Acer opalus Mill.

Sponsor

Kindly sponsored by
The Wynkcoombe Arboretum

Credits

Dan Crowley (2020)

Recommended citation
Crowley, D. (2020), 'Acer opalus' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/acer/acer-opalus/). Accessed 2024-04-18.

Genus

Acer
Sect. Acer ser. Monspessulana

Common Names

Italian Maple

Synonyms

Acer italum Lauth
Acer montanum Dalechamps ex Lamarck
Acer opulifolium Vill.
Acer vernum Carriere ex Lamarck
Acer rotundifolium Lamarck
Acer apolifolium Persoon
Acer opulifolium var. opalus (Miller) Koch
Acer italicum Lauche
Acer rupicolum Chabert
Acer hispanicum Pourret

Infraspecifics

subsp. obtusatum

Other taxa in genus

Glossary

subspecies: (subsp.) Taxonomic rank for a group of organisms showing the principal characters of a species but with significant definable morphological differentiation. A subspecies occurs in populations that can occupy a distinct geographical range or habitat.

References

Credits

Dan Crowley (2020)

Recommended citation
Crowley, D. (2020), 'Acer opalus' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/acer/acer-opalus/). Accessed 2024-04-18.

Acer opalus, Monodendri, Greece, May 1994. Image Philippe de Spoelberch.

A deciduous tree to 28 m in the wild. Bark grey, turning pale brown to orange grey and broadly plated, sometimes flaking with age. Branchlets stout, glabrous or tomemtose, reddish-brown. Buds ovoid, acute tipped, with many pairs of imbricate scales, brown. Leaves chartaceous to subcoriaceous, broadly pentagonal in outline, base cordate, (3–) 5-lobed, 7–12 × 8–16 cm, lobes shallow, ovate, lateral lobes spreading, basal lobes smaller, apex acute to acuminate, margins entire to remotely serrate, upper surface mid to dark green, lower surface paler, pubescent or not, often along veins but soon glabrous, or persistently villose; petiole 7–18 cm long, green or red, faintly grooved and broadening towards the base; autumn colours yellow to brown. Inflorescence terminal, umbellate, peduncle and pedicels pubescent at first, pendulous, 6–16 flowered, 8–15 cm long. Flowers yellow, 5-merous, sepals ovate, 0.6 cm long, petals narrow-oblong, 0.5 cm long, stamens 10–12, inserted inside the nectar disc. Samaras to 2.5–5 cm long, wings spreading variously; nutlets rounded. Flowering from March to April, before or with the leaves, fruiting from September to October. (van Gelderen et al. 1994; le Hardÿ de Beaulieu 2003; Gregory 2019).

Distribution Albania Algeria Croatia France Germany Greece Hungary Italy Montenegro Morocco Serbia Slovenia Spain Switzerland

Habitat Cool areas in humid forests, often on limestone, up to 2100 m asl.

USDA Hardiness Zone 5-6

RHS Hardiness Rating H6

Awards Award of Merit

Conservation status Least concern (LC)

Acer opalus was introduced to cultivation in 1752 (Bean 1976), though perhaps as subsp. obtusatum rather than the nominate subspecies. As noted by Bean (1976), ‘it is one of the most ornamental of early-flowering trees, producing its blossoms regularly and in great abundance in March and April’. In this regard it is comparable to forms of Acer hyrcanum, though it offers more later in the year, with good shows of yellow and orange autumn colour if afforded optimal conditions.

The typical form of the species is, perhaps surprisingly, far from frequent in cultivation, and arguably somewhat overlooked. The large specimen at Westonbirt noted by Bean (1976) appears likely to be subsp. obtusatum. The majority of plants in collections appear to be of garden or unknown origin, or scantily recorded wild accessions. A recent, fully documented collection was made by staff from Westonbirt Arboretum and Wakehurst, in Lombardy, northern Italy in 2016 under WITA 72 and is now represented at Westonbirt and Bedgebury Pinetum. The parent tree grew in association with an array of other common woody species: Acer pseudoplatanus, Castanea sativa, Corylus avellana, Fraxinus ornus, Ilex aquifolium, Ostrya carpinifolia and Quercus pubescens (pers. obs. 2016). Elsewhere in its range, in the Jura mountains, it can be found with other woody taxa including Buxus sempervirens, Prunus mahaleb and Sorbus spp. (van Gelderen et al. 1994).

A great specimen of Acer opalus at Parc de la Tête d'Or, Lyon, France, May 2018. Image Thierry Lamant.

Acer opalus in culitvation at Park van Beervelde, Belgium, one of the large specimens of the country measuring 15 m high and 72 cm in dbh in 2013, here photographed in August 1989. Image Philippe de Spoelberch.

Tha bark of Acer opalus turns pale brown to orange grey and broadly plated, Schloss Schwetzingen, Germany, June 2005. Image Koen Camelbeke.

Bark of a tree in the wild, Mont Ventoux, Vaucluse, August 2015. Image Koen Camelbeke.

Inflorescence with 6–16 yellow green flowers, Gorges du Verdon, Var, France, April 2012. Image Koen Camelbeke.

Branchlets are glabrous, reddish-brown with brown, ovoid, acute tipped buds with many pairs of imbricate scales. Mont Ventoux, Vaucluse, August 2015. Image Koen Camelbeke.

The leaf undersides of Acer opalus may be pubescent along the veins. Mont Ventoux, Vaucluse, August 2015. Image Koen Camelbeke.

Samaras, 2.5–5 cm long, have rounded nutlets. Mont Ventoux, Vaucluse, August 2015. Image Koen Camelbeke.

Acer opalus turns orange-brown in Autumn. Gorges du Verdon, Moustier, Var, France, November 2011. Image Philippe de Spoelberch.

Details of leaves showing orange and yellow colours, another ornamental quality of Acer opalus. Gorges du Verdon, Var, France, November 2011. Image Philippe de Spoelberch.

Acer opalus is probably the showiest of its genus in flower. This tree at Borde Hill, West Sussex, was grown as 'A. × rotundilobum', a taxon which might involve hybridisation with A. monspessulanum. 18th April 2015. Image Owen Johnson.

Autumn colour well underway on 22nd October 2022 on a large example of Acer opalus by the vista from Kew Palace at the Royal Botanic Gardens. Image Owen Johnson.

Leaves on Acer opalus at Westonbirt Arboretum, UK. The relatively large lamina with terminal lobe apically obtuse to acute helps to distinguish this taxon within the section Acer (24 July 2007). Image © Jan De Langhe - Arboretum Wespelaar.

Fruiting specimen of Acer opalus (Westonbirt Arboretum, UK – 24 July 2007). Image © Jan De Langhe - Arboretum Wespelaar.

subsp. obtusatum (Willd.) Gams

Common Names
Balkan Maple

Synonyms
Acer obtusatum Willd
Acer aetnense Tineo ex Strobl.
Acer neopolitanum Tenore
Acer opalus var. obtusatum (Willd.) Henry
Acer opulifolium var. tomentosum Tausch
Acer opulifolium var. velutinum Boissier non A. velutinum Boisser
Acer italum var. neapolitanum (Tenore) Dieck
Acer obtusatum subsp. neapolitanum (Tenore) Pax
Acer opalus var. tomentosum (Tausch) Rehd.

Subsp. obtusatum has obtuse lobe tips and persistently pubescent lower leaf surfaces (Crowley 2018). Branchlets may be glabrous or tomentose (Crowley pers. obs.).

Distribution

Albania
Algeria
Bosnia and Herzegovina
Croatia
France
Italy
North Macedonia
Montenegro
Serbia
Slovenia

RHS Hardiness Rating: H6

USDA Hardiness Zone: 5-6

Taxonomic note Subsp. obtusatum is treated variously at infraspecific and species rank. At varietal rank, the valid name would be A. opalus var. tomentosum, as treated by Bean (1976). Italian authors tend to treat it as distinct (e.g. Guarino et al. 2017), with some including within it two subspecies: the nominate subspecies and A. obtusatum subsp. neapolitanum (Spampinato 2014). Morphologically, the two may be separated by leaf size, these being apparently larger in so-called subsp. neapolitanum. The group warrants further study, ideally in combination with the A. hyrcanum complex.

This subspecies appears to be the more common of the two in cultivation. The large, round-crowned specimens at Kew, so attractive in spring flower and autumn colour, belong to it, the distinguishing characters of blunt, rounded lobe tips and persistant pubescence on the lower leaf surfaces easily appreciated even late in the year once the leaves have dropped and rest under the trees. The example growing to the north of the Princess of Wales Conservatory is the UK and Ireland Champion and measured 18 m in 2019 (The Tree Register 2019). The origin of these, and many other large, old examples in cultivation is unclear, though modern introductions from southern Italy were made under numbers WITA 12, 13 and 50. There, it grows on limestone soils in association with Quercus spp., Fraxinus ornus, Ostrya carpinifolia and other maples, and exhibits autumn colour stronger than any shown by specimens in Britain (Crowley 2018). A plant of Croatian origin grows at RBG Edinburgh, collected by Martin Gardner and Sabina Knees in 2003 (Royal Botanic Garden Edinburgh 2018). Subsp. obtusatum is grown in collections on both coasts of the United States, present at the Hoyt Arboretum, Oregon and the Arnold Arboretum, Boston (American Public Gardens Association 2017).

The merits of Acer opalus as a flowering tree should be more widely recognised. This is subsp. obtusatum (though labelled var. tomentosum) at the Yorkshire Arboretum, 3 April 2019. Image John Grimshaw.

A beautiful specimen of Acer opalus subsp. obtusatum in bloom at Arboretum Wespelaar (Accession ID 87320), 14 March 2019 Image Philippe de Spoelberch.

The same Acer opalus subsp. obtusatum showing good Autumn colours, Accession ID 87320, Arboretum Wespelaar, 24 October 2015 Image Philippe de Spoelberch.

The reddish bracts subtending the developing inflorescences of Acer opalus subsp. obtusatum add to its beauty (Yorkshire Arboretum, March 2022). Image John Grimshaw.

Acer opalus subsp. obtusatum flowering at Kew on 4th April 2010. Image Owen Johnson.

The British and Irish Champion Acer opalus subsp. obtusatum at Kew, March 2023. Image John Grimshaw.