Trees As Companions for Rhododendrons
By G. G. Nearing
Abstract of a short talk to the New York Chapter
Fig. 41. R. desquamatum growing under
a flowering Japanese cherry.
R. Henny photo
The majority of rhododendrons prosper best when given some shade. Deep shade improves the foliage but discourages flowering and tends to cause lanky growth. Full sun, for those rhododendrons which can endure it, induces maximum flower production and compact plant structure, but the leaves usually suffer, taking on brownish or yellowish tints, and often burning in spots. The ideal degree of shade differs with different species and hybrids, but is always a compromise between these advantages and disadvantages.
There are many kinds of shade, from the dappled light and shadow of an open woodland to the solid cast shadow from a high evergreen planting, from shade that is uniform all day to alternate periods of shade and sun. Fortunately Rhododendrons that are hardy and vigorous can adapt themselves to various combinations, and do not insist on any shade arrangement. But for landscape beauty (and what else should we consider with these most glorious of flowering shrubs?) certain planting patterns which provide both shade and background are to be preferred.
Background, as every photographer knows, can make a picture or ruin it. The same rhododendron will not look the same when seen against a brick wall, against the sky, or against a deep shadow. Usually the most satisfactory background is the shadow. Therefore planting in front of coniferous evergreens is on the whole the most effective landscape arrangement.
A bay in a dense evergreen planting can provide the most nearly perfect environment for health and beauty both. If the bay opens to the north, northeast or northwest, it will be more or less covered by cast shadows during a considerable part of the day, yet the light of the open sky will provide a favorable growing condition. And when looked at from almost any angle, evergreen boughs and the shadows under them will furnish an ideal background.
And what about the dread north wind? The evergreens to the south of the planting will serve as a windbreak. The idea of a windbreak to the south protecting against a wind that blows out of the north, may sound dubious to the inexperienced, yet in practice it is effective. The wind does not really "blow" as we blow our breath, but is actually a suction. A gust from the north starts blowing first to the south of us. But more important, the effect of a windbreak is to make the path of the wind rise high above the ground in a gradual curve, then as gradually curve back to the ground. This creates a pocket of calm on the windward side of the break as well as one on the lee side. Anyone who has spent much time at sea will be familiar with the calm spot on the windward side of the cabin.
Since the most destructive gales may blow out of the northeast, northwest or west, as well as from due north, the bay in the windbreak, protecting from three sides, is much more effective than a straight east-west line planting. And from the point of view of design, a curved line is much to be preferred.
Bays facing southward can be utilized by setting an oak or other deciduous shade tree far enough to the south so that it will cast some shade on the Rhododendrons but not much on the evergreen background.
Large evergreens with widely spreading branches must of course be kept at a considerable distance from the rhododendron planting in order not to engulf it. Norway spruce, white pine and Canadian hemlock are typical of this class of evergreen and among the most commonly planted. In small grounds where space is limited, narrower trees such as Swiss stone pine, Japanese Cryptomeria (Lobbi), Japanese umbrella pine and Colorado spruce are more serviceable. American arborvitae and the much planted Japanese cypresses are less desirable because their winter color is not deep enough green to be effective, and because their root systems are too aggressive at the ground surface.
Pines are perhaps the best suited of all the conifers for planting with Rhododendrons, since their roots tend to run far underground, rather than compete with the rhododendron roots at the surface, while the texture of pine needles contrasts well with broadleaved evergreens. The roots of spruce and hemlock are less favorable but not objectionable, while their foliage is equally ornamental. Most of the firs are deep-rooting. Red cedar, Juniperus virginiana, is one of the less desirable. But most conifers serve well enough for planting with rhododendrons.
The shade of deciduous trees, though less satisfactory from the ornamental point of view, can create an excellent condition for the well-being of the plants. But here we have a contrast between the oaks, with their deep roots on the one hand, and on the other maples and elms with roots so near the surface that it is questionable whether rhododendrons can thrive indefinitely under them. Still if the choice is between a maple tree and no shade at all, I should prefer the maple.
In the following list, the deciduous shade trees readily available in northeastern United States are arranged roughly in the order of their desirability for shading rhododendrons, though I am doubtful about the merits of some:
Oak Locust Poplar Hickory Ash Willow Magnolia Sweet Gum Walnut Tulip Poplar Hackberry Beech Tupelo Apple Buttonwood Dogwood Cherry Elm Linden Birch Maple
Different species and clonal forms of these trees may vary considerably in their rooting habits, and the same tree will act differently in soils of different structure. Also there is much variability in the demands of different rhododendrons, both for their health and for their use in the landscape. We are told, for instance, that R. 'Lady Chamberlain' should be looked at with the setting sun behind it, but I prefer to wait another three hours, and look at it only in total darkness.