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Dan Crowley (2020)
Crowley, D. (2020), 'Acer calcaratum' from the website Trees and Shrubs Online (treesandshrubsonline.
A deciduous tree to 7 m. Bark greenish-grey when young, turning darker with age. Branchlets glabrous, purplish-red or greenish, turning darker. Buds ovoid, with four pairs of scales. Leaves ovate in outline, base subcordate to rounded, 3-lobed, 6–15 × 5–21 cm, lobes broadly ovate, apically acute or acuminate, margins remotely entire, upper surface glossy green, lower surface paler, glabrous except for tufts in vein axils; petiole 2–4 cm long, red, glabrous. Inflorescence, terminal, corymbose, >10 flowered. Flowers 5-merous, usually dioecious, pedicels long and slender, sepals and petals oblong to ovate, sepals red, petals white, petals as long or longer than sepals, stamens eight, inserted inside of the nectar disc. Samaras 4–6 cm long, wings spreading obtusely to nearly horizontally. Nutlets ovoid. Flowering in April, fruiting in October. (van Gelderen et al. 1994; Xu et al. 2008).
Distribution Myanmar China Southern Yunnan Thailand Vietnam
Habitat Moist forests along water courses between 1200 and 2400 m asl.
USDA Hardiness Zone 9-10
RHS Hardiness Rating H2
Conservation status Vulnerable (VU)
This tender tree was introduced from Thailand to Sydney by the Australian botanist and horticulturist Peter Valder, and from there scions reached Plantentuin Esveld in Boskoop, from which source material was distributed to other maple enthusiasts in Europe (J. Harris, pers. comm. 2006). The species survived for a decade at Esveld but was eventually killed by frost, and A. calcaratum was feared lost from cultivation (van Gelderen & van Gelderen 1999), it seemingly not persisting in Sydney. Fortunately this was not the case as it is grown by James Harris at Mallet Court Nursery, in a polytunnel, where it seeds freely and breeds true, enabling it to be made commercially available – and it is also now growing again at Esveld. Young plants are therefore occasional in cultivation, offered periodiically by both Esveld and Mallet Court, though require a warm, sheltered position to prosper. A specimen at Arboretum Wespelaar benefits from a microclimate beneath mature Pinus nigra subsp. laricio, where temperatures can be as much as 5° C warmer than surrounding areas (K. Camelbeke, pers. comm. 2020). A specimen in central Italy was killed in temperatures of –7° C (A. Biagioli, pers. comm. 2020). Though the tree is reported as evergreen, at Wespelaar it is deciduous, appearing evergreen with tough foliage before losing its leaves in autumn (K. Camelbeke, pers. comm. 2020). Its new growth is spectacularly deep red, while its young bark is a vibrant green.