The Arnold Azalea: A Wave of Color in Spring

The Arnold Azalea, above the rock wall (photo by Aurora Santiago, May 5, 2015)

By Corinne Kennedy

The row of brightly-flowering evergreen azaleas above the rock wall at the garden’s north end is one of the Seattle Japanese Garden’s most iconic views.  In spring, they form a continuous wave of brilliant magenta-pink.  Pruned in the traditional wavelike shape known as o-karikomi, they evoke ocean waves or distant mountain ranges.

The history and name/s of this showy azalea are complicated, and not entirely clear.  It’s been variously referred to as Azalea ‘Arnoldiana’ (in use before all azaleas were reclassified from the genus Azalea into that of Rhododendron), Rhododendron ‘Arnoldianum,’ Rhododendron ‘Obtusum Arnoldianum’, and R. obtusum var. arnoldianum.

The word arnoldiana or arnoldianum appears in each of the above variants, and refers to Harvard University’s Arnold Arboretum, where our azalea was created.  It originated there in about 1910 as a seedling from the crossing of the two azaleas now known as Rhododendron kaempferi and R. Obtusum Group.  In the Arnold Arboretum’s records, it was originally known as Rhododendron obtusum arnoldianum, and its flower color was described as “a deep rosy mauve to red.”  Currently, their website uses the botanical name Rhododendron ‘Obtusum Arnoldianum’ and the common name Arnold Azalea.

Our azalea’s kaempferi parent, according to noted authority Clement Gray Bowers, is “one of the most beautiful and useful of hardy azaleas,” and was introduced to western gardens in 1892, when The Arnold Arboretum received seeds from Japan.  Thereafter, the arboretum began using it in their breeding program.  Its flowers have been described as “carmine-pink” -- without the bluish tones of the Arnold Azaleas in our garden.

The Arnold Azalea's flowers (photo by Aleks Monk, May 6, 2012)

The Arnold Azalea first came to Seattle’s Washington Park Arboretum (WPA) – probably as seeds, rather than plants -- decades before the opening of our Japanese Garden.  In 1938, as recorded in the WPA’s Accession Files, 150 seedlings were planted along Azalea Way.  [It’s fascinating to note the much more significant connection between the Arnold Arboretum and our WPA -- both were developed by the renowned Olmstead Brothers.  The WPA’s design and development were intentionally derived from the Olmsteads’ earlier Arnold Arboretum plan.]

Although we don’t know for certain, the Seattle Japanese Garden’s azaleas were probably propagated in the WPA’s greenhouses – from seeds or cuttings of the original Azalea Way plants.  Alternatively, our azaleas may have been transplanted from Azalea Way, or perhaps they came directly from the Arnold Arboretum.  If they were propagated from seedlings, rather than cuttings, their flower color wouldn’t be identical to the rosy-mauve of the original plants.

The Arnold Azalea has relatively small flowers, but it blooms profusely -- in late April and early May.  This compact-growing plant manifests two types of leaves.  The spring leaves are large, and turn reddish before dropping in late fall.  Small evergreen leaves emerge in summer, and stay on the plant throughout winter and spring.

Unfortunately, the Arnold Azalea hasn’t gone into commercial production, so it’s not available for admirers to purchase at retail nurseries.  The Arnold Arboretum apparently selected several exceptional plants from the original arnoldianum seedlings.  Unlike our rock wall azaleas, they were given cultivar names -- including ‘Dexter’s Pink’ and ‘Mossieana.’  However, like our azalea, none became popular or commercially available.

If I wanted to re-create the effect of Rhododendron ‘Obtusum Arnoldianum’ in my own garden, I would probably choose Rhododendron ‘Hino-crimson’ -- a lovely evergreen azalea with small scarlet-red flowers and attractive foliage.  Planted in Pacific Northwest gardens for many decades, it’s still widely available.  I love seeing its early spring bloom in the rockeries of our older Seattle neighborhoods.  It flowers profusely, but without the magenta tones of the Arnold Azalea -- hence it combines well with most other plants.  Planted singly or in o-karikomi waves, it will add brilliant spring color to any garden.

Corinne Kennedy is a Garden Guide, frequent contributor to the Seattle Japanese Garden blog, and retired garden designer.