Acer L.

Many of the snakebark maple cultivars that have arisen in cultivation are unattributable to species, as in the case of the superb 'White Tigress', seen here at the Royal Horticultural Society's Wisley Garden. Image K. Camelbeke.

As well as being among the largest, Acer is also one of the most important genera of ornamental trees for temperate climates and is deservedly popular and widely grown. As with Quercus there is a huge array of different forms, sizes and adaptations: from the majestic Sycamore, A. pseudoplatanus, and other temperate giants such as A. saccharinum and A. macrophyllum, to xeri-adapted shrubs such as A. hyrcanum, and species like A. laurinum with the classic rain-forest adaptations of leathery evergreen, simple leaves with a long drip-tip. Throughout this diversity the pairs of opposite leaves are a valuable clue in avoiding embarrassing errors of identification. With a few exceptions it is the foliage that provides the principal attraction to gardeners, as maple leaves provide a broad palette of colours throughout the year. The first flush of spring leaves may be yellow, red, pink or purple; during the summer there is an impressive range of green shades, and cultivated maples add red, yellow and variegated forms. However, as landscape trees, it is for their often spectacular autumn colours that the maples are most renowned. This said, several members of the genus are also quite spectacular in spring flower, including European species such as A. opalus and A. monspessulanum, which may otherwise be overlooked.

The genus is separated into sections and series, divisions which were primarily based on a variety of floral, leaf and bud characters. The proposed treatment of de Jong (1976), comprising 14 sections (eight of these including two or three series), was among the first to consider phylogenetic relationships within Acer, with his treatment developed to include 16 sections by van Gelderen et al. (1994). Later, de Jong (2002) incorporated molecular data available at that time into his updated classification, recognising 19 sections, with only six of these including two to three series within them. This treatment has informed more recent studies, though Harris et al. (2017) followed the treatment of Wolfe and Tanai (1987) (comprising 21 extant sections) rather than more recent studies, on account of its consistency with some published molecular phyologenies (e.g Grimm et al. 2006; Renner et al. 2008), though these were somewhat limited by low levels of taxonomic sampling. The most recent study is that of Li et al. (2019), and although also somewhat limited, it broadly supports the classification proposed by de Jong (2002), thus, the sectional and series level treatment adopted here largely follow these studies, only deviating to follow Harris et al. (2017) in subsuming section Hyptiocarpa within a broader section Rubra. This treatment is also largely consistent with that used by Xu et al. (2008) in their account of the genus in the Flora of China, though these authors include members of section Pentaphylla series Trifida at sectional level, in Section Oblonga, while also treating Acer wardii within Section Macrantha.

Far greater debate relates to species delimitation and while the account of Grimshaw & Bayton (2009) aligned with Maples of the World (van Gelderen et al. 1994), the approach here aligns more closely with those of recent regional floras, though not without exception. It is of course unsurprising that much of the confusion within the genus pertains to taxa found in its modern centre of diversity, China, and while not wholly satisfactory, the account of Xu et al. (2008) goes some way to resolving some of the taxonomic confusion among several of the closely related species. In numerous cases further study to elucidate intra-sectional relationships is required, and mention is made of some particularly troublesome groups in the text relating to the relevant sections elsewhere in this introduction. Of particular intrigue has been the repeated nesting of Dipteronia within Acer in numerous molecular studies (e.g Pfosser et al. 2002; Tian et al. 2002; Grimm et al. 2006; Buerki et al. 2010; Yang et al. 2010), though this is yet to be widely accepted.

While identifying some of the lesser known maple species can be problematic, familiarity with the morphological characters distinguishing the sections and series can be a great aid to narrowing down their identification. A key to the sections and series, as treated here, is provided below. Members of all sections are represented in cultivation and only Series Emeiensia within Section Trifoliata is absent at series rank. A working checklist of accepted species within the genus will shortly be published by the Maple Society, while a vegetative key to the species known in western cultivation as of 2018 was published in that year (De Langhe & Crowley 2018) and is available on the Arboretum Wespelaar website. A key to all members of the genus incorporating floral and fruit characters is currently in progress (Crowley, in prep.). An oft-overlooked phenomenon exhibited by numerous species is distinctly different juvenile foliage. Young, vigourous specimens can produce markedly larger leaves, with more or less lobing, while serration and toothing can also be significantly inconsistent. Great care should be taken when identifying young specimens, with this ideally left until plants are sexually mature, though even then aberrant foliage can be produced. Characters of several leaves from different branches should be observed, avoiding extension growth.

Section Acer

Section Acer includes trees of all sizes and shrubs. The section is characterized by buds with five to 13 pairs of imbricate scales, leaves generally three or five-lobed with margins entire, dentate, crenate or serrate, and flowers held in corymbs, 5-merous with the stamens inserted in the middle of the nectar disc. The wings of the fruits tend to spread obtusely or somewhat more broadly, though not horizontally. de Jong (2002) treats three series within the section. Series Acer, includes the type species of the genus, Acer pseudoplatanus, as well as A. caesium, A. heldreichii and A. velutinum. These species are large, deciduous trees, with buds with five to 10 pairs of scales and usually five-lobed leaves with coarsely serrate margins. They have large inflorescences carrying between 25 and 150 flowers (de Jong 1976). The series spans central and southeastern Europe, western Asia and further east, along the Himalayas as far as western China, where it is represented only by A. caesium. Series Monspessulana Pojarkova includes the large A. opalus and the smaller, or even shrubby, A. hyrcanum, A. monspessulanum, A. obtusifolium and A. sempervirens. The latter two species are semi-evergreen, while the others are deciduous. They have buds with eight to twelve pairs of scales and (unlobed to) three to five-lobed leaves, often somewhat coriaceous, with entire to coarsely serrate margins. Their flowers are held in inflorescences carrying between 10 and 50 flowers, usually with long, pendulous pedicels. The species are distributed in southern Europe, northern Africa and western Asia. Series Saccharodendron (Rafinesque) Murray includes the sugar maple, A. saccharum, and its relatives. Often large, deciduous trees, they have buds with six to nine pairs of scales, leaves with three to five (rarely seven) lobed leaves, with entire to dentate margins. Their flowers, held in inflorescences of between 10 and 60, are apetalous and have connate calyces (de Jong 1976). The series is exclusively North American and has been subject to varying taxonomic interpretations. van Gelderen et al. (1994) treated the various taxa within a single, polymorphic species, following the work of Desmarais (1952). However, not all authors have been keen to treat them as such, with Weakley (2012), for example, opting to treat the taxa found in the mid-Atlantic and southeastern United States at specific rank, while contrasting approaches can be found in many other North American texts. While the group clearly warrants further study, a new species belonging to the complex, A. binzayedii, was described in 2017 from cloud forests in Mexico Vargas-Rodriguez et al. (2017). Phylogenetic studies have found Series Acer to be polyphyletic, while Series Monspessulana and Saccharodendron are monophyletic (Li et al. 2019). A. caesium has previously been found to be sister to the rest of the section (Grimm et al. 2006; Grimm et al. 2007). Li et al. (2019) found it to be closest to A. yangbiense, suggesting that this species, morphologically fitting Section Lithocarpa (Xu et al. 2008), be included within Section Acer.

Aside from A. pseudoplatanus, members of Series Acer are mostly grown in specialist collections, their attributes comparable to those of A. pseudoplatanus which is represented by a wealth of cultivars and can also be rather variable in the wild state. The easternmost member of the section, A. caesium, is known mostly in the form of its subsp. giraldii, which is one of several maples known for its glaucous stems. Members of Series Monspessulana deserve to be more widely grown. In particular, A. opalus and A. hyrcanum in full flower just before, or with, the leaves in early spring are quite spectacular, though otherwise may be overlooked. Of semi-arid habitats, A. hyrcanum, is better suited than most to hotter and dryer areas, while the same applies to A. monspessulanum and fellow Mediterranean species A. obtusifolium and A. sempervirens. A. saccharum and its relatives are much admired for their autumn colour, particularly in nature and when grown within their native ranges. Numerous cultivars are also widely grown and suitable for a range of settings, including that of the urban environment. Aside from some of the lower taxa within A. hyrcanum and A. monspessulanum, all but the recently described A. binzayedii are found in collections.

Section Arguta (Rehder) Pojarkova

Section Arguta includes shrubs and trees, the section characterized by buds with two or three pairs of valvate scales, leaves unlobed or three to five-lobed, inflorescences of male flowers axillary from leafless buds and inflorescences of female flowers terminal and axillary from mixed buds, flowers 4-merous, with four to six stamens (where present) inserted in the middle or on the outside of the nectar disc (de Jong 1976; Xu et al. 2008). Though the buds are not described by van Gelderen et al. (1994) or Xu et al. (2008) as stipitate, they can be, and care should be taken when using this character to distinguish between this section and Section Macrantha. Though both these sections include 'snakebark' maples and exhibit racemose inflorescences, the pedicels in Section Arguta are far longer, while the flowers themselves are 4-merous, whereas in Section Macrantha they are 5-merous. The section had previously been treated as a series within Section Glabra (de Jong 1976; van Gelderen et al. 1994) though at sectional rank by de Jong (2002) and Xu et al. (2008), supported by Li et al. (2019). Section Arguta differs from Section Glabra most distinctively in its 4-merous flowers (5-merous in Section Glabra). Section Arguta contains four species, distributed across the Himalayas east to Japan.

All members of section Arguta have long been in cultivation. Acer argutum and A. acuminatum have particularly elegant foliage while the bark of the latter, along with that of A. stachyophyllum, remains green for several years recalling Section Macrantha. They are generally tolerant of most soils that are not too alkaline, and prefer light shade. They have not proved to be particularly long-lived outside of optimal conditions and like many maples are highly susceptible to honey fungus (Armillaria spp.) once in a weakened state.

Section Ginnala Nakai

Section Ginnala contains deciduous shrubs and trees, notable for their small, five to 10 paired bud scales, serrated leaves, corymbose inflorescences with distinct bracts, 5-merous flowers with eight stamens inserted in the middle of the nectar disc and somewhat flat, strongly veined nutlets (de Jong 1976). The section contains a single species, Acer tataricum, as treated by van Gelderen et al. (1994), de Jong (2002) and Xu et al. (2008), comprising four subspecies. Li et al. (2019) suggest that the section is closest to sections Indivisa and Lithocarpa. Worthy of taxonomic reapprasial, it is very widespread, extending from central and southeastern Europe to eastern Asia. Largely represented by cultivars of the subspecies ginnala and tataricum, given its broad range it is unsurprising that the species/section includes trees suited to a range of soils and climates, tolerant both of excessive heat and of cold conditions.

Section Glabra Pax

Section Glabra comprises a single, browse-adapted, polymorphic species, separable from other sections by the combination of stems often faintly striated, buds with two to four pairs of valvate scales, leaves with coarsely serrated margins and inflorescences racemose or corymbose with 5-merous flowers with eight stamens (de Jong 1976; Justice in prep.). Having formerly included section Arguta at series rank (van Gelderen et al. 1994), the two are now seen as distinct from one another (de Jong 2002; Li et al. 2019). It is considered closest to the Japanese Section Parviflora (Li et al. 2019). The sole species, Acer glabrum, is native to western North America. Despite its variability in the wild, A. glabrum is not so adaptable in the garden environment, preferring cool, damp, though well drained conditions (Justice in prep.). An interesting species, particularly for its idiosyncrasies, it is found in collections but is not widely grown outside its range.

Section Indivisa Pax

Also comprising a single species, Section Indivisa is distinguished by its dioecy, abortive terminal buds, and undivided leaves with craspedodromous venation and doubly serrate margins. Its flowers, held in racemes, are 4-merous, often apetalous and usually have six stamens, inserted on the outside or inside of the nectar disc (de Jong 1976). The section is considered by Li et al. (2019) to be closest to sections Ginnala and Lithocarpa. The sole species, Acer carpinifolium is native to Japan. It is unmistakable within the genus and in leaf is more likely to be confused with members of the Betulaceae, if phyllotaxy is not well observed.

Section Lithocarpa Pax

Section Lithocarpa is defined by its buds with eight to 10 pairs of scales (often square in cross-section), three to five-lobed leaves (rarely unlobed) with dentate to serrulate margins, racemose inflorescences emerging from leafless buds, carrying 5-merous flowers usually with eight stamens (de Jong 1976; Xu et al. 2008). de Jong (1976) included two series within Section Lithocarpa, with that of Macrophylla placed alongside the nominate series, though in 2002 he treated Macrophylla at sectional rank, which is supported by Li et al. (2019). These authors suggest that the most recently described member of the section, Acer yangbiense, endemic to Yunnan Province (Chen et al. 2003), might be better placed in section Acer alongside A. caesium. Exclusively Asian, in accordance with the treatment of Xu et al. (2008), the section containst eight species in China and two in Japan.

The section contains some of the rarest members of the genus, if not always the most ornamental. Some recent introductions have proved most intriguing. A. tsinglingense, from the mountains of central China, and currently represented more in North America than Europe, is among them. Confusion between this species and A. sterculiaceum subsp. franchetii persists among parts of the maple community, though an elucidation of the identities of recent wild collections of both is being prepared (Aiello & Crowley, in prep.) The related A. amamiense, described in 2000 (Yamazaki 2000), has thus far proved surprisingly hardy and vigorous in southern England, despite its low elevation, southern Japanese origins. Intriguingly, the petioles of that species, as well as the fellow Japanese A. diabolicum and the Chinese A. sinopurpurascens and A. tsinglingense, are lactiferous. Some species also have irritant foliage, which should be avoided by those with particularly sensitive skin.

Section Macrantha Pax

Section Macrantha includes deciduous shrubs and trees, often with striated bark, its members characterized by stipitate buds with two pairs of valvate scales (the outer pair entirely enclosing the inner pair) and racemose inflorescences carrying 5-merous flowers, with eight stamens inserted on the middle of the nectar disc (de Jong 1976; Xu et al. 2008). The section is exclusively Asian apart from Acer pensylvanicum, from eastern North America. The Asian taxa are notoriously difficult to tease apart on morphology alone, including the infamous A. pectinatum complex. The treatment of that complex here aligns with that of Xu et al. (2008) to include six, separate species, though this interpration is best considered provisional. Further work, incorporating both morphological and molecular elements on this group is forthcoming (thus far molecular phylogenetic studies (notably Zhang et al. (2010)) have been somewhat limited in their sampling and have proved inconclusive). The monospecific Section Wardiana is treated within Section Macrantha by Xu et al. (2008), though as distinct by de Jong (2002).

Members of section Macrantha are popular both for the beautiful patterns and colours of their bark, especially prominent in young trees, and their autumn colours. Several species are also attractive for their extending perules in spring, which curve like wood shavings, though are soon shed. It should also be noted that not all members of the section produce persistently striated bark, with distinctions between these groups valuable aids to their identification. Nearly all the taxa are known in cultivation. The presence of A. hookeri, treated here as distinct from A. sikkimense, requires verification, and while also seemingly absent, there is some confusion around the application of the name A. insulare. It was treated as a synonym of the Taiwanese endemic A. caudatifolium by van Gelderen et al. (1994), despite being of Japanese origin. It has also been treated as a species in the account of Acer in the Flora of Japan (Ogata 1999).

Among the most widely cultivated representatives of the section is A. davidii. Numerous selections of it have been named, of which the oldest and most common are more accurately assigned to cultivar Groups, those of the George Forrest Group and the Ernest Wilson Group. Further wild gatherings of plants fitting the Ernest Wilson Group would however be most welcome, in order to further establish the extent of the geographical variation found within (or without) the species in the northern part of its range. The same applies to its subspecies grosseri, which appears close to the more recently introduced A. metcalfii in some of its characters. A. forrestii has also produced some exceptional red fruited forms, which in mid-summer appear as spectacular as any cultivated tree at that time.

The snakebarks are tough, hardy trees in much of western Europe, and in the cooler parts of North America, although they do not flourish in overly hot conditions, whether humid or dry, which excludes them from much of the United States. On the American East Coast in particular they are susceptible to a canker (R. Olsen, pers. comm. 2007 in Grimshaw & Bayton 2009). In all areas their bark is prone to being scorched in hot sun, and it is therefore advisable to plant them in a situation where they are not exposed to the hottest part of the day. It should be noted that although sometimes described in catalogues as ‘small’ trees, and undeniably charming when young, they are capable of becoming quite large, quite quickly. It is also important to note that they often form narrow crotches and are prone to break outs in high winds or if laden with snow. Resultant wounds are not only unsightly but potentially terminal as only young, vigorous plants recover well from such damage. Such circumstances may provide appropriate opportunities to test the ability of members of the section to re-sprout from the base when cut down to grown level. Young plants can often prove overly vigorous and produce thin, weak leading stems. Justice (in prep.) advocates planting saplings at an angle in order to trigger latent buds along the stem into growth, resulting in a more robust, multi-stemmed form.

Within the section are a number of selections of debated origin and parentage. Several of these, where there is reasonable doubt regarding the identity of one or both of the parents, have here been treated as distinct and not attributed to any named taxa. These include cultivars listed elsewhere under A. davidii (e.g. 'Serpentine', 'Cascade', 'Mindavi') and some likewise treated as A. × conspicuum (e.g 'Silver Cardinal', 'Silver Ghost').

Section Macrophylla Pojarkova ex Momotani

Section Macrophylla contains a single species, Acer macrophyllum, which is a large tree with buds with five to eight pairs of scales, deeply five-lobed leaves with lobulate to dentate margins and lactiferous petioles. Its corymbose infructescences are large, terminal and axillary and with 5–6-merous flowers (de Jong 1976). The section has historically been placed as a series within Section Lithocarpa (de Jong 1976; van Gelderen et al. 1994), though Li et al. (2019) suggest that it is sister to Section Platanoidea (which also have lactiferous petioles), rather than allied to Section Lithocarpa. It is distributed only in northwestern North America.

Section Negundo (Boehmer) Maximowicz

Section Negundo has buds with two to three pairs of scales, compound leaves, trifoliolate or pinnate, with margins entire to remotely dentate (de Jong 1976). Its inflorescences are racemose or compound racemose, with 4-merous flowers with four to six stamens. de Jong (1976, 2002) treated two series within the section, Cissifolia (Koidzumi) Momotani and Negundo, with this treatment strongly supported by the data of Li et al. (2019) and the section forming a clade with Section Arguta. Series Negundo has buds usually with three pairs of scales (sometimes only two), pinnate leaves, usually three to seven (rarely nine) leaflets, female flowers on racemose inflorescences and male flowers on compound racemose inflorescences, both apetalous (and entomophilous), without a nectar disc and held on long pedicels. Series Cissifolia has buds with two pairs of scales, racemose inflorescences carrying flowers with distinct calyces and corollas surrounding the nectar disc, with 4-6 stamens (de Jong 1976). Series Negundo is represented by a single species in North and Central America, Acer negundo, while Series Cissifolia has a single species in each of China and Japan, A. henryi and A. cissifolium respectively.

Acer negundo has something of a weedy reputation in parts of our area and it has successfully colonised parts of the United States to the degree that its nativeness in some areas is hard to either prove or disprove, while as an ornamental it is less desirable than its relatives on account of its brittle wood and weakly attached branches. Mature specimens in good condition are something of a rarity. Slightly more refined are its Asian relatives, A. cissifolium and A. henryi, though A. negundo var. mexicanum is quite spectacular in its new growth. Recently discovered in Hondurus, A. negundo var. mexicanum represents the southernmost distribution of the genus in the Americas (Vargas-Rodriguez et al. 2020).

Section Palmata Pax

Section Palmata taxa have abortive terminal buds, usually with four pairs of scales, unlobed or three to 13-lobed leaves, corymbose inflorescences with cincinni terminal or axillary, pedicels of male flowers often dropping after flowering, the flowers 5-merous, with eight stamens inserted on the outside of the nectar disc (de Jong 1976; van Gelderen et al. 1994). van Gelderen et al. (1994) treated three series within the section (Palmata, Sinensia Pojarkova and Penninervia Metcalf), though de Jong (2002) disregarded two of the three (upholding only Penninervia), while Li (2011), based on nuclear and chloroplast genes, found no support for clades within the section, nor did Li et al. (2019). The series formed some convenient groupings, however, and Acer campbellii and its relatives in the former Series Sinensia are still widely referred to in that context and are thus referred to as such here and included in the key to sections and series. An identication key to members of section Sinensia is included within the account of A. campbellii. Series Palmata have inflorescences with five to 25 flowers and chartaceous leaves with five to thirteen lobes and serrated margins (de Jong 1976). Series Sinensia has inflorescences carrying 20 to 250 flowers and chartaceous or coriaceous leaves with entire to serrulate margins (de Jong 1976). Series Penninervia has unlobed leaves with margins entire to serrate though are otherwise similar to Series Sinensia (de Jong 1976).

Section Palmata is the largest section of the genus, and as well as some of the most interesting members botanically, it also contains the most popular group of maples in gardens, the ‘Japanese maples’. The majority of such cultivars, selected for their foliage characteristics, are derived from A. amoenum and A. palmatum. Treated as a single species (A. palmatum) comprising three subspecies by van Gelderen et al. (1994), the Japanese have long seen them as two separate species, with A. amoenum comprising three varieties itself, as treated in the Flora of Japan (Ogata 1999). The Flora of Japan treatment is followed here for the accounts of the wild species. As well as the two aforementioned species, several 'Japanese maple' cultivars also belong to A. japonicum, A. shirasawanum and other species, along with various hybrids. While several selections referred to A. palmatum are actually hybrids, it is notable that crosses between A. amoenum and A. palmatum are little known (van Gelderen 2018). Following the classification of Vertrees & Gregory (2009), the cultivars have recently been re-classified by van Gelderen (2018), who omits species names and treats them in some 17 Groups, defined by leaf shape and colour, as well as characters of stem and bark (van Gelderen 2018). In 2008, a checklist of cultivar names was published (Gregory & Angus 2008), with more published names belonging to Japanese maples than any other group within the genus. With an update now due, there are believed to be more than 1000 named selections of Japanese maple, with the majority of recent additions selected by North American nurserymen. They are sold in huge numbers worldwide, often to be planted in unsuitable sites where they scorch and shrivel, or get frosted shortly after shoot emergence, rather than in the sheltered, rather humid sites they favour. There are, however, more resilient species within section Palmata, such as A. pseudosieboldianum and A. sieboldianum, that have many of the same qualities and deserve more attention than they seem to receive.

The A. campbellii complex, or ‘perplex’ as aptly described by Lawrence Banks (pers. comm. 2020), includes taxa distributed over a vast tract of southeastern Asia, from Nepal to Laos and Vietnam, including much of western China. Gaps in the understanding of A. campbellii and its affinities certainly remain. Treated as a broad, single species comprising five subspecies by van Gelderen et al. (1994), these taxa are treated as distinct species by Xu et al. (2008), with one taxon, A. wilsonii (A. campbellii subsp. wilsonii sensu van Gelderen et al. (1994)), referred to its own complex by Eom et al. (2011). Collectively they are attractive, often small trees, with greenish stems in youth, though not among the hardiest of the genus.

Several recent introductions belonging to this complex have been made from northern Vietnam, the southeasternmost outpost of the Himalayan flora, including collections made under the name A. heptaphlebium Gagnepain. This species was treated as synonymous with A. flabellatum (as A. campbellii subsp. flabellatum) by van Gelderen et al. (1994), though it is anything but. However, nor is it native to the mountains of northern Vietnam, thus, collections made under this name from that region usually belong to a taxon closely related to A. campbellii (Crowley 2018), as yet to be formally described (Crowley, in prep) (see the account of A. campbellii for further details). This group of maples also includes other newer additions to cultivation, including the charismatic A. fenzelianum, and A. calcaratum. Both from south eastern Asia and thus far represented by limited stock, the extent of their hardiness in our area remains to be fully explored, though is expected to be somewhat limited.

Section Parviflora Koidzumi

Section Parviflora is characterized by having buds with two to three pairs of scales, unlobed or three to five-lobed leaves with crenate to doubly serrate margins, inflorescences paniculate-corymbose to racemose, with cincinni and 35–400 or more 5-merous flowers with eight stamens (de Jong 1976). van Gelderen et al. (1994) included Section Spicata within section Parviflora as Series Caudata Pax, though the treatment of de Jong (2002) is supported by Li et al. (2019). This includes two series, each containing a single species. Series Parviflora, represented solely by Acer nipponicum, has three to five-lobed leaves and long inflorescences with 150–400 or more flowers (de Jong 1976). Series Distyla (Ogata) Murray, with A. distylum, has unlobed leaves, with upright inflorescences carrying 35-70 flowers (de Jong 1976). The section is exclusively Japanese. Both members are desirable species, though relatively infrequent in cultivation.

Section Pentaphylla Hu & Cheng

Section Pentaphylla have buds with four to eight pairs of scales, corymbose inflorescences with distinct bracts, carrying 5-merous flowers with 8 stamens, inserted in the middle of the nectar disc (van Gelderen et al. 1994). de Jong (2002) treats the section in two series, with Series Trifida alongside the nominate series. Series Trifida usually has evergreen, unlobed or 3-lobulate leaves, whereas Series Pentaphylla has compound leaves, usually 5-palmatifoliolate de Jong (1976). Xu et al. (2008) treat Series Trifida as distinct at sectional level, as Section Oblonga, though Li et al. (2019) support the treatment of de Jong (2002), which is followed here. According to Li et al. (2019), Section Trifoliata is derived from Section Pentaphylla, though is monophyletic (see also Section Trifoliata below). Series Trifida Pax contains 15 species (Xu et al. 2008), occuring in eastern and southeastern Asia, while Series Pentaphylla has a single species, the distinctive and unique A. pentaphyllum, native to southwestern China. This species has become more frequent in collections in recent years, largely as a result of the conservation work of Bill McNamara at Quarryhill Botanical Garden, California (McNamara 2011). Series Trifida is most commonly encountered in the form of A. buergerianum, while A. yui was introduced for the first time in 2005 Aiello (2006). A. coriaceifolium and A. paxii are grown on a limited scale with most species in the series only suited to the warmer parts of our area, though their hardiness, in many cases, is yet to be fully explored.

Section Platanoidea Pax

Section Platanoidea is distinguished by its buds with five to eight pairs of imbricate scales, lactiferous petioles, three to seven-lobed leaves (rarely unlobed), with entire margins, terminal inflorescences with flowers with eight stamens, and fruits usually with flattened nutlets (de Jong 1976; Xu et al. 2008). It forms a robust clade with Section Macrophylla closest to it. It is distributed from Europe eastwards to Japan. Throughout Europe it is represented by Acer campestre, which forms a group with its Asian equivalents, the Japanese A. miyabei and the Chinese A. miaotaiense, the latter only a recent introduction to collections. A. platanoides is also very wide spread and has more than made itself at home in the temperate forests of eastern north America, where its cultivation now discouraged. The Italian A. lobelii belongs to the A. cappadocicum complex, which as a species in the strict sense occurs in western and central Asia, reaching China, where the section significantly diversifies. Several taxa from here remain absent from cultivation, while species such as A. amplum and A. longipes are under-represented. Recent introductions of members of the A. pictum complex have served to confirm the need for further study of this group. Members of the section allied to this species (and A. platanoides) are often confused with those closer to A. cappadocicum. However, they are easily separated by the colour of their second year branchlets. Those of the pictum affinity are brown, while those of the cappadocicum affinity are green, and remain so for several years. Further information on how to distinguish members of this often tricky group is included within the relevant species accounts.

Section Pubescentia (Pojarkova) Ogata

Section Pubescentia comprises small trees, with buds with six to ten pairs of scales, three-lobed leaves with margins coarsely serrate, corymbose inflorescences carrying 5-merous flowers with five or six stamens inserted on the middle of the lobed nectar disc, the fruits with flattened nutlets Xu et al. (2008). The section was absent from the study of Li et al. (2019), while de Jong (2002) acknowledged that it needed more research. As interpreted by Xu et al. (2008) it contains two species, Acer pentapomicum and A. pilosum, distributed in central and eastern Asia. Both are infrequent in cultivation and are only of specialist interest.

Section Rubra Pax

Section Rubra comprises evergreen and deciduous trees with buds with four to eleven pairs of imbricate scales, unlobed, three or five-lobed leaves with glaucous lower surfaces and cuticular waxes on upper surfaces (Harris et al. 2017). Its inflorescences are usually axillary and emerge from leafless buds in paniculate thyrses, racemes, or umbels (Harris et al. 2017). The flowers are five-merous, though sometimes apetalous, with five to 12 stamens inserted on or outside the nectar disc, which may be reduced or absent (Harris et al. 2017). Members of the Asian Section Hyptiocarpa Fang, as treated by de Jong (2002) were not included in the study of Li et al. (2019), though Harris et al. (2017) included them within an expanded Section Rubra, using molecular and morphological methods to confirm previous studies using chloroplast and nuclear DNA with several methods of analysis to support the relationship between them. de Jong (2002) had previously acknowledged the affinity of the (then) two sections. Thus the treatment of Harris et al. (2017) is followed here. The expanded section therefore includes five species, with two in North America and three in eastern and southeastern Asia. It is the only section to extend south of the equator where it is represented by Acer laurinum, considered the only truly tropical maple in parts of its range.

Unsurprisingly, it is the North American members of Section Rubra that are most common, with A. rubrum particularly renowned for its autumn colour and of which numerous selections have been made. The Asian equivalent of this species, A. pycnanthum, is endemic to Japan, where it is geographically restricted and considered Vulnerable in the wild (Harvey-Brown 2020). A connoisseur's maple, it is equally exceptional as A. rubrum at its autumnal best. The section

As well as the sugar maples, North America is also home to the most common members of section Rubra. A. rubrum, also renowned for its autumn colour, is widespread across eastern States and well represented in collections by numerous selections as well as hybrids with A. saccharinum, which occur both in the wild and in cultivation. The east Asian equivalent of A. rubrum, A. pycnanthum, is more restricted, occurring in a small part of Japan and Vulnerable in the wild (Harvey-Brown 2020). One for the connoisseur, it is equally exceptional at its autumnal best. The incorporation of section Hyptiocarpa within section Rubra expands the range of the section significantly, incorporating A. laurinum it becomes the only section to extend south of the equator. This species is considered the only truly tropical maple, at least in southern parts of its range. It is now represented in cultivation from collections made in the northern parts of its range and can only be considered hardy in the very mildest areas, though experimentation with higher elevation origin material (the species has a broad altitudinal range) is encouraged.

Section Spicata Pax

Section Spicata has buds with two or three pairs of scales, three to five-lobed leaves with serrated margins, inflorescences dense, racemose and held upright (Xu et al. 2008). The flowers are 5-merous and have eight stamens inserted on the middle of the nectar disc (Xu et al. 2008). Li et al. (2019) found that the section forms a clade with sections Negundo, Arguta and Palmata, and not close to Parviflora as de Jong (2002) had supposed. Section Spicata is another that is represented in both eastern and western hemispheres. As treated by Xu et al. (2008) and as followed here, it includes two species in eastern Asia and a sole species in eastern North America. These are morphologically similar and largely restricted to collections.

Section Trifoliata Pax

Section Trifoliata has buds with 11 to 15 pairs of scales, trifoliolate leaves with subentire to serrated margins, corymbose inflorescences bearing 5 or 6-merous flowers with 10 to 13 stamens inserted on the middle of the nectar disc. de Jong (2002) includes three series within the section. Section Grisea, with pubescent floral and foliar parts, the inflorescences with three to five flowers, contains Acer griseum, A. maximowiczianum and A. triflorum. Section Mandshurica, also with few flowers per inflorescence though largely glabrous, contains only A. mandshuricum. Section Emeiense contains A. sutchuenense, which has inflorescences with 20–30 flowers.

Though monophyletic, Li et al. (2019) found that section Trifoliata was derived from Section Pentaphylla and suggested it be treated as part of a broader interpretation of that section. It is treated as distinct here, however, folllowing the treatment of de Jong (2002) until such time that further supporting evidence becomes available. The section contains five species distributed in eastern Asia (China and Japan).

Within Section Trifoliata, A. griseum is most notable, largely for its characteristic bark, though all the species are worth growing. Only the enigmatic A. sutchuenense remains absent from collections, yet to be successfully introduced. Wilson’s collections of this species turned out to be A. henryi, as have others in collections (van Gelderen et al. 1994). It was reportedly introduced on the first SABE expedition, in 1980 from the Shennongjia forest district (SABE 222) (Nicholson 1997), though evidently no plants resulted from this gathering (Dosmann & Del Tredici 2003). It has been observed since too, in Sichuan by Western collectors, though not in seed (K. Camelbeke pers. comm. 2018).

Section Wardiana (de Jong) de Jong

Containing only one species, Acer wardii, Section Wardiana is distinguished by its paniculate-racemose inflorescence, with conspicuous bracts and small cincinni (Xu et al. 2008). Treated within Section Macrantha by Xu et al. (2008), it was considered closer to Section Palmata by de Jong (2002). It was not included in Li et al. (2019) and further work is required to elucidate its phylogenetic position. It is native to parts of eastern Asia.

Of great interest to many an Acerophile, the diminutive and exceptionally elegant A. wardii is distinct in both leaf and flower and is seemingly hardy in parts of our area, though is scarce in collections. However, material of Myanmar origin has recently been available through Pan-Global Plants (Pan-Global Plants 2019). Like Crûg Farm Plants, Pan-Global have been consistent sources of some of the more intriguing, and highly sought after members of the genus.

Hybrids

Hybrid maples are common in gardens and seedlings raised from cultivated material should always be examined carefully for hybrid characters. Crosses occur frequently in section Macrantha, for example, and should be expected from seed collected from open pollinated individuals. Of those described, the cross of Acer davidii with A. pensylvanicum to give the hybrid group A. × conspicuum is most common and its selections are now amongst the most popular of garden maples, especially in Europe. Several selections within this section and others previously assigned to a species are not unattributed to a species here, reflecting an origin where usually only one of the parents is known with any degree of certainty. Of the several, notable hybrids of known parentage, those between A. maximowiczianum and A. griseum are proving themselves as ornamental trees with good heat tolerance for the southern United States, while the same can be said for A. truncatum × platanoides. Some some hybrids, particularly in section Acer, regularly occur in the wild as well as having arisen in cultivation, while others seem altogether more unlikely. Perhaps the unlikeliest of all known is that of A. griseum and A. pseudoplatanus. Described as ‘mysterious’ by de Jong (2011), and appearing very close in leaf to A. pseudoplatanus, with equally unspectacular bark, suppositions that it is only a form of A. pseudoplatanus, or the common hybrid of that species with fellow European A. heldreichii, appear to have been quelled by the raising of the hybrid from A. griseum seed (Banaszczak. pers. comm. in de Jong 2011). If species are desired, in the absence of wild-origin seed it would seem wisest to propagate individuals of known provenance by cuttings, most of which root readily.

Given its horticultural popularity, it is certain that Acer will remain an important target for collectors. Multiple taxa, particularly some introductions from Vietnam, await description, while the warming climate may facilitate the establishment of hitherto tender species. Maple collections abound, and many arboreta have an excellent range of species. In the Netherlands the ‘Aceretum’ at the nursery of Firma C. Esveld in Boskoop, owned by the van Gelderen family, has an extremely comprehensive collection, although limited space means that many taxa are not able to grow to full size. Less hardy species are maintained under glass. In Belgium, Herkenrode and Arboretum Wespelaar hold a good range, as does the de Belder property of Hemelrijk. Over many years a good number of maples have been attempted at the Rogów Arboretum in central Poland, and useful information about their success and hardiness has been published (Tumilowicz 2002; Banaszczak 2007), though is perhaps worthy of review in the context of a warming climate. In the United Kingdom, Westonbirt Arboretum holds the National Plant Collection of species and Japanese maples. Here grow a number of early or original introductions of European and western Asian taxa, as well as the majority of the hardy taxa from China and Japan. Further National Plant Collections of maple species are held at Hergest Croft, Herefordshire, which has a strong representation of species in section Macrantha in particular, and at Blagdon, Northumberland, where many species thrive in the cooler conditions of northern England. Also in that county, Howick Hall Arboretum holds a hugely impressive collection of documented wild origin maples from across the temperate world, resulting from the collections of Lord Howick and Bill McNamara. These include the SICH collections, many of which are replicated at RBG Kew and Windsor Great Park, Surrey, as well as at Quarryhill Botanical Garden, California, where Bill McNamara held the directorship for several years. Elsewhere in North America, there is a significant collection of maples at David C. Lam Asian Garden in Vancouver. Here they also specialise in sections Macrantha and Palmata, with most plants of documented wild origin. Washington Park Arboretum in Seattle is home to the newly named 'Daniel J. Hinkley Asian Maple collection', acknowledging the longstanding contribution to that collection of Hinkley through his introductions, which are represented not only there but on both sides of the Atlantic. In the eastern United States the Arnold Arboretum, Massachusetts, has a formidable collection, including original Wilson material as well as offspring from numerous collections made across multiple NACPEC expeditions. Many NACPEC and other Asian gatherings grow at the similarly impressive collection of the Morris Arboretum, Philadelphia, while the Dawes Arboretum, Ohio, also boasts an impressive range of hardy maples.

With such diversity in the genus it is difficult to give a simple summary of cultivation requirements, but it is clear that maples do better in cooler, moister sites than in hot or dry localities. Lime tolerance is an issue sometimes discussed, but it may be that a well-prepared, moisture-retaining soil is more important for success than the presence of lime is for failure (although certain species remain firmly calcifuge). Diseases are relatively few, and often superficial without lasting effect (for example, powdery mildew on foliage in late summer), but Verticillium wilt can be troublesome, especially on Japanese maples, and there is usually no option but to remove the affected specimen. Armillaria can be a serious problem in ground already infected with it, as noted above. Insect pests are more numerous, especially in North America, where several species of moth and sawfly can cause serious damage to foliage, and borers penetrate shoots, causing dieback. Any establishment of the bacterial pathogen Xylella fastidiosa would likely be devastating for maple collections, as it would be for many other genera. Aphids can be a problem, often producing large volumes of honeydew that may be a nuisance to objects below the trees, especially when it develops sooty mould. Scale insects also affect Acer, but healthy specimens are usually able to tolerate moderate infestations. Vine Weevil is damaging to potted maples but less significant for specimens already in the ground. Where they are present Grey Squirrels can be a terrible nuisance, often seriously damaging the bark and sometime girdling the tree in the process.

Identification Key

Acer argutum, Lake Chosenji, Nikko, Japan (10 October 2008). Image Philippe de Spoelberch.

Acer distylum (label ID 149, Arboretum Robert Lenoir, Belgium, October 2012). Image Charles Snyers.

Acer davidii from SABE 1125 at the David C. Lam Asian Garden, Vancouver. Image © Arboretum Wespelaar – Koen Camelbeke.

The distinctive bloomed shoots of Acer tegmentostum. Image H. C. Angus.

Autumn colour change of Acer buergerianum in Sheffield Park Garden, East Sussex, UK. November 2017. Image Ron Kemeny.