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John Grimshaw, Ross Bayton & Dan Crowley (2020)
Grimshaw, J., Bayton, R., & Crowley, D. (2020), 'Acer okamotoanum' from the website Trees and Shrubs Online (treesandshrubsonline.
A deciduous tree to 20 m in the wild. Bark pale brown to yellowish-grey. Branchlets glabrous, turning grey to brown and woody by the end of the first year. Buds ovoid, with imbricate scales. Leaves broadly pentagonal in outline, base cordate to truncate, 7–9-lobed, 12–15 cm across, margins entire, lobes shallow, broadly triangular, apically acute to acuminate, upper surface mid-green, lower surface pale green; petiole, slender, glabrous, exuding milky sap when broken; autumn colour yellow. Samaras yellowish green, 3.5–4.5 cm long, wings converging, nutlets flattened. Flowering from April to May, fruiting from September to October. (van Gelderen et al. 1994).
Distribution South Korea Ullung-do
Habitat Remnant forests.
USDA Hardiness Zone 6-7
RHS Hardiness Rating H5
Conservation status Vulnerable (VU)
Taxonomic note Treated as a subspecies of Acer pictum by van Gelderen et al. (1994) and, following them, by Grimshaw & Bayton (2009), the leaves, fruits and flowers of A. okamotoanum are larger than those of A. pictum. Meanwhile Takayama et al. (2012 p.326), using eight polymorphic microsatellite markers to analyse seven populations of A. okamotoanum in relation to seven mainland South Korean A. pictum populations (with at least 15 samples per species), concluded that ‘that clear genetic distinction exists between the two species and that A. okamotoanum has had a single origin from A. mono’ (sic). Other than elevating this taxon to species status, the text below remains unchanged from New Trees (Grimshaw & Bayton 2009).
Rarity has added piquancy to the interest in this species, but fortunately it would seem that the report of only 15 surviving specimens (van Gelderen et al. 1994) is an underestimate, as Flanagan & Kirkham (2005) recorded it as a principal constituent of woodland they visited in South Korea in 1989. There, on the island of Ullung-do, it is a large tree up to 20 m tall, with wide-spreading branches. Seed from this expedition (KFBX 168) has given rise to a very vigorous specimen at Kew, 14 m tall in 2016 (The Tree Register 2018) (from 8 m tall in 2005 (Flanagan & Kirkham 2005)), but there are several other large specimens in the United Kingdom as well, including one that was 9 m tall when measured by Owen Johnson for The Tree Register in 2003, at Batsford Park, Gloucestershire, and one that has reached 10 m at Hergest Croft (planted in 1985). The Hergest Croft tree is a fine individual with ascending branches, although it is prone to depredations by squirrels. Another large specimen (9–10 m) grows at Chevithorne Barton in Devon. Both of the latter trees are derived from a collection made in 1982 by James Harris and Joe Witt, from which only two seeds germinated (J. Harris, pers. comm. 2006). Viable seed is now produced in cultivation. In North America it is growing at the Arnold Arboretum and elsewhere. At the David C. Lam Asian Garden, Vancouver, a 12 m tree, grown from seed collected by Ferris Miller in 1980, has an 18 m spread of branches; Peter Wharton commented that it is ‘very, very vigorous’ (pers. comm. 2007). It is not fully hardy in Poland (P. Banaszczak, pers. comm. 2007).
The leaves of all the A. okamotoanum seen have had an elegant poise with upturned lobe tips, enhanced by a slightly undulate margin. They emerge light red and turn a good yellow in autumn. It grows very well on alkaline soils (J. Harris, pers. comm. 2007).