Rhododendron Collection: May

About Rhododendron

Rhododendron / Azalea • シャクナゲ / ツツジ • 石楠花 / 躑躅 • shakunage / tsutsuji

Rhododendrons and azaleas come in many sizes and colorful varieties, some even fragrant (R. ‘Daviesii’). From dainty pastel blooms that appear before the leaves emerge, to large pinks and purples to the more fiery reds, oranges and yellows, they are guaranteed to be an eye-catching item in the garden. Flowering can occur at many different points during the year, however, in our PNW climate, it is common for most species of rhododendron and azalea, including hybrids, to bloom during the spring.

Seattle Japanese Garden is a home to some hybridized in Europe rhododendrons and azaleas (R. ‘Eddy’, R. ‘Altaclerensis’), but a special attention is given to traditional Asian varieties: some of them were taken from the wild (R. yakushimanum, R. augustinii), others are conclussions of years of complex crosses that resulted in bicolored blossoms (R. ‘Quakeress’, R. mucronatum ‘Sekidera’).

Most of them burst in flowers in May. The bright colors of the flowers, massed in such a large planting (many of them donated by the WA Park Arboretum in midst of which the Garden sits) is unusual in a Japanese Garden.

In addition to the various blooming times, the length of time a rhododendron/azalea blooms also differs. The amount of time it takes for it to grow from a bud to a full-bloom is about three weeks, but some species take longer. This can depend on the climate and temperature, so the flowering period can vary extensively from year-to-year. Once the rhododendron has fully flowered, in our climate the bloom can last from one to several weeks, depending on weather conditions.

Rhododendron 'Mars', Evergreen Azalea

Area V, 4 plants along west side path, near Yukimi-Doro Lantern. Also planted in Areas F, G, I, L & X

This evergreen azalea is a compact plant. Vivid carmine-pink flowers with a darker blotch.

The history & naming of this plant will probably never be clarified. ‘Mars’ is one of the Sander Hybrids developed in the 19th century by gardener Charles Sander. They were compact plants of a twiggy habit, but most of the named hybrids have been lost. There are also azaleas in the Japanese Garden listed as ‘Sanderi Mars,’ but their flowers are “hose-in-hose” (double rather than single flowers).

To make matters more confusing, there is also a true rhododendron named ‘Mars,’ a totally different plant. When taxonomists eliminated the genus Azalea, in many cases an identical name was used for both a rhododendron & an azalea.

Rhododendron ‘Eddy’, Evergreen Azalea

Area ZZW, a long row of 13 plants. Also planted in Area E

This is a tall, upright plant with scarlet-red flowers, borne singly or in clusters of 2 or 3. Beginning in 1999, the Japanese Garden plants have been attacked by Botrytis, a soil fungal disease that causes flower wilting and dramatically shortens the flowering period.

‘Eddy’ is an Exbury Evergreen Hybrid, introduced in England about 1933. It was considered the best of the large-flowered R. kaempferi hybrids created at Exbury. With sufficient light, it flowers profusely and has bright red fall foliage.

Rhododendron ‘Altaclerensis’ (also known as ‘Aureum grandiflorum’), Deciduous Azalea

Area E &F, lining the second shortcut. It also grows in Areas U, Y & Z

This deciduous azalea has a tall, upright growth habit. Yellow-orange buds open to fragrant flowers, bright yellow with an orange blotch.

The leaves appear during and after bloom. Powdery mildew often appears by autumn, a problem common among deciduous azaleas. It does not affect the next year’s spring leaves.

Our plants were acquired by the Arboretum in the 1940’s, and probably planted in the JG about 1961.

A different azalea is called ‘Altaclerense,’ which is white with an orange blotch. It was developed in the 1880’s at Highclere Castle, the location of the TV series Downton Abbey.

Rhododendron ‘Bouquet Rose’, Kurume Hybrid Evergreen Azalea

Area L, group of 7 plants along east side path. Also grows in Area K

The ‘Bouquet Rose’ evergreen azaleas in the Japanese Garden came from the Arboretum, and were acquired in 1958. They’re kept pruned to a low, rounded shape, and are covered with blooms in May. The flowers are rose-pink with deeper pink centers.

The Kurume azaleas were found more than 300 years ago as natural hybrids of Japanese species, in mountains on the island of Kyushu. About 700 hybrids have been developed since then, most of them not available today. Authorities differ on the identity of the original species that hybridized, but in general the plants are upright shrubs with small flowers. The name Kurume was used to describe these plants much later, beginning in 1918.

Rhododendron ‘Obtusum Arnoldianum’ (aka R. kaempferi), Evergreen Azalea

Area P, all along the top of the Fishing Village’s rock wall. Also grows in Areas G, L & S

This azalea is from the Arnold Arboretum in Massachusetts, although its exact name and cultivar or species identity is in dispute. Tit was planted in several areas of the Japanese Garden, as well as in our own Arboretum.

The small flowers are bright carmine-pink, and are prominent in the very long planting above the wall in the Fishing Village. They’re kept pruned to maintain the contoured shape. The bright color of the flower, massed in such a large planting, is unusual in a Japanese Garden.

Rhododendron ‘Moonbeam’, Glenn Dale Evergreen Azalea

Area V, by the Moon-viewing Platform. Also in Areas G, I & Z

Moonbeam’ has single, funnel-shaped, frilled white flowers with pale green dots.

‘Moonbeam’ is one of 454 Glenn Dale hybrids developed for hardiness as well as beauty in Glenn Dale, MD (a plant introduction station of the U.S. National Arboretum). They were released in the years preceding the opening of Seattle’s JG. Many of these were planted in our Arboretum, as well as in the Japanese Garden.

Some Glenn Dale Hybrids in our garden are listed by letters & numbers, rather than cultivar names. Because many National Arboretum records were lost in a fire, we may never know the true names of some of these or the names of many azaleas listed simply as “hybrid azalea.”

Rhododendron augustinii, August Rhododendron

Area W, near the entrance to the Teahouse Garden. Also grows in Area B

Rhododendron augustinii is a tall evergreen shrub with soft leaves, scaly on the undersides. The elegant flowers are 5-lobed, bell-shaped, and in clusters of 2-6. Their color ranges from white to pink, lavender, violet-blue or purple. Color on the same plant can vary from year to year. There are many named forms, including ‘Electra’ and ‘O.S.U. Form.’

This plant is hardy, easy to grow, available, and worthy of being widely grown.

It was discovered in 1886 by Augustine Henry, a medical officer in Chinese Customs, at elevations of 4500 – 13,000 feet. It’s also native to Tibet.

Rhododendron yakushimanum, Yaku Rhododendron

3 of them grow in Area ZZE, along the service path; indumentum in the pic below

This choice rhododendron has a rounded, compact form with white-felted new growth. The undersides of the leaves

are covered with woolly brown or tan “indumentum.” The bell-shaped flowers are usually rose-pink in bud, opening to white or pale pink. It has been used a great deal in hybridizing because of its excellent foliage, flowers & compact habit. When grown in shade, the form is more upright & open.

R. yakushimanum is native to the mountainous areas of the Island of Yakushima, and has been known outside Japan for only about 50 years. The island is a Unesco World Heritage site, resulting in the protection of these plants in the wild.

Rhododendron ‘Daviesii’, Deciduous Azalea

Clump of 3 in Area H, on the second shortcut

A deciduous azalea with an open, upright habit, bluish-green leaves and funnel-shaped, very fragrant cream flowers with a yellow flare. It was originally acquired by the Arboretum in 1959, before being planted in the Japanese Garden.

It belongs to the group “Ghent Hybrid Azaleas,” which refers to breeding efforts that began in Ghent, Belgium, in the early 1800’s. The “Viscosepalum Hybrids,” developed in England, include some of the earliest results of these efforts, with Rhododendron ‘Daviesii’ being a “reverse cross” of the cultivar named ‘Viscosepalum.’ In other words, the seed parent of the latter is the pollen parent of ‘Daviesii,’ and vice versa. The name Rhododendron viscosepalum ‘Daviesii’ has been used to refer to it, but our Arboretum records correctly refer to it as simply Rhododendron ‘Daviesii.’

Rhododendron japonicum, Japanese Azalea (Deciduous)

Area O, several plants on the hillside, different colors

This is an upright deciduous plant, with open, funnel-form blooms that are sometimes fragrant. The species has a great color-range -- from yellow to bright orange or salmon, to brick red or peachy/ pinkish, usually with an orange blotch. It blooms before the leaves open. Growth is vigorous, but pruning successfully maintains a lower height.

Rhododendron japonicum is an important hardy azalea native to open areas of central and northern Japan. It has been used as a parent in creating some of the English deciduous azalea hybrids. The related Chinese Azalea (R. mollis) is not as hardy.

Rhododendron ‘Quakeress’ and R. mucronatum ‘Sekidera’, Evergreen Azaleas (bicolored)

‘Quakeress,’ Area V (foreground), ‘Sekidera’ (closer to the bridge, still in buds) along west path,. Also in Area Q

Both of these plants, and some unnamed ones in Area V, consist of two colors in striking patterns. Stripes or sectors of contrasting colors occur in some of the 454 Glenn Dale Hybrids, which were developed in the 1930’s at the U.S. Arboretum Plant Introduction Station, and later released throughout the U.S. ‘Quakeress’ is one of these, created by complex crosses that included one of the parents of the Japanese Satsuki Hybrids, which originated much earlier, about 500 years ago.

The original Satsuki azaleas were natural hybrids of Rhododendron indicum & R. tamurae. Usually compact & twiggy plants, they bloomed in late May or June. Many of these plants displayed interesting color patterns, which initiated a long tradition of breeding for multiple patterns on the same plant, including solids, stripes, flakes, lines, sectors and margins. Not particularly stable, these patterns would vary significantly from year to year – even on the same plant.

Rhododendron selense, Rhododendron

Area V, west side path & R to the Princess Michiko Birch tree

This is an upright, evergreen treelike shrub, with an open habit and leathery oblong foliage. Flower color is variable, but the Japanese Garden plant has pink buds, opening to creamy white flowers with pink markings. The funnel-bell-shaped blooms are in clusters of about 8.

It is native to Tibet, Western Yunnan and Szechwan at elevations of 10,000 – 14,500 feet.