JARS v48n4 - Plant Portrait: R. dauricum and R. mucronulatum

Plant Portrait: R. dauricum and R. mucronulatum
Watts with the Species
Lynn Watts
Bellevue, Washington

"Watts with the Species," a series by Lynn Watts, has appeared in the Seattle Chapter newsletter for several years

Two very desirable rhododendrons which deserve a place in every collector's garden are Rhododendron dauricum and R. mucronulatum . Although these two species are very similar in many respects, they exhibit sufficient diversity to warrant separate status according to the experts 1 .
According to H. H. Davidian, R. mucronulatum was first described in 1837. The range of distribution extends from Japan eastward and north through Korea, China, Manchuria, Mongolia, Siberia and to the Altai Mountains located in the southernmost portion of the West Siberian Plain.
Rhododendron dauricum was evidently first described by Carl Linnaeus in 1753. The range of R. dauricum overlaps that of R. mucronulatum in Siberia, Mongolia, and north central China but is also found in southern Siberia, the River Lena region of southeastern Siberia and on Hokkaido Island, Japan.
Most currently recognized authorities agree that there exists a continuum of forms from R. dauricum to R. mucronulatum , and many of the plants distributed in the trade may well represent intergrades. Rhododendron mucronulatum is generally considered to be a larger growing shrub with acute leaves which are completely deciduous and of a thinner texture than the leaves of R. dauricum which has smaller rounded leaves. However, these differences may not be sufficient reason to classify these two rhododendrons as two separate species. In a recent conversation with Frank Doleshy 2 he indicated that further work needs to be done on the Asian mainland before we can be sure of a clear line of distinction between R. dauricum and R. mucronulatum . Both R. dauricum and R. mucronulatum flower in shades of rose-purple but cultivars are available in white. One of the most striking color forms of R. mucronulatum is 'Cornell Pink'. Another color form is 'Mahogany Red'* collected in the wild by Koichiro Wada. This form has dark purplish-red flowers.

R. mucronulatum on Cheju Island, Korea
R. mucronulatum on Cheju Island, Korea
Photo by Warren Berg

In 1976 Warren and Pat Berg and Hideo Suzuki collected a dwarf form of R. mucronulatum on the top of Mt. Halla on Cheju Island, Korea. This form blooms a little later than the usual R. mucronulatum and in some clones appears to have a darker flower color. One very distinctive color form which Warren named 'Crater's Edge'* is especially compact and blooms profusely as a very young plant. During most winters here in the Pacific Northwest these dwarf forms of R. mucronulatum retain at least some of their leaves.

R. mucronulatum 'Crater's Edge'
R. mucronulatum 'Crater's Edge'
Photo by Warren Berg

A dwarf form of R. dauricum found on Hokkaido Island has been described. This attractive dwarf rhododendron has much smaller leaves than the larger growing form. It is a very compact growing dwarf which blooms later than the more common larger growing form. On one of their many trips to Japan Frank and Kay Doleshy were visiting Hokkaido Island for the purpose of establishing that R. brachycarpum was a single species. On this particular visit they were told by a forest superintendent that a population of dwarf R. dauricum was to be found near the southern tip of the island and just north of Mount Apoi. Later Frank and Kay did visit a location in central Hokkaido near Onneyu Spa where they saw a population of very dwarf growing R. dauricum . However, this population was in a birch forest which was frequently burned over so they were uncertain whether these were true dwarfs or merely the result of fire rejuvenation.
Here on the mild side of the Cascade Mountains in the Pacific Northwest both rhododendrons will frequently start flowering in December but reserve the bulk of their display for the month of February. At that time of the year when very little color is evident in the garden their floral displays are a welcome relief to a drab landscape and a promise of spring to come.
Both R. dauricum and R. mucronulatum are highly desirable species, especially valuable in colder areas. Although the bright rose-purple, pink or white flowers are relatively small the profusion of bloom provides a striking display.

Editor's Notes
1 The Cullen and Chamberlain revision classifies R. dauricum and R. mucronulatum as separate species of the Rhodorastra subsection, Rhododendron section.
2 Frank Doleshy of Edmonds, WA, is an authority on distribution of rhododendrons in Japan.
* Name is not registered.