JARS v52n3 - Garden Renovation at the RSF

Garden Renovation at the RSF
Steve Hootman
Curator, Rhododendron Species Foundation
Federal Way, Washington

Botanical gardens, like all organizations, must keep up with the times, adjusting to current thoughts and ideas while utilizing and developing new techniques. In 1974, the collection of the Rhododendron Species Foundation was moved from Oregon to its present site in Federal Way, Wash. The following years were years of growing pains and change as a small group of species rhododendron aficionados matured into an organization with a worldwide membership appealing to rhododendron enthusiasts of every ilk. Now fully under the scrutiny of the public eye, we have become a "must see" garden with one of the finest and most complete collections of Rhododendron species in the world. In the process of maturing, we have learned from our successes and failures, developing our abilities in the management of a public institution and, more specifically, the cultivation of rhododendrons for public display and long-term conservation.

The recent and ongoing renovation of the garden is a prime example of how we have learned from past mistakes and have capitalized on the opportunity to undertake some major changes. For years, in certain parts of the garden, the staff has struggled to keep the collection alive and healthy under conditions extremely hostile to Rhododendron cultivation. As the garden has aged, many of the original sawdust beds have decomposed into shallow quagmires of decomposed muck. The poor drainage and extreme water-holding capacity of these sites has resulted in unhealthy root systems, negligible growth and even death. In drier areas of the garden the decomposing sawdust has all but disappeared. Often this situation results in large specimen plants lying on their sides because of lack of a proper soil in which roots could grow deeply and vigorously enough to provide the necessary support. Fortunately, these problems are not insurmountable and steps are being taken to remedy the situation. As funding and opportunity become available, we are replacing the old sawdust beds with raised buds of a custom coarse sand and bark mix.

Why was sawdust used to build raised beds in which to establish a permanent collection of shrubs and trees? Why weren't the rhododendrons planted directly into the native woodland soil? The reasons are many although ice is generally singled out as being the primary culprit. As most of you are aware, vast regions of Canada and the northern United States were heavily glaciated during the geologically recent ice ages (10,000 years ago and more). Like much of the resulting Pleistocene landscape, large areas of the Pacific Northwest were left with very little native topsoil because of advancing glaciers scraping away most of the upper soil profiles. In conjunction with this physical assault you have to keep in mind that organic matter breaks down fairly slowly in the highly leached acidic soils of temperate forests where conifers are dominant. The primary decomposers of organic materials are bacteria and fungi with bacteria generally much more efficient at this process in most situations. However, bacteria are dominant in neutral or near-neutral soils whereas the slower acting fungi are dominant in acidic soils like those found in high rainfall regions such as the Pacific Northwest. As a result, soil development through organic decomposition is much slower in our acidic coniferous forest than in the deep, highly developed neutral soils found in the deciduous hardwood forests of the Midwest, for example. Thus in the Pacific Northwest, because of a combination of chemistry and the Pleistocene, one commonly finds a very shallow soil profile on top of a rock-hard layer of hardpan clay. In summary, at the RSF there is very little native soil in which to grow rhododendrons.

When the garden was first established on this site, the answer to the problem of inadequate soil was to spread 37,000 yards of sawdust on the nearly 22 acres of beds. This was the obvious solution at the time, since there was little or no funding to purchase soil and our newly established relationship with the Weyerhaeuser Company provided access at no cost to large quantities of these seemingly suitable raw material. Fairly raw or partially decomposed sawdust is an excellent medium in which to grow rhododendrons and other ericaceous plants. It is coarse enough to provide adequate aeration at the roots while still retaining sufficient amounts of water. It is generally pH correct and is even somewhat anti-pathogenic. However, like all organic materials, sawdust eventually breaks down. This occurs much more quickly when one adds nitrogen to the equation to offset the "loss" of nutrients being consumed by the various microbiota doing the decomposing. Of course rhododendrons, especially species rhododendrons, require comparatively little in the way of supplemental nutrients and so the "loss" of nutrients should not really be taken into consideration (in most situations). It is far better to maintain the open porous nature of the sawdust than to feed a burgeoning population of decomposers which will quickly reduce your coarse, relatively inactive and stable organic medium into fine particles of humus.

This, however, is what occurred at the RSF. Large amounts of sawdust were incorporated into the shallow layer of native soil or piled directly on top. The rhododendrons flourished for several years until time, supplemental nitrogen and large amounts of water turned some areas of the garden into shallow beds of muck. One bed after another declined, resulting in a loss of material and aesthetic appeal. This also caused a major back-up of plants in the nursery with placement of new plants in the garden a virtual death sentence. What was once a yearly maintenance issue had turned into a full-scale garden makeover. Until recently, the constant lack of adequate funds left the problem unresolved.

To resolve the problem we have, over the past couple of years, initiated a bold program of renovation. Beginning in those areas which are evaluated to be at highest risk, we remove all of the existing plant material excluding the native and ornamental trees. We then salvage those rhododendrons which are still of specimen quality or for which we have no replacements. Next the decomposed sawdust is partially or completely removed before the application of a 1 to 2-foot (0.3-0.6m) layer of our sand/bark mix. In certain situations, we will spread the new mix directly over the old decomposed sawdust resulting in a stable supply of moisture deep down which can wick up through the new mix to keep the root zone more uniformly moist.

The new mix is actually a "soil-less" medium which we designed and have custom mixed specifically for these projects. It is composed of two-thirds coarse sand mixed with one-third medium bark although this ratio varies somewhat depending upon individual site conditions. It is perfect for species rhododendrons as it contains no clay or "topsoil" and so remains a fairly sterile mix with which we have greater control over nutrients and moisture retention. The sand and bark mix is quite coarse and drains quickly, preventing constant saturation in our wet winters and providing the required amounts of air in the root zone. The coarse sand is permanent and inorganic, providing the base substance for roots to grow into and from which to gain support. The bark, of course, is organic and eventually decomposes. However, this is not a problem since it is constantly replenished by the macrobiota (moles, earthworms, etc.) moving coarse organic materials into the soil profile. These organic elements are regularly applied as top dressings (mulches) to protect the root zone from temperature and moisture fluctuations. As they are broken down by the natural microbiota in the soil they release the nutrients necessary for proper plant growth. We have had remarkable success with this process and have noted tremendous root growth on weak and sickly plants once they have been moved into the new beds.

These renovations will result in a more attractive garden with healthier plants reflecting the current understanding of Rhododendron taxonomy. For most of its history the garden has been laid out on a taxonomic basis with closely related species arranged within their respective subsections or sections. This served to illustrate the differences and relationships between the subsections and individual species as well as provide the opportunity to compare individual clones within a species. Since the time when most of the collection was originally laid out, however, a great deal of knowledge has been gained about the genus Rhododendron . This has resulted primarily from the reopening of China in the early 1980s as well as recently renewed access to some of the relatively unexplored mountains of Tibet and southeastern Asia. The information gained from expeditions to these regions has significantly altered the thinking of Rhododendron taxonomists, geographers and other students of the genus. At the RSF we have been able to use this updated information to re-sort our collection and display the species in groupings which better illustrate the relationships among them. As each area is renovated, RSF Garden Manager Rick Peterson and I are able to utilize years of accumulated data and our own experience to evaluate the performance of each species and clone in our garden and climate. This performance rating is integrated with the latest in Rhododendron taxonomy, providing us with the necessary information to properly place each clone, species and even subsection into its ideal cultural, aesthetic, and taxonomic position within the newly renovated garden. Eventually most major plantings in the garden will be reworked in this manner, providing a long-term if not permanent solution to our greatest cultural problem.

Thus far we have initiated and completed renovations in only a few small sections of the garden because of lack of funding and staff time. However, these areas were some of the more visibly degraded sections with the greatest loss of plant material occurring. In subsection Neriiflora we have recently completed a small area containing the Forrestii Alliance ( R. forrestii and R. chamaethomsonii ) as well as R. catacosmum , coelicum and pocophorum of the Haematodes Alliance. This involved moving numerous large rocks into natural-appearing positions simulating the subalpine and alpine habitats in which these species are native. Small areas of the old decomposed sawdust were left uncovered to provide low-lying areas in which to plant Primula ssp. and other appropriate companion plants requiring boggy conditions.

Other areas which have been completed include the Gift Shop Courtyard which was planted with a variety of interesting and showy species to illustrate the diversity of the genus. Species such as R. degronianum ssp. yakushimanum , bureavioides , edgeworthii , forrestii, kesangiae , roxieanum , calostrotum 'Gigha', longesquamatum and hemsleyanum now flourish alongside numerous unusual ferns and perennials. The R. eclecteum area, and the beds displaying the species R. smirnowii , ungernii, brachycarpum, caucasicum and aureum of subsection Pontica have also been renovated. In the azalea area of the garden we have renovated those beds containing section Rhodora ( R. vaseyi and canadense ) as well as parts of sections Pentanthera and Brachycalyx .

The largest renovation project undertaken thus far has been the elimination of the old upper Study Garden. The upper and lower Study Gardens (lepidotes and elepidotes respectively) were planted in the earliest stages of the garden's development when the primary garden was arranged geographically. The area known as the Study Garden was planted taxonomically so that students of the genus could easily compare and contrast the various species within each subsection. Now, of course, the entire collection is presented in this manner. As this area of the garden had become redundant and contained some of the worst of the mucky beds and paths, it was an obvious place to begin a large scale renovation. This area was in such sorry shape that we were forced to close off public access for the past three years. For such a large scale project a tremendous amount of our custom soil mix was required as well as a great deal of time on the part of the staff and volunteers. Fortunately we were able to procure a substantial grant from the Pendleton and Elisabeth Carey Miller Charitable Foundation to purchase the necessary supplies and hire two student interns to assist in completing the project.

Work began in late 1997 with the removal of the old rhododendron and companion plantings. After this, approximately 525 yards of the coarse sand/bark mix was spread with wheelbarrows by Rick Peterson, the interns, and several hard-working volunteers. New paths were laid out, scenic rock work was added, and planting began this past spring. The renovated area formerly known as the upper Study Garden is approximately one acre in size and is located adjacent to an area of the main garden which was renovated in early 1997. This area had been replanted with several members of subsection Taliensia . With the sudden increase in space available for the collection, Rick and I decided to complete our reworking of subsection Taliensia . Through the spring and fall of 1998 this area will be replanted with the remaining members of the subsection including such species as R. lacteum , beesianum , wightii , aganniphum , phaeochrysum , przewalskii , bhutanense , traillianum , faberi , prattii , wasonii , clementinae , pronum and principis . Companion plantings will include Meconopsis betonicifolia (Himalayan blue poppy), Primula ssp., Acer palmatum cvs. (Japanese maples), Ariseama ssp., ferns and various magnolias.

The next large-scale project is to renovate the lower Study Garden, replanting it with the hardy members of the big-leafed subsections Falconera and Grandia . The resulting groves of rhododendron trees growing as an understory beneath the towering forest of Douglas fir should be spectacular. Please come and visit us to see the remarkable changes and exciting new plantings. If not sooner, I would recommend that you visit the Rhododendron Species Botanical Garden while you are in Seattle for the 1999 International Symposium and ARS Annual Convention next spring.

Steve Hootman is the curator at the Rhododendron Species Foundation.