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Species In Our Midst
Rhododendron degronianum

by Susan Clark


Rhododendron degronianum 'Enamotot'
Picture by S & J Perkins
Rhododendron degronianum 'Enamotot' in Salem, NH


Rhododendron degronianum

Rhododendron degronianum and its subspecies and their varieties comprise some of our favorite indumented plants for northern gardens. But applying the correct name of this species to plants in our collections has required some hard work recently. We have had to make adjustments to both our labels and our thinking, as the taxonomists have combined what we used to call R. degronianum, R. metternichii, and R. yakushimanum into one species, split off R. makinoi again as a separate species, rearranged some relationships, and jettisoned the familiar name R. metternichii -- they have changed their minds more than once. The dust seems to have settled, at least for the moment, and we have been told that R. degronianum is the correct species name, with three subspecies and some varieties to remember. Metternichii has been declared an invalid name, yakushimanum has been reduced to a subspecies, and the earlier R. japonicum given to the azalea, in spite of David Chamberlain's preference for it.

For those enthusiasts who really want to know the history of the changes and the reasoning behind them, the ARS Journal Fall 1988, Volume 42:4 has an article by Frank Doleshy called "How the Japanese Pontica Fit Together". Doleshy describes how the type specimen of R. metternichii first collected by Siebold in 1835 'from the alps of north Japan' was in fact a southern species, probably a R. brachycarpum and as such an invalid specimen on which to base the species identification. The article carefully sorts out all of the closely related Japanese species and their geographic distribution. It is not for the faint-hearted. Once you have waded through Doleshy's exposition, a two-part article, Yamoto Shakunage, in the ARS Journal Summer and Fall 2003 on R. degronianum ssp. heptamerum var. hondoense f. micranthum offers further up-to-date discussion. Even the title of that article should help make clear just how complicated the relationships of the various versions of this desirable but complicated species can be.

To complicate matters further, even using the new taxonomy, R. degronianum is a very variable plant. Almost every hillside in Japan seems to have plants that differ from those on the other side of the hill. The Japanese have lovingly named each form. Plants brought into Europe and this country have been labeled with such a bewildering assortment of out-dated, illegal or inventive names that discussing this species or comparing hardiness or bloom time is quite difficult. The conservative gardener, talking stubbornly about his beloved R. metternichii, may still not be talking about the same plant as his neighbor's metternichii.

Rhododendron degronianum now is made up of three subspecies: ssp. degronianum, ssp. heptamerum (and its varieties heptamerum, hondoense and kyomaruense) and ssp. yakushimanum. R makinoi, at the moment, is a separate species. These plants are all elepidotes in Subsection Pontica and their close relatives are other favorites of New England gardeners: R. hyperythrum, R. aureum, R. ponticum, R. maximum, R. catawbiense, R. brachycarpum, R. ungernii, R. smirnowii and R. caucasicum.

All the various plants included now in R. degronianum are found only in Japan, where they grow on hilltops and mountains on Honshu, Kiyushu and Shikoku Islands, with ssp. yakushimanum only from the island of Yaku. The mainland forms grow at altitudes up to 6,000' (1800 m.) and often form dense thickets. They are generally slow growing and often wider than tall. The leaves are oblong-elliptic, usually recurved, with upper surfaces shiny dark green, sometimes olive-green, and glabrous and the undersides covered with a felted, bistrate (two-layered) indumentum, the color varying with the subspecies and variety from fawn to almost rufous. New growth is covered with a silvery tomentum. Branchlets and petioles (leaf stems) are usually glandular. Flowers are held in a full, terminal truss and are usually some kind of pink. The plants seem to be tolerant of both part sun and considerable shade. The species was named in honor of M Degron, Director of the French Posts in Yokohama in 1869.

Rhododendron degronianum ssp. degronianum (the old R. degronianum and before that R. japonicum var. pentamerum) was first described by Carriere in 1869. The obsolete varietal name 'pentamerum' was usefully descriptive, since it noted the '5 parts' of the flower. It is a slow-growing shrub forming a dense mound about 4' (1.2m) tall and 6' (1.8m) wide in early maturity. It has glossy dark green leaves up to 6" (8-14 cm) long, up to 1 3/4" (2.5 -3.5 cm) wide, and recurved at the edges. It has beautiful, thin or fairly thick felted indumentum, buff to reddish brown. New growth is attractively covered with felty, grayish tomentum. It blooms in early midseason in a domed truss of 12 -14 flowers, usually a clear rose but also found in other shades of pink. Leach rhapsodizes about their "exceptional clarity and delicacy". There are 5 corolla lobes, 10 stamens and 5 cells in the ovary. The pedicels (flower stems) are distinctly short. As befits a plant from central and northern Japan it is hardy and blooms well in the warmer parts of Zone 5, although generalization is dangerous since there are so many archaically and illegally labeled plants in US gardens. Leach rates ssp. degronianum as hardy to -15°F (-26°C) although Greer rates it only as -5°F (-26°C).

Rhododendron degronianum ssp. heptamerum used to be known as R. metternichii. It and its three varieties differ from R. degronianum ssp. degronianum in its 7 corolla lobes (with some specimens having 6, 8 or 9 lobes), 14 stamens and a 7-8 celled ovary. 'heptamerum' means 'having 7 parts'. It should be clear now why the loss of the old name for R. degronianum ssp. degronianum is a pity, since 'pentamerum' (having 5 parts) would help the poor gardener distinguish these two subspecies. ssp. heptamerum was introduced from Tokyo in 1894. Dr. Hiroshi Hara, perhaps the greatest Japanese rhododendron expert, of the University of Tokyo, thinks it the best of the Japanese evergreen rhododendrons, high praise indeed given the competition. Its identification in the trade and in gardens has been a problem since its introduction and all sorts of indumented plants have been labeled incorrectly, mistakes that are often detectable by counting those corolla lobes. ssp. heptamerum has lush reddish brown indumentum, usually thicker than ssp. degronianum, rather flat leaves with less curl on the edges, and a taller height at maturity, up to 8' (2.4m) (although Greer puts its height at only 4' (1.2 m). The flowers are widely campanulate (bell-shaped) up to 3"/8 cm across and range from the less common white to pink through rose. It makes a dense, mounded plant, spectacular not only when in bloom but the rest of the year. Leach says it is marginally hardy in the Boston area, not surprising since it comes from the more southerly parts of Japan, although my plant (if the label is correct) seems to have no problems. Greer rates it hardy to -15°F (-26°C).

R. degronianum ssp. heptamerum var. heptamerum has the most southerly distribution in the species and often the most spectacular indumentum. The Coxes, in their Encyclopedia of Rhododendron Species, consider it the most desirable variety "with superior foliage and flowers". var. hondoense (formerly R. metternichii var. hondoense) differs from var. heptamerum most notably in it thin, light-colored indumentum and its more northerly origin. The third variety, kyomaruense, differs from var. hondoense, in having (take a deep breath now!) only 5 corolla lobes, and from ssp. degronianum in its thin, plastered pale indumentum. It found only on a single peninsula on Honshu. There is also a variety of ssp. heptamerum var. hondoense f. micranthum; this form grows only at high elevations in the Ohmine mountain range in southwest Japan. It is especially prized in Japan for its small stature, leaves and flowers, as well as for its reddish-brown indumentum, which is almost as thick as ssp. yakushimanum. Forma micranthum has seven corolla lobes and is closely related to ssp. heptamerum.

Old Name Current Name
R. degronianum R. degronianum
R. metternichii R. degronianum ssp. heptamerum var. heptamerum
R. metternichii var. hondoense R. degronianum ssp. heptamerum var. hondoense
R. metternichii var. kyomaruense R. degronianum ssp. heptamerum var. kyomaruense
R. metternichii var. micranthum R. degronianum ssp. heptamerum var. hondoense f. micranthum
R. yakushimanum R. degronianum ssp. yakushimanum
R. yakushimanum ssp. makinoi R. makinoi

The seemingly endless variations of R. degronianum found on every Japanese mountainside provide the taxonomists with the challenge of making order out of this array of related plants. The taxonomy given above is the current version, but probably not the last. I have to admit to a leaning towards the view of the German horticulturist, T. Nitzelius, who in 1961 summarized his extensive field work on Japanese rhododendrons by saying that he thought that all the 5 and 7 lobed plants differed only in that feature and should not be viewed as even varietally distinct. Lump them all together and that way you can avoid the absurdity of having 5-lobed varieties of a subspecies labeled 'having 7 parts'.

R. degronianum ssp. yakushimanum is much easier to handle, once you accept that our beloved yaks are now considered a mere subspecies. Most of us still refer to them as 'yaks' and can't bring ourselves to mutter R. degronianum spp. yakushimanum. Yaks have been known as R. metternichii ssp. or var. yakushimanum. R. makinoi used to be considered a narrow-leafed subspecies of R. yakushimanum. As their name makes clear, this plant comes from Yaku Island, a small mountainous island south of Japan.

Yaks were first described by Nakai in 1921 and introduced into Europe by Lionel de Rothschild at Exbury in 1934, to whom 2 plants were sent by the famous Japanese botanist, Koichiro Wada. From Exbury yaks were grown at the Royal Horticultural Society's garden at Wisley. It was a Wisley yak, in full bloom, in the 1947 Chelsea Flower Show, that took a First Class Certificate and started the yak craze. Rhododendron fanatics fell in love with this fabulous plant and they precipitated an exuberant breeding spate that is still producing hundreds of named yak crosses.

Like the other subspecies in R. degronianum yaks are quite variable, even considering the small size of their home island. There is considerable variation in plant size, from 6-8' (1.8-2.4 m.) to 18" (46 cm) and habit, lax and open to compact and dense, leaf size 4-6" (up to 15 cm) to 1 1/2" (3.8 cm), flat to recurved, Plants from the lower elevations are more open and upright. The forms considered most desirable are compact, heavily indumented and surprisingly hardy for mountain plants from so far south; most seem bud hardy to -20°F (-29°C). Most compact specimens have dark green, glossy leaves 3 1/2" (9 cm) long and 1 1/8" (2.75 cm) wide with recurved edges. The undersides are coated with a glorious woolly indumentum, usually tawny, and thick enough to obscure the leaf's midrib. The petioles and new shoots are densely coated with a lighter tomentum, while the upper surface of the leaf starts with a floccose (cottony) tomentum that wears off during the summer. Their widely campanulate flowers (bell-shaped) come in early midseason, about 1 3/4" (4.5 cm) wide, up to10 in a dome-shaped truss. Yak flowers have 5 corolla lobes. Buds tend to be much darker than the open flowers, a deep pink, and contrast beautifully with pale pink or white flowers for the famed 'apple blossom' effect.

There has been much argument about whether some of the most popular yaks are forms of the species or natural hybrids with ssp. degronianum or R. smirnowii. 'Ken Janeck', 'Mist Maiden' and 'Pink Parasol' are all spectacular, free-flowering garden standouts, big impressive plants up to 8' (2.4 m). They have large, light-colored leaves 6-8" (up to 20 cm) quite different from the darker leaves of the high altitude plants. With their free-flowering habit and tolerance of partial sun or deep shade, they are garden mainstays. At the other end of the spectrum the 'FCC' yak and 'Koichira Wada' are very compact, mounding plants only 3' (1 m) with small, glossy dark leaves. Dick Brooks grew them side-by-side and there seemed to be very little difference between the two.

The arrival in Europe and North America of R. degronianum ssp. yakushimanum caused great excitement among rhodie growers and breeders. The yaks have been crossed with every available species and cultivar, as breeders looked to add their surprising hardiness, thick indumentum, compact habit and darker buds to their offspring. A list of all registered yak crosses would be in the hundreds and every rhodie catalog is full of wonderful sounding hybrids. All of us have our personal favorites, not to mention a tendency to want more.

R. degronianum in all its forms offers the New England gardener an abundance of hardy, gorgeous plants. Their willingness to grow in part sun makes them valuable elepidotes in the landscape. Their genes make them excellent plants for hybridizing. Whether to worry about determining the exact label for every plant depends on each gardener's temperament. Those who value accuracy and precision, as well as those who desire to use the current correct name, have some work to do to keep up with the taxonomists. The plants will look splendid in our gardens, regardless.

Susan Clark, Concord, MA


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