Azaleas as Refugees and Colonists on the Volcanoes
(and how to make them equally happy in a garden)
Frank Doleshy, Seattle, Washington
From a paper given at the 1975 Annual Meeting in Bellevue, Washington
In Japan, many of the azaleas and other rhododendrons have climbed onto the ash and lava fields of the volcanoes and survived there, but have disappeared from the intervening lowlands. To describe this process would require a long discussion of volcanic activity and weather changes. However, by looking at the plants now growing on volcanic material, we can find some clues to successful cultivation in our gardens.
What is a good rhododendron soil?
To start with the fundamentals, we often hear that rhododendrons favor a porous, acid soil with cool temperatures for the roots and with adequate moisture, at least during the growing season. Also, we are usually told that some shade is desirable.
This is good enough advice, but it would apply equally well to cedar or dogwood, or to buttercups or bracken fern. Yet we find wild rhododendrons thriving in places where there are few, if any, cedars, dogwoods, buttercups, or ferns; indeed, where these plants might have difficulties. This raises some questions: What are the special features of the soils where wild rhododendrons thrive? Should we try to furnish similar soils and planting sites in our gardens? And, if we want to try this, how do we go about it?
Young R. macrophyllum in the middle of a former logging
road; now a little-used truck trail on a Christmas tree farm
35 miles southwest of Seattle. There is no shade.
Photo by Frank Doleshy
Wild rhododendrons grow in soils which are anything but rich. Considering first the Seattle area, the native, evergreen R. macrophyllum is mainly a plant of gravel soils derived from the deposits of the ice sheet. In some places the rhododendrons have their upper roots in a layer of humus. However, young plants are most often seen, and appear most invasive, on old roads and other denuded areas with very little humus.
Japan has large areas of volcanic materials, and these generally do not break down into rich soils. Yet, on the lava itself and the volcanic ash, rhododendrons grow profusely. These include the evergreen azaleas of the far south, forming thickets on mountains which have erupted repeatedly during historic times, and they are even to be found in places where the land is being built up at present by the ash and other materials blasted out of the crater above. In the north, ash deposits just 350 years old on Mt. Zao support large, healthy plants of R. brachycarpum and the best R. albrechtii that Mrs. Doleshy and I have ever seen.
In central Japan, on Mt. Yatsugatake, a fresh-looking deposit of porous, clinker-like lava blocks (composed of andesite scoria) is more or less dominated by R. brachycarpum. The elevation there is 7500 feet, and the rhododendrons form dense, knee-high thickets with pine. Also, any slightly sheltered spot is likely to be full of small rhododendrons growing in moss, as crowded as the contents of an over planted flat.
The famous rhododendron of Yaku Island grows on a "soil" which is mainly composed of grit from weathering of the granite. A little to the north, on Mt. Kuju, erosion has uncovered granite beneath a lava field, and the granite blocks are about the size and shape of small van trucks standing on their noses - with very good plants of R. mettemichii growing from the other end (their roots snaking down the sides of the granite blocks under a coat of moss).
How can rhododendrons grow on such materials?
Students of plant nutrition have learned a great deal about the root activity of ordinary plants in ordinary loam soils, where the clay or humus particles conserve moisture and serve as sites for pick-up of nutrients. Here, on the other hand, we are concerned with extraordinary plants, i.e., rhododendrons and other members of the Heath Family, which grow on gravel, volcanic ash, granite grit, and other materials containing little or no clay or humus. I do not know of any fully adequate explanation of their mode of existence, but we can perhaps get some inkling by looking at their roots. These are usually a shallow mat and include a large number of small rootlets. This type of root system probably protects soil moisture and, when cool, it tends to trap water from any vapor which rises from below. Also, the numerous root fibers provide a relatively large amount of root surface area, and this may be one of the reasons why they can take up enough water and nutrients from coarse soil which contains only small amounts of clay and humus.
If clay content is large enough to have a noticeable effect on the physical character of the soil, rhododendrons are likely to die, with symptoms which suggest malnutrition, disease, or both. Perhaps the basic factor is that this kind of soil is unfavorable for some mycorhizal fungus (or "mycelium") which furnishes nutrients to the rhododendrons. Our knowledge of these relationships rhododendrons require a completely clay-free soil. A small-to-moderate amount is probably useful to them, provided that the basic character of the soil is determined by coarse materials. But the trouble is that this clay is also a plus factor for various other plants which may be able to crowd the rhododendrons cut. Therefore, it is on the clay-deficient volcanic, glacial gravels, and granite grits that rhododendrons have their competitive edge and are able to outgrow other plants, thus forming nearly pure stands.
Humus is more obviously beneficial than clay, and most growers try to give their rhododendrons as much humus as possible. Yet, as mentioned above, small plants of R. macrophyllum are often thickest on the bare gravel soil of abandoned roads, where there is little humus. Evidently the shortage of humus is not a primary factor for the rhododendrons, and the other conditions are sufficiently favorable for them that they get ahead of competing plants.
The lesson from these stands of wild plants is clear. For any heavy, high-clay-content soil, or slow-draining soil, gravel is a very useful soil additive. However, it won't do much good if the individual pieces are separated by a cushion of clay, and it should be mixed into the soil in such quantity that there is some direct contact between the pieces. Soil can also be opened up by adding leaf mould, compost, and other humus material. These have two effects. First, the fibers and other coarse debris open up passageways in heavy soil. Second, the humus gives off organic acids and other substances which act on clay, producing a more friable, crumbly structure. (As an added benefit, the humus will supply some nutrients, but it may not entirely eliminate the need for other fertilizers.)
Actually, gravel alone may solve the problem, and this solution is usually long-lasting. Size and type of gravel, in my opinion, are not very significant. I often use pea gravel, and I'm chided by a friend who prefers 7/8-inch gravel (on a somewhat heavier soil). However, I think that our theories and results are practically the same. Crushed rock may be slightly more effective than the rounded stones of ordinary gravel, but I don't think that the difference would be enough to justify a substantially higher cost.
Sun and shade
Turning from soils to the overhead environment, we are often told that rhododendrons thrive best in some shade, or quite a lot of shade. Whenever I hear this said, I think of a discussion with a member of the Shelton Chapter. Shelton is located about 50 miles southwest of Seattle, in tree farm country. This Shelton member is in the business of raising Christmas trees, and he first became interested in rhododendrons because he wanted to find out how to get rid of them in open, un-shaded, logged-off land where they were crowding his trees! The rhododendron involved is, of course, the Pacific Northwest native, R. macrophyllum. Since this is a member of the Ponticum Series, it probably gives us a good indication of how the ordinary commercial hybrids would behave in a similar area.
In Japan also, Mrs. Doleshy and I have seen R. mettemichii and R. sataense systematically chopped off at ground level to eliminate competition in reforested areas where the trees were still small. But the rhododendrons have often come back quickly from the roots and continued to crowd the small trees.
The trees, if not cut down, will eventually grow above all other plants. This is a disaster to the rhododendrons, as we see if we go back, after twenty years, to a tree farm where rhododendrons once crowded the young trees, but where the tree tops now form a canopy over the rhododendrons. Most of the latter will have died, leaving gaunt, chalky stems 10-12 feet tall - the victims of over shading. (Or, more precisely, they are probably the victims of an overhead canopy which didn't let them cool off enough to get any dew at night, Either on the leaves or in the top layer of the ground.)
One lesson from this is to avoid the planting of rhododendrons where there is a tree or a roof or a deck directly above. The other lesson is that many rhododendrons thrive in locations where they get a considerable amount of direct sun, at least in the Pacific Northwest climate. In Seattle, for example, R. fictolacteum grows well in 40% sun on a north slope, and the members of the Sanguineum Series appear more robust in a partly sunny place than in shade.
To encourage your rhododendrons to grow with the vigor which enables them to crowd out weeds and other competition, you should either avoid clay soils or else improve these with gravel or humus. When picking a planting site, avoid the places where a tree or a structure is directly overhead, and also try to select a place where there is at least a small quantity of direct sun. You can learn a great deal by going to a place where rhododendrons grow wild. While there, dig into the soil enough to see what it contains, and look carefully at the locations where small rhododendrons are coming up or large ones seem particularly healthy. Visit a volcano if you can, but a plateau of glacial gravel is also very good.