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Tree to 12 m, though often a shrub in cultivation. Bark greyish-black, slightly white-striped. Branchlets red or brown, glabrous. Leaves deciduous to semi-evergreen, papery to leathery, 10–14 × 5–8 cm, entire or rarely 3-lobed, upper surface dark green, lower surface pale green with barbed hairs in the vein axils, margins entire to serrulate, apex caudate; petiole 2–4 cm long, glabrous; autumn colour rather limited. Inflorescence terminal, racemose, with 40–50 flowers. Flowers rather small, 5-merous, staminate or hermaphrodite; sepals ~0.3 cm long, oblong-ovate, petals greenish-yellow, same length as sepals, stamens 8, inserted inside the nectar disc. Samaras 2–2.5 cm long, brown, wings spreading obtusely, rarely horizontally. Flowering from March to April, fruiting in September (China). (van Gelderen et al. 1994; van Gelderen & van Gelderen 1999; Xu et al. 2008; Grimshaw & Bayton 2009).
Distribution Bhutan Myanmar China southeast Xizang, Yunnan India Assam, Sikkim Nepal Vietnam
Habitat Mixed forest between 1700 and 3000 m asl. Also grows as an epiphyte.
USDA Hardiness Zone 8-9
RHS Hardiness Rating H3
Conservation status Not evaluated (NE)
While far from being common in cultivation, representation of Acer sikkimense has greatly increased in recent years, however it remains restricted to areas with mild winters. Though Xu et al. (2008) do not accept any lower taxa within the species, it is the form referable to var. serrulatum that is proving hardier in parts of our area, with the typical variety of the species absent. Indeed, recent experiences suggest that the species is a little less tender than supposed even little more than a decade ago (Grimshaw & Bayton 2009), though the effects of an increasingly unpredictable climate should of course not be disregarded, nor indeed should the question of provenance, as infered in New Trees.
The species here is treated as separate from Acer hookeri, (within which Bean (1976a) placed it), the presence of which in collections is yet to be confirmed. Plants bearing the name A. hookeri have been introduced to collections including Bedgebury Pinetum in Kent (M. Dovrak, pers. comm. 2019) and collections in Cornwall but their identity remains unconfirmed. A. hookeri is distinguishable from A. sikkimense, including var. serrulatum, in its incised leaf margins and long slender pedicels, which in A. sikkimense are short and stout. This character also separates A. sikkimense from A. davidii and its affinities.
Acer medogense T. Z. Hsu & Z. K. Zhou
Subpecies serrulatum differs from subsp. sikkimense in its serrulate leaf margins (van Gelderen et al. 1994).
RHS Hardiness Rating: H4
USDA Hardiness Zone: 8-9
Taxonomic note Although reduced to synonymy by the Flora of China (Xu et al. 2008) plants grown as var. serrulatum in gardens appear to have greater hardiness than the type. It is on account of this useful horticultural trait, and its serrulate margins, that we treat these plants as a distinct entity.
Of the several collections made of Acer sikkimense var. serrulatum, the vast majority have been made from the Hoang Lien Mountains of northern Vietnam. As noted by the late Peter Wharton it seems somewhat variable in its deciduousness (Grimshaw & Bayton 2009), with a tendency for leaves to persist in favourable climes, and while some unexpected hardiness has been found within the species it should still be afforded shelter where possible.
For the several plants growing at Westonbirt, Gloucestershire, the winters of 2010–2011 and 2011–2012 proved the biggest test thus far. Though five plants were lost across these winters, where temperatures of -11.6°C were recorded (A. Vry, pers. comm. 2019), examples of both WWJ 11689 and WWJ 11703 survived, the latter being reduced to ground level then successfully resprouting. It now stands as a multistemmed plant, more than 3 m tall in January 2019 (pers. obs.). This plant has arching stems and foliage that flushes a deep red, fading to dark green though retaining its redness on the undersides of the leaves throughout the season. The neighbouring WWJ 11689 remained on a single stem following the winter of 2009–2010, though with some dieback. This plant has since begun to flower regularly, in late April. Both collections were made on Y Ty in northern Vietnam, close to the Yunnan border, by the late Peter Wharton and Bleddyn and Sue Wynn Jones. Other collections from this region include WWJ 11853 and WWJ 11613, as well as FMWJ 13166, which has survived -5.5°C at Arboretum Wespelaar, Belgium. This however also grows as a multistemmed plant, having been struck down by an April frost (J. Ossaer, pers. comm. 2019). Collections made under the VIET code in 2014, on an expedition led by Logan Botanic Garden, Scotland, may now also be found in gardens including Royal Botanic Gardens, Kew and Westonbirt.
Examples at Tregrehan include the UK and Ireland champion, measuring 13 m tall in 2014 (The Tree Register 2019). Thriving in the Cornish climate, these specimens are an arresting sight for the discerning acerologist and plants of both Chinese and Vietnamese origin grow here. Outside of our area, the species is grown in Australia and California (Grimshaw & Bayton 2009). Though several of the plants in gardens exhibit reddish foliage, similar to that of WWJ 11703, this character is not consistent in the variety, as plants with entirely green foliage are common in parts of the Hoang Lien Mountains of northern Vietnam, with both often seen growing epiphytically.