Azaleas and Rhododendrons at the U. S. National Arboretum
By Henry T. Skinner
Director, National Arboretum
Agricultural Research Service, U. S. Department of Agriculture
Fig. 11. The National Arboretum
in the rhododendron area.
Fig. 12. The National Arboretum.
Mollis hybrid azalea with dogwood
and an under planting of Scilla
Fig. 14. Glen Dale hybrid witchery
in the Clonal Collection in the
Azalea plantings at the National Arboretum in Washington, D.C., are becoming so well known that automobile traffic during late April, when the Arboretum is open to visitors, presents a critical problem to staff and visitors like.
Of principal attraction at this time are the color massed slopes of Mt. Hamilton planted with some 65,000 Glenn Dale hybrids in brilliant groupings of thirty five or forty plants to a clone. These sweeps of three to seven foot azaleas ascend the curving hillside beneath a high cover of oaks and tulip poplars, are set off by an intermixed abundance of native dogwood, and at the height of their blossoming provide a color spectacle of unqualified popular appeal.
Initial plantings of these Glenn Dale azaleas were made by B. Y. Morrison, first director of the arboretum, in the early part of World War II and they were greatly extended in 1945 and '46. In the planting process trenches were dug around the hillside more or less on contour and abundant peat was used. In the normally acid clay-gravels of this coastal region the plants have succeeded beyond expectation so that no further attention has been needed, apart from spraying, even through the dual drought periods of the past season.
On the approach to this hillside display is a brick walled garden, more or less formal in design, which in a small ceremony this spring will be named The Morrison Azalea Garden in honor of the originator of the Glenn Dale Azaleas. It is already planted to a named collection of the 400 odd clones which Mr. Morrison selected. It is the initial unit of a series of such gardens which have been planned to accommodate standard reference collections of all the principal hybrid azalea groups.
At a curve near the highest point of the Mt. Hamilton roadway is a brick overlook commanding a fine view down a steep valley with its tributary streams and dissections. With the Glenn Dales behind, the view in early May from this point is across broad drifts of deciduous azaleas set off by planted conifers, dogwoods and clumps of native laurel. These deciduous azaleas in 120 clones and several thousand plants form the noteworthy collection of Ghent and Mollis hybrids presented to the Nation by the people of the Netherlands. For landscape effect they have been planted in bold masses over a broad and often inaccessible terrain. For this reason three plants of each clone have again been assembled in a restricted area where they can be conspicuously labeled and displayed in color groupings for comparative study.
The first Ghent and Mollis hybrids were planted in the spring season of 1947 with additional plantings made the following year and while they have been slower to establish themselves than the Japanese azaleas most varieties are now making good growth and set abundant flower buds. They are heavily leaf-mulched and thrive under a current program of spring feeding with high nitrogen fertilizer.
Below the main planting of Ghent and Mollis hybrids is a smaller valley devoted to the oriental species azaleas. Fine plants of R. reticulatum , flavum , mariesii , etc., may be seen in this area, while farther south is another small valley in which have been assembled the 17 odd American species with their variants and hybrids, being the partial product of research work with these species during recent years. A number of Gable, Kurume and a fairly complete set of Chugai hybrids are included in the plantings. While some of the Knap Hill and Exbury clones are in the nursery area, the several other hybrid groups are not as yet too well represented.
Fig. 13. Native azaleas.
foreground in the National Arboretum
Fig. 15. Slopes of Glenn Dale Azaleas
on the road to Mt. Hamilton in the
Fig. 16. Entrance to the Morrison Azalea
In the evergreen rhododendrons plantings are for the most part quite recent. They occupy the lower portion of Azalea valley below Mt. Hamilton in gullied wooded terrain adjacent to a crescent shaped pond constructed during the past year. Within this area is a steep sided gully devoted to the Dexter hybrids, a higher flat area for the testing of clones resultant from an arboretum breeding program and more scattered surrounding plantings of the standard and newer hybrids at present totaling approximately 150 clones. Most of the latter are small plants set out only during the past year so that their success is as yet a matter of conjecture. The only larger specimen rhododendrons occur as accents in the Dutch Azalea area and represent chiefly the older Catawba forms together with a set of some 20 of the hardy
hybrids raised in Germany several years ago by T. J. H. Seidel. About forty maturing plants of
flower well as a screen planting in the Dogwood collection and prospering specimens of
and a few other low forms are currently being moved to permanent positions. In the main, however, the better species are as yet very poorly represented.
In spite of the warm and rather dry summers of the Washington area there are good indications that correspondingly mild winters combined with the easily adapted soils and varied topography of the National Arboretum may provide the conditions under which a great many rhododendrons, as well as azaleas, will succeed here with as good prospects as perhaps anywhere on the eastern seaboard.