not take place. The trees must not be so densely planted, however, that a frost pocket is created. In addition, if too much sunlight is excluded, the plants will become leggy; and fewer flower buds will be differentiated.
It is equally important to prevent excessive transpiration during the summer months. As has been mentioned, most Asiatic species have evolved in a humid summer climate. Thus the rhododendron, although it has a very large leaf area, is not adapted to excessive water loss through the leaves, and has little inherent protection against a dry atmosphere. Physiological drought, therefore is likely to cause more difficulty in our climate than the temperature extremes themselves.
From this it can be seen that an open woodland of mixed coniferous and' broad-leaved trees standing on a slight slope will best approximate the growing conditions required by most rhododendrons.
We have thus far considered only the functional value of trees, but their aesthetic qualities are no less important. Both of these facets will be dealt with in the informal discussion that follows. The trees discussed do not comprise an exhaustive list, as this would require several volumes but a selection has been made which included those which should be most satisfactory under our conditions.
All rhododendron growers have doubtless at some time or another been told of the benefits of Pine needles and Oak leaves as a mulch for, their plants. The Pacific Coast has many native Pines and Oaks, and in addition many exotic species are well adapted here. Since both these genera fulfill all the requirements as shelter trees for rhododendrons, they should be the first to be considered. Used together, they form the best combination for our purpose.
Large Broad-Leaved Trees
The Oaks are without doubt the most useful of this group. Since they are deep-rooted, they compete very little with shallow-rooted shrubs for nutrients and moisture. They grow very well in the area, and are not unduly susceptible to pests. A garden possessing native Oaks 'such as Q. garryana is very fortunate, since they are amongst the slowest growing of the Oaks. Incidentally, the lower limbs of this Oak, and of many other trees often create a visual barrier and are best removed as this will make the garden more spacious and interesting.
The Red Oaks, Q. borealis and Q. coccinea, are rather faster growing than the White Oaks, and both have excellent form, besides often displaying brilliant fall colour. Among the White Oaks, Q. alba is usually easily obtained, and is a tree which improves greatly with age. Q. roburfastigiata, the upright form of the English Oak, is ideal for smaller gardens, and is seen too seldom here.
In California the native Live Oaks, especially where they occur with Pinus pines, form an excellent association for rhododendrons. Some of the Live Oaks, such as Q. chrysolepis, and Q. agrifolia, should be tried more frequently in the warmer areas of the Northwest. Q. ilex, the Holly Oak, though very slow growing, is more hardy than is generally supposed.
The Birch is another familiar group of garden trees. They display grace and beauty at all seasons. In the winter the woods are lit up by their trunks, in spring by their fresh, green, new growth, and in the fall by their distinctive golden-yellow leaves. Their roots, although shallow, are not voracious, but it is usually better to plant at least ten feet from their trunks. The European Birch, Betula pendula, is more in character with most woodland gardens than any of its weeping forms. Many other Birches, such as B. lutea, B. lenta, and B. papyrifera, make good shade trees for rhododendrons, although offering more root competition than the Oaks.
Further good deciduous trees for both protection and fall colour are the Black Tupelo, Nyssa sylvatica, and the Sweetgum, Liquidambar styraciflua. Both are deep-rooted trees of interesting texture and habit. They hold their foliage well into the fall in the Northwest, and the leaves eventually turn marvelous shades of scarlet and gold.
Fig. 25: Rhododendron sheltered to the south by
a flowering cherry and dogwood (Cornus nuttalli)
both trees are less than twelve feet away.
Fig. 26: A group of trees sheltering rhododendrons
and azaleas on their north sides. Such widely
different types as Weeping Willow, Elms and
noticeably Pinus Monticola in the background.
Staphlea, Sciadopitys and forms of Japanese Maple
are close associates of the shrubs. Note the relief from
Western sun which is offered by the Cunninghamia in
Among the taller flowering trees, my own favorite is the double form of the English Roadside Cherry, Prunus avium var. plena (Fig. 25) which is rarely seen in the Northwest. It is without doubt one of the world's most beautiful trees and should be far more widely grown than it is. The root system is comparable to that of the Birches. The tulip tree, Liriodendron tulipifera, which grows so well in this area, can be used on the fringe of large gardens, but it has roots which are too competitive for close contact with shallow-rooted shrubs.
There are many other trees which are suitable for this outer fringe area in large gardens. They may be used to provide shelter, variety, and fall colour, while keeping their shallow, voracious roots at a safe distance from the sheltered shrubs. The Beeches are great soil improvers, but not only do they have a greedy root system, they also cast a very dense shade. Aspens and Maples can contribute fall colour, provided they are kept a good distance away from the shrubs. Other vigorous-rooted trees such as the Lindens and Elms are hardly in character with rhododendrons. As a wind break, English and Portugal Laurel; or a selected form of R. ponticum can be used, for they are fast growing and effective.
LARGE CONIFEROUS TREES
The Pines comprise the largest and best group of coniferous trees to use with rhododendrons. Besides providing the evergreen foliage necessary for winter protection, and supplying very beneficial mulch, the Pines, unlike the Douglas Fir, the Hemlocks, Spruces, and true Firs, are usually deep-rooted. Most species are tap rooted when young, and later develop deep, spreading laterals which will not interfere greatly with the roots of shrubs in its shelter. Botanists recognize about eighty species of Pine in all. No less than thirty-five of these are native to North America exclusive of Mexico, while twenty are native to the Pacific Coast, and eight to Oregon. Although they do not exhibit the great range of habit shown in the genus Rhododendron, there is an amazing amount of variation between the species, and I suggest that a combined collection of Pines and Rhododendrons might be very interesting. (Fig. 26)
All coniferous trees may be considered useful to provide shelter, but if too many garden varieties are used, a spotty, restless development will result. If a number of conifers are to be used, - by making the Pines predominate a unity will be obtained which will allow much more variation in the undercover than would otherwise be possible. As the trees mature, a fine effect is produced by removing the lower branches. An illustration of this is shown facing page 33 of the Royal Horticultural Society Rhododendron Yearbook for 1948.
If you are fortunate enough to have native Pines in your neighborhood, use them, but if they have five needles in a fascicle beware of the twin scourges of White Pine Blister-rust and Pole Blight. For the same reason, the exotic five-needled Pines are not to be recommended for use here.
Among many excellent Pines are the Scots Pine, Pinus sylvestris, with its brilliant orange bark and blue-green needles, and the Japanese Black Pine, P. thunbergii, which has a distinguished horizontal branching pattern and dense tufts of dark green foliage. Both of these will ultimately make large trees of forest size, as will P. densiflora, the Japanese Red Pine. Its variety, P. densiflora umbraculifera, however, is a dwarf form suitable for small gardens. Among the native Pines, P. ponderosa and P. contorta are probably the most useful, although many others have features of value. P. coulteri from southern California is completely hardy here, and grows to forty feet with large cones the size of pineapples, while the various forms of P. cembroides are interesting small trees which should be worth trying. The well known Monterey Pine P. radiata is very handsome, but many were lost in the winter of 1949-50 in the Northwest.
Fig.27: Hardy hybrid rhododendrons growing well with their roots
less than 10 feet from a 120 foot western red cedar.
Most of the other native and many exotic conifers are excellent shelter trees for rhododendrons, providing the shrubs are not planted too near them. Among the best of these are the Larches, especially Larix decidua, the European Larch, with its yellow green foliage and scarlet female cones. The Redwoods are also good, and the Hemlocks, true Firs, Douglas Firs, Cedars, (Fig. 27) and False Cypresses, although shallow-rooted, are useful trees if their power to compete for moisture is carefully considered.
Native Spruces are also quite numerous, and have interesting form, but many are subject to the Spruce Gaul Aphid (Adelges Cooleyi) and should be avoided by those unwilling to spray them. Koster's Blue Spruce is definitely out of character with rhododendrons.
Here, of course, the list is almost endless, and only a few are considered. Many trees used as undercover in the larger gardens could be used to shelter the rhododendrons in the smaller gardens. The Flowering Crabapples and Hawthorns are well suited for this purpose. The Japanese Maples in their green forms, and occasionally in their red forms, are always interesting, and combine particularly well with evergreen azaleas. Acer palmatum var. septemlobem is a fine tree rarely seen. The Vine Maple is a good native tree, though not always so easily handled due to its trailing habit. Many other small Maples, such as A. davidi, A. griseum, and A. carpinnifolium are fine in bark, branch pattern, and leaf, and light up a whole woodland with their autumn hues. Camellias, although too stiff in most of their varieties, provide a good background for Azaleas. The closely-related Stewartias and Franklinias are excellent for woodland planting with other acid-loving plants.
A tree of marvelous colour in relatively dry autumns is Cercidiphyllum japonicum, the Katsura tree, Oxydendrum arboreum, the Sorret Tree, and a member of the family Ericaceae, is another tree with outstanding fall colour, and is very suitable for all sizes of gardens. It has the additional advantage of bearing Pieris like flowers during the summer. Styrax japonica, the Snowbell Tree, is at its best when it is pruned so that its arching branches may be seen from below. Davidia vilmoriniana, the Dove Tree of Ernest Wilson fame, is spectacular during its short period of bloom. It has a graceful appearance and appears quite resistant to pests and diseases.
In the Dogwood family there are a number of good trees for woodland planting, with our own Cornus nutallii bidding for a position among the world's most beautiful flowering trees. The forms of C. florida and C. kousa are also very attractive.
The Flowering Cherries as a group have shallow roots and are best used at a safe distance from rhododendrons. They are breathtaking in bloom with an evergreen background, and since they are deciduous and often similar in scale to the larger rhododendrons, they provide a pleasing seasonal change. Such superb trees as Yoshino, Shirotae, Shirofugen, and Shogetsu are worthy of a place in every garden.
The last group of small trees that I will mention are the Magnolias, and they are probably the most important. There are many rhododendrons which, in our favored climate, are not worth growing. But there is scarcely a Magnolia which would come in this category. A good account of the Magnolias can be found in Mr. Millais' excellent book on the genus, and also in the 1950 Conference Report on Magnolias of the Royal Horticultural Society.
As a rule Magnolias require about the same conditions of shelter as do rhododendrons, although in some Cornish gardens they are grown as shade trees instead of Oaks. All deciduous Magnolias benefit from a background of coniferous trees, which bring out the beauty of their bloom by contrast. Magnolias possess shallow, fleshy, brittle roots, and must be moved and planted with care. Both these operations are best performed in early spring as the plants are coming into growth.
Most people know the early flowering Magnolias, M. denudata and M. liliflora, and their hybrid, M. x soulangeana, also the evergreen species M. grandiflora, and M. kobus, the common understock. But how many grow the giant pink Magnolia, M. campbelli, or its hybrid with M. denudata, M. x veitchii, both of which are hardy in the more sheltered gardens of the Northwest, or that other magnificent tree, the large-leaved Magnolia, M. macrophylla, with leaves up to thirty inches long, which thrives on the Pacific Coast as long as it is protected from wind? How many have seen the rose-pink hanging blossoms of M. dawsoniana, M. sargentiana, and its variety M. sargentiana robusta, or the huge cream-coloured flowers of M. x watsoni
Another exceptional Magnolia, which flowers all summer, is M. par. viflora. During the course of one season in Mr. Millais' garden, a single plant bloomed continuously from the middle of April until the first of September. It had a maximum of three hundred flowers on June 20th, without having less than fifty at any other time during the season of bloom.
We have discussed only a few of the wealth of trees which may be associated with Rhododendrons. It is to be hoped that these and all the other suitable trees will eventually find their way into a wider range of Pacific Northwest gardens.