Acer diabolicum K. Koch

Introduced by Charles Maries for the Veitch Nurseries in 1880, Acer diabolicum is one of two Japanese endemics belonging to Section Lithocarpa (the other is A. amamiense). The species is named for the devil’s horn-like persistent styles attached to the insides of the winged fruits, and for the stinging hairs present on the fruits (Jackson 1927), also present in other members of the section. The ‘horned’ fruits help distinguish A. diabolicum from its relatives, as does its clear sap. The similar-leaved A. sinopurpurascens and A. tsinglingense both have lactiferous sap. The species is typically dioecious, but like other so-called dioecious maples can produce flowers (though likely sterile) of the opposite sex to that which the tree apparently belongs (pers. obs.).

Acer diabolicum is relatively infrequent in collections, owing at least in part to its functional dioecy and consequent difficulties in seed propagation. Also, while numerous maples will graft onto A. pseudoplatanus well enough when a more closely related rootstock is unavailable, that species has been found unsuitable for use with A. diabolicum (van Gelderen et al. 1994).

The Westonbirt tree noted by Bean (1976) as being nearly 14 m tall in 1966 was perhaps the same described by Jackson (1927) as being the one of the largest in UK cultivation at that time (and a male). Growing along Main Drive in the Old Arboretum it was felled in 2013 having significantly deteriorated (pers. obs.). A male tree growing elsewhere in the Old Arboretum, planted in 1937, measured 13 m tall in 2024 (The Tree Register 2025). Younger plants have also been added to the Westonbirt collection, including from seed sourced from the Chiba Faculty of Horticulture. Two trees planted side by side in 2011 have turned out to be male and female and may provide a useful source of seed in the future. Similarly, young examples at RBG Edinburgh, grown from seed collected by RBG Edinburgh staff in 2005 under BBJMT 190, are also male and female (pers. obs.). Material from another RBG Edinburgh-linked expedition to Japan, under BCJMMT 66 and 175, made in 2007, is grown at collections on both sides of the Atlantic. A plant of BCJMMT 66 at RBG Edinburgh has made a multi-stemmed tree c. 3.5 m tall by early 2026 (T. Christian, pers. comm. 2026); the same collection grows at The Morton Arboretum, Illinois. Both collections are also at Arboretum Wespelaar, where a tidy, upright specimen had attained 4 m in September 2025 (pers. obs.).

The tallest example of the species known in the UK was a tree at the Sir Harold Hillier Gardens, Hampshire, that measured 18 m tall in 2010 but has since been lost (The Tree Register 2025). A tree at Tortworth Court in Gloucestershire was recorded at 14 m in 2015, though was described then as ‘retrenching’ at the time (The Tree Register 2025). An old tree from a Charles Sargent introduction grows at the Arnold Arboretum, Massachusetts, while an old tree accessioned at the Royal Botanic Garden Edinburgh in 1906 appears still quite healthy in 2025 (T. Christian pers. comm.).