The five recognised species of Rehderodendron are found in southwest China, Myanmar and Vietnam. They are deciduous or semi-evergreen trees, with winter buds enclosed in scales. The leaves are alternate with serrate or serrulate margins; stipules absent. Inflorescences are axillary, usually below the current leaves and paniculate or racemose. Flowering occurs before or as the leaves emerge. The flowers are hermaphrodite and five-merous; the calyx tubular, five- to ten-ribbed, the corolla campanulate, divided almost to the base and there are ten stamens, five long and five short. The fruit is a woody, indehiscent capsule with 5–10 prominent ribs and a short beak; each fruit has one to three fleshy seeds (Hwang & Grimes 1996).
Discussing Rehderodendron is difficult because its diversity is poorly understood, both in the wild and in horticulture. Enthusiasts in southern England and in western North America grow a range of accessions from China and Vietnam for which no precise identifications are as yet available, and which cannot therefore be discussed in any meaningful way! Determinations will require flowering and fruiting material. One of the exciting aspects of these ‘unknown’ species is that some are precocious-flowering, the floral display appearing before the leaves unfurl – offering the promise of a stunning spectacle when the trees mature.
The only well-known Rehderodendron is R. macrocarpum, which makes a solid, wide-limbed tree whose masses of white flowers are succeeded by red, sausage-shaped fruits. It deserves the adjective ‘magnificent’ (as applied, for example, by Hillier & Coombes 2002), its only slight demerit being that the flowers appear with the foliage and are thus somewhat obscured. At the David C. Lam Asian Garden there are a number of plants derived from cuttings taken from seedlings of a Rehderodendron collected by Steve Hootman on the Daliang Shan, Sichuan in 1995, under his numbers SEH 98 and SEH 126. These have been grown under the label R. aff. macrocarpum and have made fine trees, currently up to 5–6 m high, each derived from different numbered individual seedlings. They are now flowering, and produce spectacular rose-coloured capsules. The flowers are smaller and fewer in number in the raceme than in familiar specimens of R. macrocarpum, but agree in their typical exserted stamens, and in flowering as the leaves develop. They almost certainly fall within the range of variation of R. macrocarpum, but may possibly equate to the concept of R. mapienense Hu, now sunk into R. macrocarpum. The specific epithet mapienense derives from Mabian, a region of southern Sichuan (P. Wharton pers. comm. 2008).
Styracaceae in general were great favourites of the late Peter Wharton’s, and his comments on Rehderodendron were among the last e-mails received from him in May 2008. He also wrote about R. indochinense H. L. Li: ‘We [the David C. Lam Asian Garden] have a young plant of this species derived from a collection made on the southwestern side of the main Fan Si Pan peak, near Sapa, northern Vietnam. This collection is derived from a large leaning riverside tree (23 m, 60 cm dbh) at 1480 m in a gorge of the Nam Bon river. The conspicuous red-brown barrel-shaped woody seed capsules are very distinctive, some reaching 10 × 5 cm. The impressive ovate-oblong leaves were also on average considerably larger (22 × 15 cm) than described in the Flora of China. The hardiness and the quality of the flowers of this species are at present unknown. I suspect [it] will thrive in regions with reasonable summer heat, moderate winters and ample ground moisture. The fall colour was quite a respectable clear yellow’ (pers. comm. 2008).
It seems certain that any newly introduced Rehderodendron will require similar conditions to those enjoyed by R. macrocarpum, and most Styracaceae, namely a moist acidic soil and a warm, sheltered site. Propagation is by seed, which is formed only after cross-pollination, so it is important to plant groups of any collection; or by cuttings, though these need care to make a good specimen (Wharton et al. 2005).
All Rehderodendron can exhibit exceptional growth rates once germination has been achieved, exceeding 60 cm in the first year and 45–60 cm in the second (Blackhall-Miles pers. comm. 2018) which may prove a challenge in areas where they are borderline tender and would perhaps benefit from being kept in a pot for a few years until they have built up some meaningful woody tissue. Rehderodendron are not hardy in much of the UK, with the best examples being found in the southwest of the UK and in southern Ireland, but they may be worth attempting in very mild gardens up the Atlantic seaboard of these islands.
Recent introductions of Rehderodendron species, including R. macrocarpum from a 2014 expedition to North Vietnam by the Royal Botanic Gardens Kew and Edinburgh, together with Longwood and the University of British Columbia Botanic Garden, have yet to prove themselves hardy in the UK. The plants being cultivated at Logan Botanic Garden are semi-deciduous, as they are in the wild, with the young plants being over wintered in a polytunnel retaining their leaves. Robbie Blackhall-Miles (pers. comm. 2018) confirmed that this was his experience growing Rehderodendron, and stated that they lost their foliage once planted out.
|1a||Leaves with stellate pubescence||(2)|
|1b||Leaves glabrous except for veins or when young||(3)|
|2a||Underside of leaves abaxially densely stellate-tomentose; fruit stellate-tomentose, coarsely rugose between ribs||R. kweichowense|
|2b||Underside of leaves abaxially sparsely stellate-pubescent; fruit glabrous, smooth between ribs||R. gongshanense|
|3a||Fruit brown, not spotted; leaves often with some hairs at least on veins; branchlets dark purplish||(4)|
|3b||Fruit reddish-brown with prominent pale brown spots; leaves always glabrous; branchlets reddish brown||R. indochinense|
|4a||Flowers opening at same time as leaves; corolla lobes 1.5–1.8 × 0.5–0.8 cm; stamens slightly longer than corolla||R. macrocarpum|
|4b||Flowers opening before leaves expand; corolla lobes 2–2.5 × 1–1.4 cm; stamens shorter than or equal to corolla||R. kwangtungense|