Acer

Maple

A large and important genus composed chiefly of deciduous trees, some being of the largest size, many middle-sized or small, a few shrubby. The hardy species are widely spread over the three northern continents, the finest trees being natives of N. America. A large number come from E. Asia, many of which, however, are small trees.

The most constant and distinctive characters of the genus are the opposite leaves and the form of the fruits. Each fruit consists normally of two sections, known as samarae (commonly as ‘keys’), attached to each other by their bases, and each ‘key’ consists of a nutlet, containing one, sometimes two, seeds, and a large, thin, membranous wing. These wings assist in the dispersal of the seed. The flowers are sometimes unisexual. The typical maple leaf is broad and flat, with five palmate lobes. But there is a great diversity of shape in the genus: some species have as many as eleven or thirteen lobes to each leaf, many have but three lobes, and there is a distinct group with leaves not lobed at all. Finally comes the section of maples with compound leaves consisting of three or five distinct leaflets, sometimes kept genetically separate as Negundo.

Most of the maples have tamely coloured flowers, varying from yellow to greenish white; a few have purple flowers (like A. circinatum), and are very ornamental when in blossom; whilst others, like A. opalus, flower in early spring before the leaves expand, and although not highly coloured make, at that season especially, a pleasing display. Still, on the whole, the attractions of the maples generally are in the large or handsomely cut foliage, and in the red or yellow tints many of them assume in autumn.

Few trees are more easily cultivated than these, their chief requirements being a rich moist soil and a moderately sunny, or at any rate not unduly shaded, position. Some of the smaller species, however, like A. rufinerve, A. capillipes, and A. argutum, like their stems shaded. All the maples should, if possible, be raised from seeds; if grafting has to be resorted to, as for the numerous coloured-leaved and variously habited varieties, the scions should be worked on stocks of their own species.

The number of species of maple has so largely increased in this century by introductions from China that even the largest garden could not accommodate them all, though no other genus of hardy broad-leaved trees is so varied or has so many species that are worthy of cultivation. The following is a short selection:

Large and Medium-sized Trees: A. cappadocicum ‘Aureum’, ‘Rubrum’ and var. sinicum; A. heldreichii; A. lobelii; A. monspessulanum; A. opalus; A. platanoides, A. p. ‘Goldsworth Purple’, ‘Crimson King’ or ‘Faasen’s Black’, A. p. ‘Drummondii’; A. pseudoplatanus ‘Atropurpureum’ (syn. ‘Purpureum Spaethii’); A. rubrum; A. saccharum; A. saccharinum (but the brittle wood makes it unsuitable for town-planting); A. trautvetteri; A. × zoeschense.

Small Trees and Shrubs: A. argutum; A. circinatum; A. cissifolium; A. davidii; A. forrestii; A. griseum; A. grosseri var. hersii; A. japonicum, A. j. ‘Aureum’ and ‘Vitifolium’; A. negundo and its cultivars; A. nikoense; A. palmatum and its cultivars; A. pensylvanicum; A. pseudoplatanus ‘Brilliantissimum’; A. rufinerve; A. triflorum.

From the Supplement (Vol.V)

It was remarked on page 185 that ‘no other genus of hardy broad-leaved trees is so varied or has so many species that are worthy of cultivation.’ Fifty-four pages were devoted to describing those in cultivation, but many more would have been needed to do full justice to the genus had the treatment been prepared at the present time. This is largely thanks to the new introductions by Gordon Harris, who has built up a remarkably comprehensive collection of maples at Mallet Court in Somerset, which includes numerous cultivars of A. palmatum previously unknown in this country. Most of these are mentioned in this supplement.

It is indisputable that many species recognised in the present edition are closely related to others named earlier and merit only subspecific rank. The relevant synonyms are added in this supplement, but it must be emphasised that not all the new combinations made by Dr Murray would necessarily be accepted by other botanists working on the genus, though the majority probably would be.

Recent Literature

(I.D.S.Y.B. International Dendrology Society Year Book)

Banks, R. A. – ‘Some Maples at Hergest Croft, Herefordshire’, I.D.S.Y.B. 1971 pp. 8-13.

de Jong, P. C. – ‘Flowering and Sex Expression in Acer L.’ Medel. Landbouwhogeschool, Wageningen, No. 76-2 (1976). An important contribution to our knowledge of the genus, with a proposed classification.

Harris, J. G. S. – ‘Maples in my Garden’, I.D.S.Y.B. 1971, pp. 14-23.

Harris, J. G. S. – ‘Maples in Taiwan and Hong Kong’, I.D.S.Y.B. 1972, pp. 59-62.

Harris, J. G. S. – ‘Propagation of Acers’, I.D.S.Y.B. 1973, pp. 57-61.

Harris, J. G. S. – ‘Maples from Japan’, Journ. R.H.S., Vol. 199, pp. 394-9 (1974).

Harris, J. G. S. – ‘Growing Maples from Seed’, The Garden (Journ. R.H.S.), Vol. 101, pp. 503-6 (1976).

Harris, J. G. S. – ‘Japanese Maples’, The Plantsman, Vol. 3, pp. 234-50 (1982).

Harris, J. G. S. – ‘An Account of Maples in Cultivation’, The Plantsman, Vol. 5, pp. 35-58 (1983).

Lamb, J. E. D., and Nutting, F.J. – ‘Propagation Techniques in the Genus Acer’, The Plantsman, Vol. 5 (3), pp. 186-92 (1983).

Lancaster, Roy – ‘Maples of the Himalaya’, The Garden (Journ. R.H.S.), Vol. 101, pp. 589-93 (1976).

Mulligan, B.O. – ‘Maples in the North Western U.S.A.’, I.D.S.Y.B. 1970, pp. 13-19.

Murray, Edward A. – The author, a leading authority on the genus, has published numerous notes, keys and new combinations in his cyclostyled publication Kalmia.

Ogata, Ken – A Dendrological Study on the Japanese Aceraceae … Inst. For. Bot., Univ. of Tokyo (1965).

Vertrees, J.D. – Japanese Maples. Timber Press, Forest Grove, Oregon, USA (1978). See further under A. palmatum.

AcerL.

Maples

Acer is one of the most significant tree genera of the northern hemisphere. The 156 species occur in North America south to Mexico, in Europe and North Africa, and from the Middle East through the Himalaya to southeastern Asia (de Jong 2002). Though it is primarily a temperate genus, species of Acer occur in tropical Asia, crossing the equator on Java. They are trees or shrubs, grown primarily for their leaves, which display an array of shapes, sizes and colours. Bark is extremely variable and often diagnostic; it may be smooth, ridged or flaky, seldom rough or forming plates, an exception being the coarse exfoliating plates of A. yui. Bud scales are often large and foliose with 2–15 pairs, eventually caducous. The leaves are deciduous or evergreen, opposite and typically palmate with five lobes, although they may be entire or with 3–13 lobes, or they may be pinnate with three, five or rarely seven leaflets. Hairs are usually present in the vein axils of the leaf undersides and often elsewhere. Stipules are absent (present in A. saccharum subsp. nigrum). Inflorescences are extremely variable in form – compound or simple, corymbose, paniculate, racemose or umbellate – and are produced in terminal and/or lateral positions. Flowering occurs before or as the leaves emerge. The flowers are hermaphrodite or unisexual (trees sometimes dioecious). They have (four to) five (to six) sepals, (four to) five (to six) petals (rarely none), the sepals and petals sometimes fused together, usually yellowish or green, though some species have contrasting white petals and green or red sepals; (4–)8(–13) stamens, nectaries forming a fleshy disc or ring in the centre of the flower; the stamens may be inserted inside or outside the ring or, more commonly, emerge from the ring itself. The fruit is a two-seeded schizocarp, a dry indehiscent structure that breaks up at maturity into single-seeded sections. These sections each have a single wing and are known as samaras (van Gelderen et al. 1994, de Jong 2002). Development of the samara proceeds directly after flowering, and large but immature fruits are often a conspicuous feature of the trees through the summer. In the following account the entries for fruiting time refer to ripening.

Despite the interest of horticulturists, Acer has not been a subject for recent academic botanical study, and a modern revision is urgently needed to clarify the systematic position of many taxa. The taxonomy in this account is generally based on Piet de Jong’s classification in the current standard reference Maples of the World (van Gelderen et al. 1994), although this differs in some respects from other major authorities such as the recently available Flora of China account (Xu et al. 2008); some modifications have been included in the present work on the advice of Peter Gregory, in advance of his own forthcoming book on maples (Gregory, in prep.). A summary table of the infrageneric classification of Acer is provided below, which helps place un familiar species with better-known relatives, but for more detail Maples of the World should be consulted. A number of familiar species with multiple subspecific taxa are included, as some of the names are unfamiliar and represent recent collections. For these species simple keys covering the important characters are provided. Almost all species mentioned in this account are illustrated (mostly by colour photographs) in Maples of the World, its companion volume Maples for Gardens (van Gelderen & van Gelderen 1999), and An Illustrated Guide to Maples (le Hardÿ de Beaulieu 2003).

ACER L.

A guide to the infrageneric classification of Acer (derived from van Gelderen et al. 1994)

One of the most important genera of ornamental trees for temperate climates, Acer 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 A. hyrcanum subsp. reginae-amaliae, 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 red or pink; 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.

Enthusiasm for the genus is perhaps most apparent in the popularity of the ‘Japanese maples’, of which the majority of cultivars, selected for their foliage characteristics, are derived from A. palmatum, with some derivatives of A. japonicum and A. shirasawanum. 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.

Another group receiving much attention are the ‘snakebark’ maples of section Macrantha, popular both for the beautiful patterns and colours of their bark, especially in young trees, and their often good autumn colours. Well-known examples include A. davidii and A. pensylvanicum, which in its cultivar ‘Erythrocladum’ has strikingly red young stems in winter. These two have hybridised in cultivation to give the hybrid group A. ×conspicuum (described in more detail below), selections from which are now amongst the most popular of garden maples, especially in Europe. In recent years a number of snakebark maple cultivars have been selected and named for distinct characteristics of habit, bark pattern, leaf shape or colouring. Some are attributed to A. davidii, but may well be garden hybrids. Two such, distinguished by the Royal Horticultural Society’s Award of Garden Merit, are the George Forrest Group, with green and white bark and simple leaves, and ‘Serpentine’, a small, rather stiff tree with a somewhat sinuous appearance, purplish-tinged bark and rather dark foliage – its red samaras a mark of possible hybridity (L. Banks, pers. comm. 2007). ‘Karmen’ has chocolate-coloured leaves on emergence and darkish green bark. Acer pectinatum subsp. forrestii ‘Sirene’ has dark, purplish-tinged bark with pinkish striations, and the leaves emerge with purple flush before turning dark green. Both of these may also be hybrids. In the United States A. tegmentosum ‘Joe Witt’, selected from a tree in the Washington Park Arboretum, is highly valued for its very ‘white’ stems and yellow autumn colour; it seems to be unknown in Europe. ‘White Tigress’ is another American selection, probably derived in part from A. tegmentosum, but now prowling the world and proving to be a most beautiful tree, with crisp white striations on dark green stems.

Members of this section are particularly promiscuous, and as the ‘new’ species described below – such as A. morifolium and A. rubescens – become better known and more widely grown, it seems probable that their genes will become more frequent in the mixing pot and further hybrids will arise. In the absence of wild-origin seed it would seem wisest to propagate individuals of known provenance by cuttings, most of which root readily. 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 hot and humid or dry conditions, 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 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.

The evergreen maples with simple leaves are an interesting group for the gardener in milder areas to explore. While most are indubitably tropical or subtropical and decidedly tender, some are surprisingly hardy, even in the eastern United States. Careful collection from the coldest provenances will be particularly important.

Found over a vast tract of southeastern Asia, from Nepal to Laos and Vietnam, and including much of western China, are members of the Acer campbellii complex. This remains poorly understood, both in the wild and in cultivation, although the authors of Maples of the World recognise five separate subspecies of A. campbellii (van Gelderen et al. 1994). It requires a focused study. They are attractive small trees, usually with greenish stems and somewhat leathery three- to seven-pointed leaves, but they have a reputation for not being very hardy. In recent years the group has been joined by material from northern Vietnam, distributed quite widely through specialist nurseries in both the United States and Europe as A. campbellii var. fansipanense Gagnep. There is some uncertainty over the validity of this name (it is not listed by IPNI), and the exact status of the material it represents has not been thoroughly investigated. It is said to differ in having narrower leaf lobes and smaller fruits than subsp. campbellii (Ho 2000). This area of northern Vietnam has an extraordinary diversity of series Sinensia, with A. campbellii subsp. campbellii, subsp. flabellatum (and its var. yunnanense) and subsp. wilsonii all growing in the Hoang Lien Son Range, and in the same general region also are found A. chapaense, A. fenzelianum, A. oliverianum and A. tonkinense (P. Wharton, pers. comm. 2007). It would seem wise to attempt the identification of any maple from northern Vietnam with a flexible mind! Probably the majority of specimens labelled var. fansipanense in cultivation derive from collections made by Sue and Bleddyn Wynn-Jones, and sometimes Dan Hinkley as well, from a single tree growing near a campsite at 2710 m on Fan Si Pan (for example, BSWJ 8270, HWJ 569 – although this was sold through Heronswood Nursery as subsp. campbellii) (B. Wynn-Jones, pers. comm. 2007; Crûg Farm Plants 2007–2008, Heronswood Nursery catalogue 2004). Saplings from this provenance have been observed at Hergest Croft and elsewhere, and are certainly attractive and worth cultivating, apparently being reasonably hardy in western Britain and in the Pacific Northwest. They have firm, three- or five-lobed leaves carried on conspicuously red petioles, and as young trees at least the stems remain green to the ground. The leaves flush in various shades of red and brown, and turn a good colour in autumn (Crûg Farm Plants 2007–2008). At Hergest Croft there is one in which the stem is golden-yellow, that deserves propagation (L. Banks, pers. comm. 2007). At lower altitudes (1400–1500 m) the trees can be up to 20 m tall in riparian situations, bearing ‘remarkably large’ leaves with three large forward-pointing lobes and two smaller side lobes (P. Wharton, pers. comm. 2007).

Acer is hugely popular in the horticultural world and it is certain that the genus will remain an important target for collectors. Among very recent introductions is A. yui, with unique bark characteristics, collected in Gansu in 2005 by a NACPEC expedition (Aiello 2006). Germination of the seed collected was poor, but a few seedlings are growing steadily at the Morris Arboretum and elsewhere (A. Aiello, pers. comm. 2007). Hybrid maples are relatively rare, but seedlings raised from cultivated material should always be observed carefully for hybrid characters. In addition to those described here a few good garden hybrids are in circulation, among these being A. ×hillieri (A. miyabei × A. cappadocicum), with cultivars ‘West Hill’ and ‘Summergold’. Dirr (1998) mentions the promising development of hybrids of A. maximowiczianum and A. griseum with some excitement, as potentially offering very ornamental trees with good heat tolerance for the southern United States. The cultivars ‘Cinnamon Flake’, ‘Ginzam’ (Gingerbread) and ‘Girard’ are available in the United States, but are not common. It is probable that they originated in the Highland Park Arboretum in Rochester, New York, where there are plants of both parents that date to the beginning of the twentieth century. Young hybrid plants exfoliate in varying degrees, with some approaching the extent seen in A. griseum, but this feature does not develop at such an early age (on seedlings from a controlled cross made at the US National Arboretum, exfoliation started at age 10) and they are therefore less popular with the nursery trade, despite their greater heat tolerance than A. griseum. At age 19 the USNA seedlings are showing very good bark, but with age it tends to become greyer, with exfoliation confined to smaller patches (R. Olsen, pers. comm. 2007).

Maple collections abound, and many arboreta have an excellent range of the 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. In the United Kingdom, the National Arboretum at Westonbirt holds the National Plant Collection of Japanese maples, but also has a good range of other species. National Plant Collections of ‘other’ maples are held at Hergest Croft, Herefordshire, and at Blagdon, Northumberland, where many species thrive in the cooler conditions of northern England. 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 is available (Tumilowicz 2002, Banaszczak 2007). In North America there are good collections of maples at the David C. Lam Asian Garden in Vancouver (mostly of documented wild origin), the Washington Park Arboretum in Seattle and the Arnold Arboretum in Boston, but according to Richard Olsen perhaps the most outstanding collection of hardy maples in the United States is at the Dawes Arboretum in Newark, Ohio – information that unfortunately came too late for it to be used as a source of data for this book.

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. Insect pests are more numerous, especially in North America, where several species of moth and sawfly cause serious damage to foliage, and borers penetrate shoots, causing dieback. 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 sometimes girdling the tree in the process.

A. acuminatum Wall. ex D. Don B186, S37, K68

A. aidzuense (NOW A. tataricum subsp. aidzuense (Franch.) P.C. de Jong) S41

A. albopurpurascens Hayata S49, K68

A. amplum Rehder (OR A. longipes subsp. amplum (Rehder) P.C. de Jong, NT89) B192, S39, K68

A. amplum subsp. tientaiense (C.K. Schneid.) Y.S. Chen (OR A. longipes subsp. amplum (Rehder) P.C. de Jong, NT89) B192, K68

A. argutum Maxim. B186, K68

A. barbatum (NOW A. saccharum subsp. floridanum (Chapman) Desmarais) B232

A. barbinerve Maxim. B187, K68