Although most of the rhododendrons grown in Japanese gardens came originally from China, they have been an integral part of Japanese landscape design for many centuries, prized for their lovely trumpet-shaped spring flowers. Japanese have been growing azaleas for centuries and have traditionally classified azaleas into two groups, the Tsutsuji and the Satsuki. This classification system was codified in 1692 in their first book on azaleas, Kinshu Makura (A Brocade Pillow). The Tsutsuji azaleas bloom one lunar cycle after the spring equinox and comprise the most popular azaleas in the U.S.; the Satsuki bloom two lunar cycles after the vernal equinox. The word Satsuki means "fifth month" in relation to the fifth month of the lunar calendar.
Satsuki evergreen azaleas have been hybridized in Japan for at least 500 years. The first Satsuki were probably natural hybrids of R. indicum and R. tamurae. In general, they are late-blooming (mid-May and June), with 1 to 5 inch single flowers, although some have hose-in-hose, semi-double and fully double flowers. Satsuki flower shapes range from rounded overlapping lobes to narrow wide-spaced lobes, with lobe edges ranging from flat to frilled. Flower colors vary from white to pink, yellowish pink, red, reddish orange and purple. Color patterns include solids, and stripes, flakes, lines, sectors and margins of color on a lighter background. The complete range of color patterns can appear on the same plant, differently each year. These color patterns are highly prized by the Japanese, and were carefully described and classified starting as early as 1692 in the book A Brocade Pillow.
One of the great classical gardens, built for the imperial villa at Shugakuin in the 17th Century, contains a remarkable two-tiered clipped tapestry hedge composed partly of rhododendrons of many colors planted in a band 30 feet wide across a hillside. The Japanese still like to plant rhododendrons massed on sloping sites, but they also use them as single specimens along the edges of pools and streams, under trees or against rocks. Several of the small-leaved species are pruned into rounded bushes or low cushion shapes. Not surprisingly, the Japanese have a long history of azalea breeding, and some of the hybridized azaleas most familiar to Western gardeners, such as the Kurume azaleas, are Japanese in origin.
Examples of Japanese rhododendron include: R. keiskei (Keisk rhododendron); R. metternichii (leather-leaf rhododendron); R. indicum (indica azalea); R. kuisianum (Kyushu azalea); R. linearifolium (spider azalea); R. mucronatum (snow azalea); R. degronianum ssp. yakushimanum; and the Satsuki and Kurume evergreen azaleas.
R. keiskei (Keisk rhododendron) grows no higher than 2 1/2 feet tall and has yellow flowers 2 inches across. Its oval, pointed evergreen leaves, about 2 inches long, are olive green on top, rusty brown on the undersides; in autumn, they turn bronze red.. [Return to Top]
R. metternichii (leather-leaf rhododendron) grows 8 feet tall and has oval evergreen leathery leaves, 6 inches long, glossy on top, downy and rusty brown on the undersides. The flowers are rose pink.. [Return to Top]
R. indicum (indica azalea) is one of the oldest Japanese azaleas, bred since ancient times, and it is the type most commonly used for ornamental pruning. It is a slow-growing evergreen shrub that eventually becomes 3 to 6 feet tall. Its 1 1/2-inch-long oval leaves grow on finely twigged branches. In the species, the flowers are red tinged with purple, about 2 to 3 inches wide, but it has been bred in many other colors.. [Return to Top]
R. kuisianum (Kyushu azalea) is a semi-evergreen spreading shrub, growing 3 feet tall, but up to 5 feet wide. Its leaves are bright green, about 1 inch long. The plant is covered in late spring with a solid mass of purple 1-inch-wide flowers.. [Return to Top]
R. stenopetalum 'Linearifolium' (spider azalea) is an unusual species. This species is normally 4-foot-tall shrub with narrow, ribbon-like evergreen leaves, 3 inches long and 1/2 inch wide, and rose-pink flowers with deeply divided petals. The variety 'linearifolium' (formerly R. macrosepalum 'Linearifolium' or R. linearifolium) is somewhat smaller, 3 feet tall, and the leaves are deciduous and 3 inches long. But the flowers are even more curiously shaped than those of the species; they have 1 1/4-inch-long sepals that overshadow the petals. The flowers of this variety are fragrant and have been bred in several colors; in one, 'Polypetalum', flower petals and leaves are so similar in size that the plant seems to have pink and green leaves. [Return to Top]
R. mucronatum, the snow azalea, has Chinese ancestry but in not found in the wild. It is commonly used for ornamental pruning, grows 6 feet high and 4 to 6 feet wide. It is hardy to Zone 5, -15 F. Its many branches are densely covered with oval evergreen leaves, 1 1/2 to 2 1/2 inches long, matted with hairs. Its fragrant white flowers, 1 to 3 inches wide, often bloom in pairs and may be stained with green, lilac or pink at the base, depending on the variety.. [Return to Top]
R. degronianum ssp. yakushimanum. (aka. yakusimanum) first described by Japanese botanist T. Naki in 1920. Fortunately a Japanese botanist and nurseryman Dr. Wada sent two plants to Lionel de Rothschild's garden at Exbury in the south of England in 1932. Many forms of R. yakushimanum have been identified and most are considered to be subspecies of R. degronianum. Some are identified as subspecies of R. makinoi and R. keskei. Yakushimanum has fuzzy indumentum on the undersides of its leaves and it is fairly hardy. It forms a compact well shaped plant. It is not only a popular plant, but it has by hybridized extensively for its compact habit, hardiness, and great foliage.
The Satsuki azaleas are late blooming evergreen azaleas from Japan with highly variable flower colors. They typically exhibit sporting, a characteristic where the flowers are composed of two colors and no two flowers are exactly the same except those that are all one of the two colors. Most Satsukis have flowers that will be predominantly one color with blotches of the other color. Satsukis vary in hardiness but most are not very hardy. The hardiest are hardy to -10F and some are only hardy to +30F. Since they bloom fairly late, it is best to plant in partial shade. The Gumpo azaleas are popular Satsuki azaleas in the US. This is a huge family of plants, most of which were hybridized in Japan. [Return to Top]
Kurume azaleas which originated in Japan are hardy to Zone 7, +5° F. After five to eight years in the garden, most Kurume azaleas become dense 18- to 30-inch bushes with small glossy leaves. In spring the evergreen foliage is hidden by 1-inch flowers. Typical varieties are 'Coral Bells', medium pink; 'Eureka', similar to 'Coral Bells' but hardier; 'Glory', salmon pink; 'Hershey's Red', rose red; 'Hino Crimson', crimson; 'Hinodegiri', bright red; 'Lorna', bright pink; and 'Polar Bear', white. [Return to Top]
Plants bearing the code designation H-1 survive to -25°, H-2 to -15°, H-3 to -5°, H-4 to 5°, H-5 to 15°, H-6 to 25° and H-7 to 32°. Keiskei has an H-2 rating and is hardy in Zones 5-9. Metternichii is less cold resistant and has an H-3 rating and grows best in Zones 6-10. The remaining azaleas have an H-2 rating and do well in Zones 5-10.
Rhododendrons grow best in partial shade, since full sunlight tends to bleach the flowers. They need an acid soil with a pH of 5.0 to 6.0, well mulched with organic material. Mix garden loam with equal parts of coarse sand and ground bark or oak leaves before planting. Soil around the rhododendrons' shallow roots must be kept cool and moist but well drained. All except leatherleaf rhododendrons transplant well in the spring, or in the fall if mild winter weather does not damage the shallow-rooted plants. Fertilize once a year after the plant blooms with a mixture of 5-10-5 and cottonseed meal. Prune rhododendrons after the flowers have faded to induce new growth. Most evergreen rhododendrons are propagated from cuttings rooted in sand or a mixture of sand and peat. Deciduous rhododendrons are propagated by seed, grafting or cutting.
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