QBARS - v17n1 A Native Population of Rhododendron macrophyllum

A Native Population of Rhododendron Macrophyllum
F. L. Doleshy, Seattle, Washington

R. macrophyllum
Fig. 1. R. macrophyllum
Curtis photo

Real estate development is a destructive enterprise, carried on with machete, cruising axe, chain saw and bulldozer. If the owner is a rhododendron enthusiast, developing a rhododendron area, he feels like a vandal. But, if he does much of the work himself, he may learn something.
Boundary surveys open up a cross-country sample of vegetation, contour mapping requires a look at every ridge and dip, and the road cuts uncover soil structure in depth. Some of the lessons from a Hood Canal waterfront development have been obvious enough for immediate garden application; others are more subtle. Also it is hard to sort out the effects of soil, shading and other environmental factors. But these individual factors will serve as a framework for discussion.

Location, Companion Flora and Soil
Our area faces west at latitude 47°37'. Dominant trees are Douglas Fir, Madrona and Western Hemlock, with some Western White Pine. Salal and Evergreen Huckleberry are profuse. Small plants of interest are:

  • Indian Pipe ( Monotropa unifiora ), abundant in alluvial pads of forest duff.
  • Candystick ( Allotropa virgata ), the most decorative of the non-green-pigmented Ericaceae, scattered but not rare.
  • Coralroot ( Corallorrhiza species ), everywhere sending up sprays of tiny, brilliant orchid flowers in early summer.
  • Calypso Orchid ( Calypso bulbosa ), seen in one location.
  • Indian Paintbrush ( Castilleja species), infrequently found just above the beach-an exceptional form with upper leaves like red-tipped cogwheels.
  • Licorice Fern ( Polypodium vulgare ), common on trunks of large maples near the shoreline.

The soil appears to be glacial outwash material, with layers of sand, silt clay and various sizes of sorted gravel. Rhododendrons are generally abundant on the gravel and may form almost pure underbrush stands, especially on rather level, hard-packed surfaces. They shun the sand and silt clay. But a fine gravel with some clay content seems very satisfactory.
These differences are so sharp that one can estimate road-building costs by looking at the rhododendrons, but explanation is not so simple. Is gravel desirable because of air content, surface stability, lack of competition, the structure of the material, the amount of moisture, or all of these factors?
The best clues are found by comparing the gravel with another natural habitat, i.e., old stumps and logs. Seedlings are common on the decayed surfaces and, in one case, we found a windfall Douglas Fir log 3½ feet in diameter with healthy 4-foot rhododendrons growing out of grooves in the bark. This log turned out to be rotten for two inches under the bark, and the sound interior was cruised as a No. 1 plywood peeler.
With rhododendrons growing side by side on gravel and old logs, one may wonder whether they have any strong preferences. But the situation seems a little more clear after looking at certain common characteristics of the gravel and decayed wood, not shared by sand and/or silt clay.
First, the gravel and decayed wood seldom hold puddles of water, and they do not soak up enough water to exclude air. (Even in mid-winter, a crumbling old log will burn like a slow fuse.)
Second, gravel and decayed wood both have a stable surface which does not wash or erode like sand or silt clay. This could be critical for young rhododendrons, with their slow initial growth and lack of deep roots. But sand soils are not noticeably hostile in gardens, and this surface problem would only be important to the grower who enjoys having his plants seed themselves naturally.
Third is the factor of competition. This is elusive. When there are no rhododendrons in an area which seems lushly favorable, it is hard to decide whether they dislike some unobserved environmental factor or whether they cannot compete with other plants at this site. The lack of competition may be significant on both gravel and decayed wood, because these are rather specialized materials supporting mosses, a few ferns, conifers, and the Ericaceae . No lush weeds or grasses crowd out young shrubs before they get started.
Fourth, the structure of gravel and decayed wood may seem most dissimilar to a fire warden or staking crew. But, to a rhododendron, both materials offer openings for fine roots, and both provide a rigid structure which will not often settle and strain these roots. Therefore we have no basis for arguing with the many people who have held that soil structure is more important than soil content. Fifth and most interesting is the matter of water supply. Rhododendrons, certainly, will never be reclassified into the Cactus Family, and it is generally assumed that they cannot survive long without external supplies of water. The decayed logs are good reservoirs of dampness during much or all of the summer, but the gravel seems like a different proposition. Yet the open gravel of an abandoned logging road is often a preferred site, with young rhododendrons surviving through 40-day summer droughts which halt all forest operations. This seems extraordinary, but possible explanations are suggested in the following paragraphs.

Shade, Shelter and Moisture
There is obviously some suffering during the summer. Leaves wave limply in the dry north wind of mid afternoon. But, by morning, most plants look plump and happy. Recovery depends on location; those in open areas usually bounce back to health, while those under trees remain dejected.
An obvious effect of the open site is heavy dew. Plants which look up at the open night-time sky are able to radiate their heat and collect a full load of condensed moisture, while their neighbors under shelter remain warm and dewless. The difference is very striking during clear weather and seems at least partially responsible for the health of the Rhododendrons. More about this later. First, another problem: Even if leaves are watered by dew, what about roots in gravel soil? Can they tolerate dust-dryness? And, if they can't, where is the water coming from?
We suspect (but are not sure) that the roots also need liquid water, and we have a tentative idea as to source. Water vapor flowing up through the gravel may condense just below the surface when night-time cooling occurs. The supply of vapor should be adequate, because a deep cut through the gravel layer almost always reveals dampness at the lower margin. And, if substantial condensation can be proved, this would go far toward explaining the healthy little seedlings on open gravel surfaces. A conclusive study, with necessary instruments, would take a good deal of work, but the results would be interesting.
Returning to other observations of shaded vs. open environment, it is clear that the life history of a stand of Rhododendrons in our area is brief. After cutting or burning, Rhododendrons and other Ericaceae come in quickly. At the same time, the shade-hating Douglas Fir seedlings colonize the open mineral soil. All live happily together at first. But, in 20-30 years, the firs form a canopy and the rhododendrons start to die. First they try to become trees themselves and succeed remarkably well to a height limit of about 25 feet. Then, in a few years, there is nothing left but a tangle wood of dead trunks and branches.
If the forest lives on to a climax phase, the Douglas Fir meets the same fate, in a somewhat different way.
Young replacements of this species cannot grow under the canopy and, after several hundred years, the shade-tolerant cedar and hemlock take over the forest.
This is the "over-shading" effect, both for Rhododendrons and fir. But is the important factor a scarcity of light or of moisture? For R. macrophyllum , direct exposure to the sky seems critical. Under a canopy there is ill-health and death, even with good light from clearings at the side. Therefore it appears that the lack of dew (or other condensed vapor) is the intolerable condition.
Turning now to gardens, we have seen evidence of the same reaction. Overhead canopies appear to have damaged or killed R. wiltonii , several members of the Lacteum Series and, surprisingly, R. fictolacteum in the Falconeri series. Other species, however, remain healthy and shapely with no overhead exposure. Examples are R. orbiculare , strigillosum , makinoi and uvarifolium . Therefore it is not clear that canopies are bad for all rhododendrons. Also, one must consider another factor: The conditions favoring heavy summer dew are equally conducive to heat loss and frost in the winter. To some rhododendrons this may be more of a hazard than dewless summers. And, going a step further, it is likely that some thrive best with daily summer dampening and winter frost protection. That is, they would prefer overhead protection during the winter only, and the grower's problems are quite formidable.
We suspect that this last category includes some species in the Boothii, Glaucophyllum, Edgeworthii and Moupinense Series, and we can only suggest the following solutions:

(a) Plant them under open sky near a house, greenhouse or chimney which radiates more heat in winter than summer and thus keeps off frost without preventing dew.
(b) Plant them under a canopy of trees and apply light fog-nozzle watering each dry summer morning.
(c) Use brush protection during the winter.

Returning to the Lacteum Series, several species in our garden have reacted severely to overhead shelter. We have had no experience with R. lacteum itself, but it seems at least possible that the difficult behavior of this species stems from the same cause.

As stated earlier, the favorite germinating sites at Hood Canal are gravel soil and decayed Douglas Fir logs and stumps. (Douglas Fir is specifically mentioned because similar reproduction has not been seen on pine or hemlock, but we suspect that hemlock is equally suitable.) This suggested a trial of the same materials under civilized conditions. However, germination on gravel seems dependent on environmental factors which cannot be arranged in any ordinary greenhouse. Therefore we have experimented with decayed wood only, and this has been so successful that we no longer buy sphagnum.
For our first comparative test we had fresh seed of two species well known in the Seattle area but very dissimilar. These were:
R. camtschaticum - the popular little creeper from Bering Sea areas, listed as elepidote.
R. buxifolium - high-altitude lepidote from Borneo, the type species of the large Buxifolia Series. For each, we prepared two seed boxes which were identical except for sphagnum on the surface of one and decayed wood on the other. Both species germinated well on both materials. But, in the following weeks, it became clear that those on the decayed wood were getting somewhat ahead. And the pay-off came when transplanting from the wood. The roots of individual plants were usually centered in one small cube of this material, which could be moved to the flat with no trouble.
We have continued to use the wood without disappointments but are not certain that it would work so well under other conditions. Therefore we give the following outline of our seed-growing procedure:
All seeds are started in wooden strawberry boxes, with the germinating surface about ⅝" below the top. Coarse material is used in the lower part of the box for good drainage. The seeds are watered into the surface, then the box is covered with a 5" by 5" plate of glass and plunged into a 70° frame if there is room - otherwise it is plunged in the 50° -60° sand of the greenhouse floor. Newspaper shading is used at first but removed if it seems to interfere with germination (as with R. semibarbatum ). All watering is capillary: if practical we simply pour water on the plunging medium and let it soak up into the seed boxes. Or, if necessary, the boxes are set in a shallow pan of water until soaked. We do not interfere with condensation and drip from the cover glass, as this seems harmless and provides a nearly closed water cycle, which reduces the frequency of watering.

The impressive thing about the native Hood Canal rhododendrons is that they usually recover from diseases without serious harm. In the spring of 1961 many leaves showed symptoms of the common native Chrysomyxa piperi rust. But these leaves did not drop off or become noticeably disfigured. And, in the spring of 1962, this disease was rare or dormant. We made no detailed survey, but saw no symptoms at any time.
Also, in both years, Exobasidium infection was fairly common, causing new leaves to be colored light-ivory or flame-orange. But, in the case of one closely-observed plant, this disease was less serious in 1962 than 1961. Moreover, by midsummer of each year, the effect on new growth was barely noticeable.
Symptoms of bud blast are infrequently seen, and we have found no individual plants with a high percentage of affected buds. This supports the view that bud blast is a secondary infection, attacking buds already weakened by mechanical or other injury.
In summary, all of these diseases appear to be limited or controlled by natural factors, to such an extent that they do not seriously threaten the rhododendron population.

Pruning or Mechanical Damage
As far as we know, the usual objective of pruning and pinching is to reduce tall or irregular plants to simple spherical or dome shapes, with no geometric complexity. We don't do this, but have no criticism of those who do.
If compact plants of R. macrophyllum are desired it seems best to cut them off just above ground level and then give more sun. While re-running old survey lines we found several plants which received this treatment. Looking at them after 8-20 years. one can see that:
(a) Stems usually died back if they had been topped or cut off at an intermediate height.
(b) After cutting, numerous stems grew from near ground level, producing a dense plant.
(c) The natural habit of the plant eventually reasserted itself, producing a tall cluster of growth or a column of congested stems. But, with greatly increased sun, they remained broad and well clothed with foliage to the base.
These observations parallel the reports of plant explorers in Asia, who have often remarked on the neat little shrubs which spring up from the roots after fire or other devastation. Some of these gentlemen have suggested similar garden treatment, and their advice apparently should be heeded.

A Note on Layering
We have seen layered branches in the ground on Whidbey Island but not at Hood Canal. However, in the latter area, we have found two plants which rubbed against rotted logs and rooted into these logs. In both cases these roots were growing some distance above the ground from a main trunk more than one inch in diameter, but we have no way of knowing the size of the trunk when rooting started.

A Note on Natural Variation
Variation in R. macrophyllum is perhaps more pronounced than most people think. Flower color obviously differs from plant to plant at Hood Canal, and the variations seem to center around two main forms:
(a) Those with a uniform rose-pink color.
(b) Those with a very light base color (sometimes almost white), striped and edged with a deeper pink.
Other noticeable variations are the bright red buds seen at Mt. Jupiter, on the other side of Hood Canal, and the rather pure pink color which can be found on Whidbey Island. Also, on the western outskirts of Bremerton, we have seen some individuals with a distinctly bluish leaf color.
We are not aware of any experiments with further development of these variations. However it seems likely that good parent plants could be selfed or selectively bred to isolate and fix the most attractive characteristics. R. macrophyllum is quite a good candidate for such experimentation because it requires little care and will usually flower at a reasonable age.

In Conclusion
We want to state most emphatically that we should like to hear about other similar observations. Particularly, we are interested in overhead shading and the utilization of dew or other local condensation.
If the Editor is willing to publish our tentative views he would doubtless welcome confirmation or opposing ideas from other sources.