JARS v52n4 - Rhododendron uwaense and Related Species in Japan

Rhododendron uwaense and Related Species in Japan
Yiji Kurashige
Laboratory of Botany
Akagi Nature Park
Gunma, Japan


In 1984, the discovery of a new species of Rhododendron in Japan surprised many people. It was found in Kochi Prefecture, Shikoku, and was named Rhododendron uwaense by Hara and Yamanaka (1984). The name "uwaense" refers to the Uwa district where it was first discovered. The Japanese name for this plant, Tokiwa-baikatsutsuji, translates as "evergreen resembling R. semibarbatum ." As the Japanese name suggests, this evergreen species has lateral inflorescence buds identical to those of R. semibarbatum , but because the calyx lobes are as long as the capsule and the presence of only five stamens, R. uwaense is classified in subgenus Azaleastrum , section Azaleastrum . Although five species of this section inhabit southeastern China, Taiwan, Laos, Vietnam, and Myanmar (Philipson and Philpson 1986), the discovery of R. uwaense greatly expands the distribution of subgenus Azaleastrum , section Azaleastrum .

Rhododendron uwaense in Japan
Figure 1. Capsules of Rhododendron uwaense in Tsushima-cho, Kochi
Photo by Yiji Kurashige

Field Studies

The author visited Tsushima-cho, Kochi in October 1993 to conduct field studies. Rhododendron uwaense is known to inhabit a very limited area, only 500m in length along a river at an elevation of 160-170m. At that site, however, the population density is quite high, with many seedlings growing on sunny dry slopes. It grows in a prominent Quercus glauca-Castanopsis cuspidata forest, sometimes mixed with cultivated Chamaecyparis obtusa and Cryptomeria japonica trees, and is associated with Rhododendron weyrichii , Acer palmatum , Deutzia crenata , D. scabra and Pieris japonica . There were some large specimens of Rhododendron uwaense up to 5m tall growing along the river. The tallest plants may be due to the fact that this was a secluded area. The leaves were pseudo-whorled at the shoot apex and alternate in the lower part or the turions, ovate-lanceolate, thin, lucid deep green, and sized 5 x 2cm. The young leaves were fringed with 0.5mm long hairs. The petioles were 1.2cm long at the mature leaves, and 5mm long at the leaves around the shoot apex. The calyx was persistent and outstanding during the fruiting season. The calyx lobes were 4mm long. The peduncles were 1.5cm in length. The capsules were semi-globose, 5mm long, divided into five parts and some had styles as long as 2cm (Figure 1). It is known that corolla is rose colored, with darker blotches on the upper lobes, rotate, 3cm across, and divided into five lobes (Figure 2). Wild populations flower in late April to early May.

Flowers of Rhododendron uwaense
Figure 2. Flowers of Rhododendron
at the habitat in
Tsushima-cho, Kochi Prefecture.
Photo by Akinori Kaneko.

There are two species of subgenus Azaleastrum , section Choniastrum , R. moulmainense (from Okinawa Prefecture) and R. latoucheae (from Amami-Ohshima, Kagoshima Prefecture) in southern Japan. The author conducted field studies for other Azaleastrums in March 1992. Rhododendron latoucheae is a small evergreen tree to 8m tall. Its only habitat is found along a river flowing from Mt. Yuwandake, the highest mountain in Amami-Ohshima, Kagoshima. There it grows in a prominent Castanopsis sieboldii subsp. lutchuensis woodland and also on sunny slopes in association with Rhododendron scabrum , R. tashiroi , Acer insulare and Deutzia neseana . Its corollas are 4-5cm across, tubular funnel-shaped, colored pale pinkish white with yellow blotches on the upper lobes (Figure 3). Rhododendron moulmainense is distributed throughout Ishigaki-jima and Iriomote-jima. It grows 5-6m tall, and in an evergreen Quercus-Castanopsis forest in association with Dendropanax trifidus , Terminalia catappa , Lasianthus japonicus , Mussaenda parviflora and Gardenia jasminoides . This species is closely related to Rhododendron latoucheae , but its leaf shape is different and it has conspicuous lateral nerves on the lower surface of the leaves.

Flowers of Rhododendron latoucheae
Figure 3. Flowers of Rhododendron latoucheae at the habitat
in Nishi-nakama, Amani-ohshima, Kagoshima Prefecture.
Photo by Yiji Kurashige.

Rhododendron semibarbatum has lateral inflorescence buds below pseudo-whorled terminal vegetative buds, and Sleumer recognized this species within subgenus Azaleastrum . This late-flowering (June-July), deciduous shrub bears small white flowers hidden in foliage which turns reddish brown to yellow in autumn (Figure 4). It is endemic to Japan, from Hokkaido in the north to Kyushu (Yakushima) in the south. The author visited several habitats in Tochigi, Gunma, Aichi, Yamaguchi, Fukuoka and Miyazaki. It grows at the edge of woodlands, in woodland, or sometimes on sunny cliffs, at elevations ranging from 135-1400m. The species grows to 2-3m tall. Figure 5 shows the distribution of subgenera Azaleastrum and Mumeazalea in Japan.

Flowers and the previous year's
capsules of Rhododendron semibarbatum
Figure 4. Flowers and the previous year's capsules of
Rhododendron semibarbatum
in cultivation at
Akagi Nature Park, Gunma Prefecture.
Photo by Yiji Kurashige.

Taxonomic Account

Sleumer (1949, 1978) classified Rhododendron species with lateral inflorescence buds in subgenus Azaleastrum , which was divided into four sections: Azaleastrum , Choniastrum , Mumeazalea , and Candidastrum based on the deciduous character of the leaves and stamen number. Philipson and Philipson (1986) revised Sleumer's classification of subgenus Azaleastrum , and they recognized three subgenera: subgenus Azaleastrum consisting of sections Azaleastrum and Choniastrum, as well as monotypic subgenus Mumeazalea ( R. semibarbatum ) and subgenus Candidastrum ( R. albiflorum ). The basis for removing R. semibarbatum from the subgenus Azaleastrum (sensu Sleumer) is mostly due to its deciduous character, dimorphic stamens and its geographic isolation. However, the author believes that subgenus Azaleastrum , section Azaleastrum is more closely related to subgenus Mumeazalea than to section Choniastrum , based on observations of R. moulmainense , R. latoucheae , R. semibarbatum , and R. uwaense in their native habitats, and would like to point out their morphological similarities. Members of section Azaleastrum have one flower per inflorescence bud (there are usually 3-5 in the section Choniastrum ), five stamens (ten in Choniastrum ), rotate corollas (narrowly funnel-shaped in Choniastrum ), and globose/semi-globose capsules (cylindrical in Choniastrum ) typical of R. uwaense and R. semibarbatum . Also, R. moulmainense and R. latoucheae are trees up to 8-10m tall, in comparison with R. semibarbatum and R. uwaense which are 2-5m tall, multi-trunked shrubs. As for the geographic separation between the subgenus Mumeazalea and the section Azaleastrum , the discovery of R. uwaense in Japan means that the distribution of section Azaleastrum and subgenus Mumeazalea overlaps.

The distribution of R. uwaense,
R. latoucheae, R. moulmainense and R. semibarbatum in Japan.
Figure 5. The distribution of R. uwaense , R. latoucheae ,
R. moulmainense and R. semibarbatum in Japan.
R. uwaense ;  □ R. latoucheae ;  ■ R. moulmainense ;  (----) R. semibarbatum .

Horticultural Significance

Rhododendron uwaense is the most northerly species in section Azaleastrum and its floristic associates are all winter-hardy species growing in the north of Japan. Therefore, this species is probably hardier than other cultivated species in the section, such as R. leptothrium , R. ovatum or R. hongkongense , which are semi-hardy when grown in the north of Japan, and it may be proven a valuable parent of hardy Azaleastum hybrids. However, we should be careful about conserving R. uwaense in its wild habitat. Although it is only thirteen years since R. uwaense was discovered, and despite keeping the location of its habitat more-or-less secret, these plants are vulnerable to collection by gardeners and nurseries.

1.  Hara, H. and Yamanaka, T. 1984. A new species of Rhododendron (Sect. Azaleastrum ) from Shikoku, Japan. Journ. Jap. Bot. 59(10):289-292.
2.  Philipson, W. R. and Philipson, M. N. 1986. A revision Rhododendron III, Subgenera Azaleastrum , Mumeazalea , Candidastrum and Therorhodion . Notes RBG Edinb. 44(1):1-23.
3.  Sleumer, H. 1949. Ein system der gattung Rhododendron L. Bot. Jahrb. Sysf. 74:511-553.
4.  _____. 1980. Past and present taxonomic systems of Rhododendron . Based on macromorphologial characters. In: Luteyn, J. L. and O'Brien, M. E. (eds.), Contributions Toward a Classification of Rhododendron , pp. 19-26. New York Bot. Gard., New York.

Mr. Kurashige is a botanist and plant collector at the Akagi Nature Park who specializes in the the genus Rhododendron.