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Species In Our Midst
Rhododendron roxieanum

by Joe Bruso


Rhododendron roxieanum var. oreonastes
Picture by S & J Perkins
Rhododendron roxieanum var. oreonastes in Westport Point, MA


Rhododendron roxieanum

R. roxieanum is in subgenus Hymenanthes (broad-leaved evergreen rhododendrons), section Ponticum, subsection Taliensia. Members of this subsection are prized for their fine foliage, which is usually indumented (having wool-like hair underneath). The plants are generally long-lived, but slow growing, taking many years to start flowering freely. They are generally hardy in our area, but sometimes succumb to our hot summers. As with most rhododendrons, they require good drainage if root pathogens are to be avoided, and dislike high levels of nitrogen. Members of subsection Taliensia originate in central Asia, occurring in parts of Bhutan, Tibet, and western China.

R. roxieanum, named after Mrs. Roxie Hanna, a friend of George Forest, is one of the more commonly cultivated species from subsection Taliensia. Some forms are considered connoisseurs' plants. The typical form seen in cultivation is compact and upright with very narrow leaves, up to 5 inches long and up to 15 times as long as wide. R. roxieanum var. oreonastes is representative of this form.

The species, however, is quite variable with the many forms growing together in the wild. For example, in habit some varieties reach maturity in cultivation at 5 feet (up to 25 feet in the wild), and in leaf shape some forms have elliptic foliage. Indumentum color ranges from white in new foliage to cinnamon as the leaf matures. Flowering occurs at a relatively young age in some forms, but others take many years to flower. Flowers are bell-shaped, 1.5" long, creamy white to white tinged with rose, occasionally crimson spotted, in trusses of up to 20.

R. roxieanum is rated hardy to Zone 6. I've managed to keep one plant alive for 3 years (from seed) in my garden (Zone 5B). All of its siblings were wiped out their first winter outside when a late freeze followed a warm period that caused bud break. Other Chapter members are growing it in our area successfully, but it needs a fair amount of shade to survive our hot summers.

R. roxieanum is related to the smaller-leafed R. proteoides, with the two species merging in some areas of their natural range. It appears to cross freely with other species in the wild. Its variability, plus its tendency to form natural hybrids, has led to the naming of some of these hybrids as distinct species. Former species now recognized as hybrids with R. roxieanum include R. globigerum and R. alutaceum. R. alutaceum is a cross between R. roxieanum and any of several other subsection Taliensia species growing nearby such as R. phaeochrysum and R. traillianum. Kenneth Cox notes that R. roxieanum var. parvum is actually a hybrid between R. roxieanum and R. proteoides. He has found variety parvum growing between populations of these last two species in the wild, and has made this cross himself "to prove the point". He reports that R. roxieanum is "very promiscuous", forming attractive natural hybrids with R. sanguineum and R. vernicosum. All of this promiscuity and intergrading of forms in the wild provides fodder for the arguments between "splitters" and "lumpers" who classify rhododendrons.

The most common and only registered hybrid with R. roxieanum parentage is 'Blewbury' (R. roxieanum x R. anwhiense). 'Blewbury' is said to have more heat tolerance than R. roxieanum, while retaining a resemblance to R. roxieanum's foliage. Unregistered hybrids include crosses between R. roxieanum var. oreonastes and: R. bureavii, R. proteoides, R. degronianum, 'Mrs. Furnival', 'Betty Hume', and 'Barbara Hall'.

Peter Cox has crossed R. roxieanum with R. spilotum, resulting in flowers with pink stripes and a deep blotch. Ken Cox, an admitted "big roxieanum fan", states that "roxieanum has some potential as a parent of hybrids with good foliage, but it would take an F2 generation to get the free flowering and root toughness that would make a good commercial plant. Our F1 hybrids don't quite cut the mustard." Sounds like an opportunity for the hybridizers among us.

Joe Bruso, Hopkinton, MA


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