Medium tree to 20 m tall. Branchlets densely covered with brownish-yellow lenticels. Young branchlets, stipules, rachises, petioles and inflorescences densely covered with yellowish-white glands and pubescence. Stipules leaf-like, 3–6 mm, sessile. Leaves alternate, imparipinnate, deciduous. Petiole 3–4 cm; leaf blade 15–30(–40) cm, rachis grooved and pubescent; leaflets (7–)9–13(–17), alternate, hairy on veins or in vein axils beneath; hairy on midrib above; lateral leaflets oblong-lanceolate to elliptic-ovate, smaller towards base; terminal leaflet lanceolate. Inflorescences axillary, towards tips of previous years’ growth, paniculiform, fascicled from many spikes, branches 15–30(–38) cm; flower clusters alternate, usually consisting of a hermaphrodite flower in the middle and sterile female flowers laterally. Flowers actinomorphic. Tepals 4, free. Hermaphrodite flowers: stamens 6; filament slender, erect; anther longitudinally dehiscent; pistil 1; ovary superior, covered with glands, 2-loculed but only 1 developing; stigmas free. Female flowers; pistil 1, staminodes absent. Nutlets brownish-yellow when mature, obpyriform, slightly compressed, 2–3 mm, with grey-brown glands, 4-ribbed on each side; wings rounded to ellipsoid, 5–8 mm wide. Seeds fleshy, elliptic, about 2 mm. Reports of flowering time vary wildly, and it is not clear to what extent this reflects reality: October–January (Fu et al. 2003), January–March (Sun et al. 2006 – these authors were engaged in field research on flowering, in Guizhou), March–August (Kozlowski et al. 2018). Fruiting time July–August (China) (Fu et al. 2003; Sun et al. 2006).
Distribution China N and W Guangxi, S Guizhou, SE Yunnan Vietnam North
Habitat Hillsides, valleys, streamside woods; 700–2500 m.
USDA Hardiness Zone 9-10
RHS Hardiness Rating H3
Conservation status Vulnerable (VU)
Native to southern China and northern Vietnam, the Horsetail Tree would be expected to thrive in areas with mild winters and warm, moist summers, such as the American southeast. With its dense, pendulous panicles it could prove very ornamental in both flower and fruit. The abundance of flowers gives rise to the specific epithet, from the Greek chiliarchus (an officer commanding 1000 soldiers), so ‘thousand flowered’. Unusually in the Juglandaceae, colour contributes to the interest, both in flowers (purplish or brown, depending on stage, Sun et al. 2006) and fruit (turning from green to jet black on ripening, Baines 2016). At present it is only cultivated outside East Asia in the most tentative and experimental way.
The form of the inflorescence in this wind-pollinated tree is unique in the Juglandaceae, as is the way in which female and hermaphrodite flowers are mixed throughout the length of the catkins. Despite earlier reports that the female flowers are sterile (Fu et al. 2003), Sun et al. (2006) clearly demonstrated in a wild Guizhou population that only these female flowers produce seed. While the hermaphrodite flowers appear to have perfectly formed female parts, no seed is set even when they are pollinated. On any given tree, the female organs of hermaphrodite flowers mature first, followed by their male organs, and finally the female flowers: this is so tightly controlled that there seems to be no overlap between male and female fertility anywhere on a single tree, and so these researchers found it impossible to ascertain whether this species is physiologically self compatible. Different trees go through this sequence at different times, so cross-pollination is the norm. The lesson for would-be growers is clear: an isolated specimen is most unlikely to produce any seed.
Rhoiptelea chiliantha seems to be a rare tree, whose populations consist of scattered individuals in mixed mesophytic and evergreen broadleaved forests (Sun et al. 2006; Kozlowski et al. 2018). In classifying it as Vulnerable, the IUCN (2019) cites ongoing, non-specific threats from logging and encroaching agriculture.
Growing interest in plant collecting at altitude in northern Vietnam has led to several seed introductions in recent years. Tom Hudson has young plants from a Vietnamese collection ready for planting outside in Cornwall; an earlier collection proved to be a Rhus (T. Hudson, pers. comm. 2019). Again in the United Kingdom, a plant at Westonbirt from a Peter Wharton collection has failed (D. Crowley, pers. comm. 2019). Seed was collected for the Vietnamese collection at Blarney Castle, County Cork, Ireland in 2017 and 2019. (Powers 2018; A. Whitbourn pers. comm. 2019). Adam Whitbourn (pers. comm. 2019) reports that they germinated well but proved ‘very slow’ and ‘challenging to bring on’. Seedlings of Vietnamese provenance have been offered for sale in North America, under NV 032, and with the sensible proviso ‘hardiness unknown’ (Milliken & Dodson 2019).
Rhoiptelea chiliantha remains untested as a garden or landscape tree. We reiterate that it seems most likely to succeed on the southeastern fringes of our North American area.