Why williamsianum?

By Emily Joseph – RSBG Retail Sales Manager & Assistant Nursery Manager
   
Of all the rhododendron species you could grow, Rhododendron williamsianum is among the easiest and most adaptable for urban and suburban gardens. We often consider it an excellent introductory choice for beginners or budding rhododendron collectors. Rounded leaves about the size of a silver dollar and an orb-like habit give this adaptable species a naturally well-shaped and tidy appearance. In the early spring, one-to-three flowered inflorescences open with large campanulate (or bell-shaped) blooms in a variety of pale (almost white) to deep pink hues.  Shortly afterward, or often at the same time, new foliage emerges from the vegetative buds in beautiful coppery salmon to bronze tones. Like most rhododendrons, this species performs best in part shade with consistent moisture. Yet unlike many rhododendrons, williamsianum can also thrive in sun and becomes somewhat drought tolerant once well-established. In ten years, one can expect this shrub to grow into a dense mound of two to three feet, though as evidenced at the RSBG, these plants continue to grow well beyond the mature estimate with age. Some of the oldest specimens in the garden, located above the magnolia grove, are now five-to-six-foot masses more than 50 years old (growing in full sun)- a testament to this shrub’s longevity in the landscape.
   
Where did Rhododendron williamsianum originate? The species’ native range is remarkably limited, confined to the rugged peaks and cliffs of just two mountains in central Sichuan, China, at elevations of 8,000–10,000 feet.This highly isolated habitat supports a very restricted natural population making this species incredibly rare and vulnerable. The species was first catalogued and introduced to Western horticulture by Ernest Wilson in 1908 during his third expedition to China, where he observed it growing in “thickets” on the cliffs of Wa-Shan in western Sichuan. J.C. Williams of Caerhays Castle, whom the species was later named for, received seeds from Wilson and cultivated the first botanical specimens for study.If you’ve ever purchased a williamsianum from, or browsed through, the RSBG nursery, you may have wondered why we offer so many different numbered forms on our labels. Each accession represents a plant selected for a particular quality or notable trait, or one with a specific history tied to the original parent plant. In living collections, plants are catalogued using unique accession numbers to track identity, provenance (origin), and taxonomy. When we propagate plants asexually (clonally), all offspring of the parent plant retain the accession number, much like a hybrid or cultivar name. Though admittedly, a number like “1966/606” certainly doesn’t provide much information about what makes this individual special. In number, this simply tells us that this was the 606th plant accessioned into the garden’s collection in 1966. So, what do these numbers mean within the RSBG williamsianum collection? There’s a method to the madness:

1966/606: A specimen originally from the Caerhays Castle collection in Cornwall, England. This form features delicate rose pink, bell-shaped flowers and is considered a highly ornamental if not simply historic selection.

1973/301: This is the lowest growing/most compact form of the species in the RSBG collection with light pink bell-shaped flowers in spring, though its provenance is unknown.

1975/277: Yu# – A wild collected form from China originally collected in 1938-1939 by Te-tsun Yu, noted Chinese botanist and co-founder of the Kunming Institute of Botany. This form is one of the larger flowering forms with pale rose pink blooms.

1975/307:For those looking for darker, more intensely saturated color in the flowers, this is one of the larger flowering forms with the darkest pink blooms, originally from the collection of James Barto, an early and avid collector and cultivator of the genus Rhododendron.

1980/055: W#1350 – The type of the species (the first scientifically described specimen) from Ernest Wilson collected on his 1906-1909 expedition to Hubei and Sichuan, China. This selection came to the RSBG from Borde Hill Gardens in the UK. It is one of the smallest-leaved forms with the palest pink flowers.

1984/004: From Arduaine Garden in Scotland, this form has the largest flowers for this species in the RSBG collection, which open until almost flat-faced. The rounded foliage is also much larger than the typical foliage form.

 

As you can see, each numbered accession has a distinct botanical provenance or defining characteristic. Features like flower color and size/habit can significantly influence garden design so considering the differences between each offering is important when designing for a specific aesthetic or space.

In addition, visual traits and hardiness in the landscape are often sought after by hybridizers aiming to breed showier, more adaptable plants for commercial use. Each parent plant contributes desirable characteristics to their hybrid offspring. Rhododendron williamsianum has long been used in hybridizing, producing some stellar cultivars originally introduced in the 1950s and 1960s that remain available today.

Rhododendron ‘Kimberly’ is an F1 hybrid (a first generation cross between two wild species) of R. williamsianum x R. fortunei ssp. fortunei. It features a compact, mounding habit, ovate foliage and deep pink, bell-shaped fragrant flowers that fade to white. In this cross, williamsianum contributes most of the habit, foliage and flower traits, while the genetics of fortunei enhance the flower size and provide fragrance. Rhododendron ‘Moonstone,’ another F1 hybrid, this time between R. campylocarpum x R. williamsianum, offers ovate leaves, a compact habit, and creamy yellow flowers flushed pink. Here campylocarpum contributes the genes necessary to achieve a yellow flowering form. ‘Cowslip,’ a cross with Rhododendrdon wardii, also produces pale yellow to creamy white blooms. Other hybrids such as ‘April Glow,’ ‘Bow Bells,’ and ‘Hummingbird’ remain popular in retail nurseries, providing gardeners with the standout traits of the species parent while having tough and tolerant constitutions in the landscape.

 

 

 

 

 

Whether you decide to grow a true williamsianum or a hybrid like ‘Moonstone,’ this attractive and adaptable group of plants deserves a notable place in ornamental gardens and designed landscapes. As always, both hybrids and species are available in our retail nursery behind the Visitor Center, which is open year-round to the public without requiring garden admission to visit. It is especially meaningful to know that when you plant an accessioned Rhododendron williamsianum, you are bringing home a small piece of botanical history, along with the RSBG’s living legacy, to continue growing in your own garden.

 
References:

Cox, P. A., & Cox, K. N. E. (1997). The Encyclopedia of Rhododendron Species. Glendoick Publishing.

Rhododendron williamsianum. Trees and Shrubs Online. Retrieved March 25, 2026, from
https://treesandshrubsonline.org/articles/rhododendron/rhododendron-williamsianum/.

Wilson, E. H., & Sargent, C. S. (1988). Plantae Wilsonianae: An enumeration of the woody plants collected in western China for the Arnold Arboretum of Harvard University during the years 1907, 1908, and 1910. Dioscordies Press.