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The genus Quercus is being revised for Trees and Shrubs Online to provide a thorough, modern account of all oaks cultivated in the northern temperate zone. Due to its large size and complexity new articles will be published in batches, defined by the eight taxonomic Sections into which the genus is divided. This multi-year revision is being undertaken by Allen Coombes and Roderick Cameron, who began work in June 2020.
The first Section to be completed was Section Ilex, comprising the following species and hybrids all published in February 2021: Q. acrodonta; Q. alnifolia; Q. aquifolioides; Q. aucheri; Q. × auzendei; Q. baloot; Q. baronii; Q. coccifera; Q. cocciferoides; Q. dolicholepis; Q. engleriana; Q. floribunda; Q. franchetii; Q. gilliana; Q. guyavifolia; Q. ilex; Q. lanata; Q. leucotrichophora; Q. lodicosa; Q. longispica; Q. monimotricha; Q. pannosa; Q. phillyreoides; Q. rehderiana; Q. rotundifolia; Q. semecarpifolia; Q. senescens; Q. setulosa; Q. spinosa; Q. tarokoensis; Q. tatakaensis; Q. tungmaiensis; Q. utilis.
Section Cerris was published in July 2021, comprising: Q. acutissima; Q. afares; Q. brantii; Q. castaneifolia; Q. cerris; Q. chenii; Q. × crenata; Q. euboica; Q. hispanica; Q. ithaburensis; Q. × libanerris; Q. libani; Q. look; Q. macrolepis; Q. × numidica; Q. × schneideri; Q. suber; Q. trojana; Q. variabilis.
The latest batch to be published are the oaks of Section Cyclobalanopsis, which appeared in August 2022: Quercus acuta; Q. augustini; Q. bambusifolia; Q. delavayi; Q. gilva; Q. glauca; Q. hondae; Q. hypargyraea; Q. kiukiangensis; Q. lamellosa; Q. lobbii; Q. longinux; Q. macrocalyx; Q. morii; Q. myrsinifolia; Q. oxyodon; Q. poilanei; Q. salicina; Q. schottkyana; Q. sessilifolia; Q. stenophylloides.
The oaks of Section Lobatae are currently being revised and will be published in due course.
This revision of Quercus has been made possible thanks to the sponsorship of the International Dendrology Society, the Wynkcoombe Arboretum, and several private individuals. We acknowledge their generosity collectively as ‘The Trees and Shrubs Online Oak Consortium’.
TC, August 2022.
The 400–450 species in the important genus Quercus are found across the northern hemisphere in North and Central America (south to Colombia), Europe (including Macaronesia) and Asia (south into Indonesia, extending into the southern hemisphere). The greatest species diversity is in the Sierra Madre Occidental of Mexico (see Valencia-A. 2004) and in eastern and southeastern Asia (Govaerts & Frodin 1998). New species continue to be named. Identification of oaks is complicated by variation and hybridity, and particular care must be taken with cultivated material, where the growth form may not be representative of the wild plant, and hybrids are particularly frequent. In addition, successful identification often depends upon the presence of acorns, requiring therefore both a reproductively mature tree and successful pollination.
Most oaks are medium-sized to large trees, though shrubby or rhizomatous species occur, but appearance and stature is often strongly influenced by growing conditions. The bark is extremely variable, but is usually thick and somewhat rough when mature; Cork Oak (Quercus suber) is famous for its thick, corky bark, and the bark of many Mexican red oaks has a bloated appearance suggestive of pachydermatous hide, at least while young. Winter buds are covered in imbricate scales and may be spherical, ovoid, terete or angled. The leaves are spirally arranged and may be evergreen, sub-evergreen (sometimes termed ‘tardily deciduous’) or deciduous; it is not always easy to decide into which category species fall, and their behaviour in cultivation is not necessarily the same as in the wild. Leaves may be lobed or unlobed and with entire, dentate or serrate margins, sometimes with short bristles or awns at the tips of the lobes or teeth. These bristles are sometimes as spiny as those of some hollies (Ilex), but species with spiny leaves often produce non-spiny leaves higher up in the canopy, and some species have completely unarmed forms within natural populations. The size and shape of the leaves may vary greatly within species and individuals, and further variation is often introduced in the secondary growth flush frequently produced late in summer. In this the leaves may appear very different to those of the primary flush, often being smaller and less dissected, although sometimes the lobes are more distinct; they may also be tinted in shades of brown, bronze or red when young – an attractive feature that is usually less conspicuous in the first flush. The secondary veins are unbranched and run parallel to the margins, or branch early to form a network before reaching the margins. The presence and form of foliar hairs can be a helpful character for identification at the species level: they are usually visible with a 10× or 15× lens. Stipules are typically inconspicuous and caducous. The inflorescences are unisexual and are borne in the axils of leaves or bud scales. They are usually produced in spring, before or as the leaves emerge. As an identification character they are not of great value, and are in most cases omitted from the accounts that follow, in favour of the fruiting characters. Staminate inflorescences are pendulous catkin-like spikes; they may be solitary or occur in clusters, and bear small, rather insignificant flowers. Pistillate inflorescences are generally short and stiff, with a terminal cupule and one to many lateral cupules. Not all the cupules develop to maturity. There is a single pistillate flower in each cupule, and the cupule (which does not have valves) may be sessile or pedunculate. There is great variation in cupule morphology; they may be cup-, bowl- or goblet-shaped and can cover most of the acorn or just the base; the scales may be adherent or reflexed, linear or conical, or reduced to tubercles. The fruit (acorn) is a nut. Acorns have a scar at the base where they were attached to the cupule and the remains of the style may persist at the acorn apex (stylopodium). Acorns mature either in the year of flowering (annual maturation) or the following year (biennial maturation) (Nixon 1997, Huang et al. 1999).
A guide to the infrageneric classification of Quercus
|Subgenus Cyclobalanopsis (e.g. Q. lamellosa, Q. gilva)
|Acorn cupule with concentric rings of scales
|Evergreen or sub-evergreen
|Section Cerris (e.g. Q. cerris)
|Acorn cupule with individual scales
|Evergreen or deciduous
|Evergreen or deciduous
|Grey or yellow fascicled hairs on leaf undersides
|Asia, Europe, North Africa, North America
|Leaf margins entire, revolute or spiny
|Asia, Europe, North Africa
|Section Lobatae (red oaks) (e.g. Q. rubra)
|Deciduous (or evergreen)
|Leaves entire, serrate or lobed
|Bristles present on tips
|Section Protobalanus (e.g. Q. tomentella)
|Leaves glaucous or waxy below
|Leaf margins entire or dentate
|Section Quercus (white oaks) (e.g. Q. alba)
|Deciduous (or evergreen)
|Leaves entire, serrate or lobed
|Asia, Europe, North America
Quercus is in need of taxonomic revision. Govaerts & Frodin (1998) recognised 531 species, but this is probably an overestimate (A. Coombes, pers. comm. 2008). Flora of China (Huang et al. 1999) and Flora of Taiwan (Liao 1996a), however, separate the genus Cyclobalanopsis, with about 150 species. Cyclobalanopsis can be distinguished from Quercus because the cupule scales of the former are fused into concentric rings while those of the latter are free and spirally arranged (Huang et al. 1999). In the present treatment a broader definition of Quercus, encompassing Cyclobalanopsis, is applied. Within Quercus the subgeneric classification is controversial, and several systems have been proposed (see Govaerts & Frodin 1998). Although the details of these are not necessarily important to oak growers, a taxonomic distinction can also be a useful horticultural guide, and for this reason an indication of the infrageneric group to which a species belongs is given in the accounts that follow. A summary table is also provided (opposite). Of particular importance is the division of the majority of American oaks into two principal groups: the red oaks (section Lobatae) and the white oaks (section Quercus). Both are in subgenus Quercus. Red oaks have smooth or deeply furrowed bark at maturity, their leaves typically have bristles at the apex of the lobes or at the leaf apex (where unlobed), and the cupule scales are flattened and can be tuber cu late (very seldom). In general, red oaks adapt well to cultivation in northern Europe, with its relatively cool summers. White oaks have scaly or papery bark at maturity, their leaves never have bristles on the lobes or at the apex, and the cupule scales are thickened and tuberculate. Many white oaks require a long, hot summer, and for this reason are less likely to flourish in northern Europe.
In addition to a taxonomic revision being needed, great steps would be made in clarifying oak nomenclature if American authors would agree on standard common names for their native species. In our accounts, names as given in Flora of North America (Nixon 1997) have been used, but there are alternatives for most species.
Within sections, Quercus species are promiscuous, and interspecific hybridisation occurs regularly both in the wild and in cultivation. Many nothospecies have been described and continue to be described. Listing all hybrids currently in cultivation is beyond the remit of this book, but some are described by Palmer (1948) and in numerous papers in the Journal of the International Oak Society (now International Oaks) and elsewhere. Acorns from arboreta are particularly unlikely to be true to name, and F1 hybrids should be expected. A complication is the collection and distribution of acorns from trees identified as hybrids in the wild, labelled with the nothospecific name for the maternal parent: these are inevitably the F2 generation or backcrosses to either parent, so seedlings may exhibit any combination of characters from the original parents. Further confusion may arise if seed is gathered from known hybrids in collections, as these may well have been pollinated by yet another species, to create a triple hybrid. Among the plethora of seedlings an occasional exceptional tree will be worth selecting for propagation, but unless regular propagation and distribution are expected it is pointless to apply a cultivar name.
Of great importance as the dominant trees in many landscapes, as a source of timber or forage, and supporting a huge diversity of wildlife, oaks have a special significance to many. In the arboretum their amazing variety makes them particularly popular, reflected in the dynamic International Oak Society and in the efforts of institutions and individuals across the world to introduce to cultivation as many species as possible. Some of the Mexican oaks are amongst the most remarkable and garden-worthy trees described in this book; indeed it is perhaps not too much to say that Quercus rysophylla is the outstanding single new introduction of the past 30 years. It is a measure of recent interest in Mexican and Central American species that in 1933 Sir Oscar Warburg and Edmund Warburg, in their review of oaks in cultivation in the British Isles (Warburg & Warburg 1933), listed only seven taxa, of which three were unidentified, and Bean (1976b) described only four; yet here we describe 29 species found principally in Mexico and Central America.
Many of the oaks from colder parts of Mexico are deciduous in the wild, losing their leaves in winter or late winter, but these tend to be more evergreen in cultivation. Some areas of Mexico, especially the Sierra Madre Oriental, receive most of their rainfall in summer, with a long, dry season from autumn through to late spring. In consequence it would be a mistake to assume that Mexican oaks are necessarily drought-tolerant in cultivation: Shaun Haddock (pers. comm. 2006) has lost specimens and seen others severely damaged by the effects of summer drought in southwestern France. Conversely, moisture in summer can be fatal to some species of Quercus from areas of the western United States with a Mediterranean-type climate.
The accounts that follow describe or discuss 95 species, on top of the 100 or so covered by Bean and Krüssmann (disentangling the synonymy makes for a certain vagueness), disregarding hybrids. In addition more than 40 other species are represented in cultivation principally by young plants (see below), bringing the total list of cultivated oaks to at least half the diversity of the genus – a remarkable number when it is considered that a large proportion of Quercus are tropical trees. Most collections also have an assortment of hybrids, of both wild and garden origin, raising the diversity total even higher.
Many ‘Quercus sp.’ accessions go unidentified for years, or are annotated ‘aff.’ and remain so for some time before their identity is revealed. For example, at the Arboretum National des Barres there is an evergreen oak of subgenus Cyclobalanopsis, grown from acorns collected by Père Paul Farges – one of the great French missionary bot anists – at Heoupin, near Chenkou in Sichuan, at an altitude of 1400 m, and received at Les Barres in March 1901 (Lamant 2004). Long considered to be Q. oxyodon, in 1998 it was identified by Zhekun Zhou as Q. liboensis Z.K. Zhou (Fourier 1999), under which name it has been propagated in recent years. With further study, however, it has become apparent that it is not Q. liboensis, which is close to Q. glauca and should have farinose leaf undersides. Rather, the French tree – now 10 m in height, dbh 23 cm, spread 10 m, and with waxy undersides to the leaves – seems to be an annual fruiting relative of Q. multinervis (W.C. Cheng & T. Hong) Govaerts (A. Coombes, pers. comm. 2008). Plants are in cultivation as Q. liboensis, from a recent introduction, but their identity has not been confirmed. More successful has been the identification as Q. tungmaiensis Y.T. Chang (section Cerris?) of Keith Rushforth’s Tibetan collection KR 5765, growing as 2.5 m trees at Tregrehan and Chevithorne Barton, and showing distinctive lanceolate, strongly toothed leaves with impressed veins on the upper surface (Coombes & Zhou 2009). On the other hand, some species believed to have been in cultivation turn out to have been misidentified. For this reason, Q. rex Hemsl. has disappeared from the original list for New Trees (although its acorn is still illustrated below, p. 698).
Some shrubby species of Quercus that are well established in cultivation, on either side of the Atlantic, are not described below because of shortage of space. These include the Critically Endangered Q. hinckleyi C.H. Mull., which forms a low mound in its semi-arid habitat in Presidio Co., Texas and northern Chihuahua, but can become taller in cultivation, and the short tree Q. john-tuckeri Nixon & C.H. Mull., from California. At the Hillier Gardens, shrubby Q. tinkhamii C.H. Mull. has made a small tree of 3.5 m (Coombes 371, San Luis Potosí, 1996), and the thicket-forming Q. microphylla Née is starting to spread by suckers (Coombes 409, Puebla, 1997).
It is almost impossible to keep up with the flow of oaks into cultivation. As a demonstration of this, the following list is of additional species (disregarding variants and hybrids) not recorded by either Bean or Krüssmann, or covered elsewhere in New Trees, but currently grown by Michael Heathcoat Amory at Chevithorne Barton (as of October 2008). Deriving from numerous sources and collectors, many are still very young seedlings or are being maintained in pots or raised beds, and most are in other collections as well: Q. argyrotricha A. Camus, Q. berberidifolia Liebm., Q. cedrosensis C.H. Mull., Q. championii Benth., Q. cornelius-mulleri Nixon & K.P. Steele, Q. crispipilis Trel., Q. cubana A. Rich., Q. daimingshanensis (S.K. Lee) C.C. Huang, Q. depressa Bonpl., Q. greggii (A. DC.) Trel., Q. havardii Rydb., Q. hintoniorum Nixon & C.H. Mull., Q. hirtifolia M.L. Vázquez, S. Valencia & Nixon, Q. hondae Makino, Q. intricata Trel., Q. invaginata Trel., Q. jonesii Trel. (syn. Q. coccolobifolia Trel.), Q. laeta Liebm., Q. liebmannii Oerst. ex Trel., Q. miquihuanensis Nixon & C.H. Mull., Q. mohriana Buckley ex Rydb., Q. multinervis (W.C. Cheng & T. Hong) Govaerts, Q. pacifica Nixon & C.H. Mull., Q. peduncularis Née, Q. pentacycla Y.T. Chang, Q. potosina Trel., Q. pumila Walter, Q. pungens Liebm., Q. repanda Bonpl., Q. sapotifolia Liebm., Q. sessilifolia Blume, Q. vaseyana Buckley.
At the Hillier Gardens are Q. delavayi Franch., grown from Coombes 481 (collected in Yunnan in 1998) and 1.5 m tall in 2008, and Q. sessilifolia Blume, from Costin & Lancaster 178A (gathered in Honshu in 1990 as Q. acuta) and now 5 m tall. A 3 m individual of Q. sessilifolia, derived from Hangzhou Botanic Garden, grows in the Jardin botanique de Nantes, France (F. Picard, pers. comm. via A. Coombes 2008). Thierry Lamant from Orléans, France has introduced Q. cupreata Trel. & C.H. Mull. and Q. tuberculata Liebm. to French gardens, from acorns collected in the Sierra Madre Oriental of Nuevo Léon in 2003, and believes that Q. cupreata will prove to be at least as hardy as Q. rysophylla (T. Lamant, pers. comm. 2008).
In the United Kingdom there are outstanding National Plant Collections at the Sir Harold Hillier Gardens, Hampshire, developed, in particular, by Allen Coombes, and at Michael Heathcoat Amory’s home Chevithorne Barton, in central Devon. Despite Chevithorne Barton being a comparatively cool site at moderate elevation, many oaks seem to flourish here in fertile soil and rather high rainfall. A number of enthusiasts have good collections in France (see Lamant 2004), including Shaun Haddock at Arboretum de la Bergerette, Saint-Sardos, Dept. Tarn et Garonne. In Luxembourg there is a specialist collection at the LTA Arboretum (Lycée Technique Agricole), Ettelbruck, for which Eike Jablonski is responsible. Still in the Ardennes, but across the border in Germany, is another collection at Kruchten, a site similar to Ettelbruck in physical and climatic characteristics. Many interesting oaks are also grown at Arboretum Trompenburg in Rotterdam. Oaks are popular in the United States and most arboreta there have a good selection, Guy Sternberg’s collection at Starhill Forest Arboretum, Illinois being of particular note. Situated in an area with a severely continental climate, Starhill Forest is a useful comparator to those gardens with milder winters and cooler summers. In New Zealand there are many notable oaks at Eastwoodhill Arboretum and also at Hackfalls, where Bob and Lady Anne Berry have an outstanding collection, growing several species not mentioned here.
Oaks receive prominent coverage in all dendrological works, but the only treatment of the whole genus is the massive Guide illustré des Chênes by Antoine le Hardÿ de Beaulieu and Thierry Lamant, published (in two volumes) in 2006 – sadly after the completion of most of the text of Quercus for New Trees. Profusely and finely illustrated, it is a major work of reference, and translations into other languages are eagerly awaited. It covers all the species described below, with illustrations, and as it is so comprehensive we do not reference these images individually in our accounts. Menitsky’s Oaks of Asia (2005), Miller & Lamb’s Oaks of North America (1985), and Diversidad del Género Quercus (Fagaceae) en México by Valencia-A. (2004) are all regionally important works. An invaluable source of nomenclatural information is the World Checklist and Bibliography of Fagales (Govaerts & Frodin 1998) and its counterpart online, available through the Kew World Checklist of Selected Plant Families website (2008). A further source of nomenclatural guidance is provided by the Oak Names Database (2007–2008), hosted by the International Oak Society, which is the International Cultivar Registration Authority for the genus Quercus. This database is particularly valuable for its coverage of hybrids and cultivars. In compiling our account we are greatly indebted to Allen Coombes of the Hillier Gardens for his enormous contribution of information over an extended period.
In cultivation oaks are relatively tolerant of a range of growing conditions, and among the recently introduced taxa certain Mexican species are proving to be reliable in many locations. The distinction in climatic preference requirements between white and red oaks should be noted, and if a noble specimen is desired, northern European planters should probably avoid the American and Asian white oaks. Some oaks are intolerant of lime, and where this is known to be the case it is mentioned in the accounts that follow. Many of the recently introduced species have yet to be tested on calcareous soils, however, and their tolerance is unknown.
Contrary to popular belief, many oaks grow extremely fast if happily situated, especially when young. Two or even three flushes of growth each year are possible, and can lead to rapid gains in stature. As the shoots are often coloured bronze or reddish, the new growth can be a very attractive feature in its own right.
They are best propagated from fresh acorns (of known wild origin whenever possible), sown individually in small pots or deep plug cells, where germination will often be prompt. Acorns should never be allowed to dry out before or after sowing, and care should be taken to prevent rodent predation. When growing vigorously young trees may be potted on into deep pots, but contortion of the taproot must be avoided. Planting should take place as early as possible, either direct from the pot or after a year or two of growth in nursery beds. In the absence of seed, rarities and selected clones can be propagated by grafting onto appropriate stocks, but cuttings are increasingly being used to obviate problems of incompatibility. This is a very exciting and rapidly developing field, with enormous potential benefits for horticulture.
The recently published Red List of Oaks (Oldfield & Eastwood 2007) has highlighted conservation issues facing the genus: 78 species have been placed in IUCN Red List categories Critically Endangered to Near Threatened, while for a further 33 there are grounds for believing they may be threatened, though they are currently categorised as Data Deficient because of insufficient supporting information to confirm their status. Only 98 could be unequivocally assigned to the rank of Least Concern – the rest having not been evaluated at all. Oaks, officially categorised or not, are threatened globally by habitat destruction and land-use change, as well as over exploitation for timber and failure to regenerate, so it is probable that many of the species currently unevaluated are also under threat. As gardens contain about half the known diversity of the genus, they may be a valuable source of material for reintroductions in future.
Oaks are famous for their hosting of a large number of other organisms, and when mature are of major importance in temperate forest ecology. While most of this biodiversity is innocuous to the host tree, there are some important pathogens that affect Quercus and can be devastating to both individuals and populations. Most alarming is the appearance of Phytophthora ramorum, the organism that causes the effect known as sudden oak death and has led to the deaths, particularly in Oregon and California, of large numbers of Quercus and Lithocarpus densiflorus, as well as other species. Symptoms are cankering of the stems, usually accompanied by a dark reddish or black trickling ooze, and the partial or total death of the canopy and eventually the tree. There is at present no known cure, and rigorous preventative measures are being put in place by national plant health authorities across North America and Europe. The water-borne pathogen also affects many other woody plants at sub-lethal levels, from which susceptible species can easily be infected. As these include popular ornamental shrubs such as Rhododendron and Viburnum, grown and sold in large numbers by nurseries, and frequently shipped long distances from their point of origin, this is a serious issue for horticulturists to address. It is of particular interest that the British Government’s fact sheet on P. ramorum (Defra 2006) states that all proven incidences in non-nursery specimens have been on plants in close proximity to rhododendrons. Governmental bodies and research institutions throughout our area publish regular updates on the state of research into sudden oak death, easily accessed by an internet search.
Another serious fungal problem affecting oaks in eastern and central North America is oak wilt, caused by Ceratocystis fagacearum. Red oaks are particularly susceptible, and can be killed very quickly if infection takes place in early summer, while white oaks are somewhat more resistant and tend to die off branch by branch, over several years. Wounded trees are especially at risk of infection, and it is thought that the recent rapid spread of the disease is in part due to activities surrounding the construction of new homes in woodland, resulting in greater frequencies of wounded trees. Transmission is by nitulidid beetles carrying spores, or by tree-to-tree contact through interconnecting roots. This information is derived mostly from the USDA Forest Service website (US Forest Service 1999), but many other websites also carry information on this disease, and should be consulted for current advice.
Major insect pests of oaks are also an increasing problem. In Europe the Oak Pro-cessionary Moth (Thaumetopoea processionea) is extending its range northwards, and larvae were found at Kew for the first time in 2006 – one of several breeding instances in the London area (T. Kirkham, pers. comm. 2006; Forest Research 2008b). The larvae are capable of causing massive defoliation of oak woodlands, and are also a human health hazard due to their hairs, that break off and float on the wind, and can give rise to severe irritation to skin and lungs. In ornamental trees the best cure is to catch the nests of caterpillars while they are still young and carefully cut them from the tree (T. Kirkham, pers. comm. 2007).
A large genus of evergreen and deciduous trees and shrubs, of which about 450 species are known. Sixty to seventy are in cultivation. Leaves alternate, a spiral of five making one circuit of the branchlet, frequently lobed somewhat deeply, but occasionally merely toothed or even entire. The down on the leaves, etc., is mostly stellate. Male and female flowers occur on the same trees, but on separate inflorescences. The males are numerous on pendulous catkins, small, green or greenish, forming sometimes tassel-like clusters; females few and quite inconspicuous. The most distinctive feature of the oak is its fruit, which consists of a usually egg-shaped or rounded nut (acorn), the lower part of which is more or less enclosed by a cup covered with woody, sometimes fringe-like scales. The acorn frequently takes two seasons to mature. The cup of the acorn develops from the involucre of the flower and is the counterpart of the husk of Fagus and Castanea, but normally it encloses only a part of the nut (acorn) and does not split into valves nor show any sign of vertical subdivision. Another distinction, of course, is that in Quercus a single nut is produced (the female flowers being solitary in each involucre), while in Fagus the fruit contains two nuts, each developed from a separate flower; in Castanea there are one to three and normally three in Nothofagus. Here it should be pointed out that the term ‘fruit’ in the descriptions of the individual species is used for the nut together with its cup, in accordance with botanical usage, and the word ‘acorn’ is used for the nut only.
Classification. – Various classifications of the genus have been proposed. The major groupings are based mainly on characters of flower and fruit, such as the length and form of the style, the presence or absence of a tomentum on the inner side of the wall of the acorn (endocarp), and the position of the aborted ovules on the seed (the ovary in Quercus contains six ovules, only one of which develops into a seed). The foliage does not always serve to place a species, since leaves of similar type occur in two or more sections, and may vary greatly in each. For example, Q. suber, the cork oak, belongs to the same section as Q. cerris, and has produced with it a hybrid of great vigour – the Lucombe oak – though the two species are superficially very unalike. On the other hand Q. ilex closely resembles Q. suber in its foliage yet is distant from it taxonomically and nearer to the common oak, with which it has hybridised (Q. × turneri).
The following synopsis is taken from Mme. A. Camus’ work Les Chênes, which is the only comprehensive work on the genus. Below the rank of section each letter indicates a subsection, but the botanical names of the subsections are not given, since they are not in common use.
Leaves persistent, toothed in a few species but commonly entire and never lobed. Acorn-cups (cupules) with concrescent scales arranged in concentric rings. This group is confined to E. Asia and Malaysia, and is almost wholly tropical to warm-temperate in distribution. The cultivated species are from W. China, Japan, and the Himalaya. The group shows far less variation than the subgenus Quercus, and its subdivisions are really equivalent to the subsections of that subgenus.
a. Q. lineata, Q. oxyodon
b. Q. myrsinifolia
c. Q. glauca
d. Q. acuta
Leaves deciduous or persistent, entire, toothed or lobed. Acorn-cups covered with imbricated scales, which are free or concrescent, and appressed, erect or recurved.
sect. Cerris. – Styles elongated, scarcely or not expanded at the apex. Scales of acorn-cup elongated and sometimes reflexed. As defined by Camus, this group includes species in which the interior of the acorn-shell (endocarp) is densely tomentose (e.g., Q. coccifera), but in the majority it appears to be glabrous or slightly hairy. With a few exceptions the acorn takes two years to ripen. In the first five subsections the leaves are persistent, in the last three deciduous, though sometimes late in falling. This section is confined to the Old World and the majority of the species are natives of Europe and W. Asia.
a. Q. gilliana, Q. phillyreoides, Q. semecarpifolia
b. Q. baronii
c. Q. alnifolia
d. Q. calliprinos, Q. coccifera
e. Q. suber
f. Q. afares, Q. castaneifolia, Q. ithaburensis, Q. libani, Q. macrolepis, Q. trojana
g. Q. acutissima, Q. variabilis
h. Q. cerris
sect. Mesobalanus. – A small group of species, considered to be intermediate between sect. Cerris and sect. Quercus, on the grounds that the styles are elongated as in the former, but swollen at the apex as in the latter. But biologically they seem to be nearer to sect. Quercus, hybridising readily with Q. robur. All are species of Europe and W. Asia, except Q. dentata.
a. Q. dentata
b. Q. pontica
c. Q. frainetto, Q. macranthera, Q. pyrenaica
sect. Quercus (Lepidobalanus). – Leaves commonly lobed and deciduous, at least in the cultivated species, but sometimes coriaceous and persistent, as in, e.g., Q. ilex. Styles very short or wanting, abruptly widened into the stigma. Acorn-cup with short, usually appressed scales; inner wall of acorn usually glabrous, but tomentose in a few sections. Aborted ovules basal. This large group is the only one that occurs in both the Old and New World. It has many representatives in Mexico, but few of these have ever been introduced and not many are likely to be hardy with us. It is convenient to classify the subsections geographically:
OLD WORLD SPECIES
a. Q. lanata, Q. leucotrichophora (incana), Q. lodicosa
b. Q. engleriana
c. Q. ilex
d. O. aliena, Q. glandulifera, Q. mongolica
e. Q. boissieri, Q. canariensis (mirbeckii), Q. faginea, Q. fruticosa, Q. infectoria, Q. tlemcenensis
f. Q. hartwissiana
g. Q. petraea, Q. pubescens. Minor species: Q. brachyphylla, Q. congesta, Q. dalechampii, Q. iberica, Q. mas, Q. virgiliana
h. Q. robur. Minor species: Q. brutia, Q. haas, Q. pedunculiflora, Q. thomasii
EAST AMERICAN SPECIES
a. Q. bicolor, Q. michauxii, Q. muehlenbergii, Q. prinoides, Q. prinus
b. Q. stellata
c. Q. lyrata
d. Q. alba
e. Q. macrocarpa
WEST AMERICAN SPECIES
a. Q. sadleriana
b. Q. douglasii
c. Q. garryana, Q. lobata
d. Q. gambelii, Q. utahensis
A. Q. glabrescens
B. Q. reticulata; also Q. warburgii (of Mexican affinity, but not known in the wild and possibly a hybrid)
sect. Protobalanus. – A small group called by Trelease ‘the intermediate oaks’. They resemble the section Quercus in several respects, e.g., the short styles and the position of the abortive ovules, but the fruits take two years to ripen and the inside of the shell of the acorn is tomentose, as in the section Erythrobalanus. It is represented in cultivation by Q. chrysolepis and the closely allied Q. vacciniifolia. Another member, Q. tomentella, grows at Exbury but does not thrive.
sect. Erythrobalanus. – This New World section is mainly represented in cultivation by the red oaks and willow oaks of N. America. In the former the leaves are lobed, and the lobes are awn-tipped and never rounded (except sometimes in Q. arkansana, Q. marilandica, and Q. velutina); in the willow oaks the leaves are entire but usually have an awn at the apex; in Q. nigra and to a lesser degree in Q. laurifolia they are obscurely lobed. With the exception of Q. kelloggii, all these species are natives of eastern and central N. America. In the live oaks of the west, the leaves are coriaceous and entire or spinose.
The styles in this section are elongated and capitate. The fruit usually takes two years to ripen; the wall of the acorn is tomentose on the inside; and the abortive ovules are apical.
Central America, and especially Mexico, is very rich in species of the section Erythrobalanus. No fewer than thirty-five of the subsections recognised by Trelease in The American Oaks are native to this region, and there is one in South America, in the Andes of Colombia. But only two Mexican species are known to have reached maturity in Britain.
a. Q. arkansana, Q. marilandica
b. Q. laevis
c. Q. falcata
d. Q. ilicifolia
e. Q. palustris
f. Q. velutina
g. Q. rubra, Q. coccinea, Q. shumardii, Q. ellipsoidalis (mentioned under Q. palustris)
h. Q. kelloggii
a. Q. imbricaria, Q. laurifolia, Q. phellos b. Q. nigra
WESTERN AND SOUTH-WESTERN LIVE OAKS
a. Q. agrifolia, Q. wislizenii
b. Q. hypoleucoides
a. Q. crassifolia
b. Q. crassipes
The oaks are amongst the finest of the large trees of temperate regions. The two native of Britain, Q. robur and Q. petraea, are the largest and longest lived of our deciduous trees, and produce the most valuable timber. Nor are they surpassed in rugged beauty and strength. Their maximum duration of life is probably not less than one thousand years. For some reason the planting of oaks in parks and gardens has fallen into desuetude in recent times. Beyond a few of the commoner sorts, they are now stocked by very few nurserymen, who cannot, of course, be expected to keep up supplies for which there is no demand. With one exception, no firm now grows oaks in such number and variety as did Lee of Isleworth, Smith of Worcester, or Booth of Hamburg, a century ago.
Some deciduous species are amongst the handsomest and most striking in foliage of all our big trees, and would impart distinction to any demesne, whilst Q. coccinea and Q. palustris give the richest touches of crimson to our autumn landscape. Q. ilex forms a class by itself among evergreen trees hardy with us. Oaks, as a whole, thrive best on good deep loams. The old conception that the value of a soil for agriculture was indicated by the size and quality of the oaks upon it has many times been verified, not only in this country but in others, especially by the early settlers in both the east and west coast regions of N. America.
Oaks should always if possible be raised from acorns, which should be kept from getting dry after gathering until sown. Grafting has, perforce, to be resorted to for special varieties and rare species; but although one may see occasionally fine grafted specimens, the practice should only be adopted where absolutely necessary, for it tends to shorten the life of the tree, and in the end retard its growth. Unless ‘wild-source’ seed can be obtained, grafting must also be used to increase those species which cross readily with the common oak. A notorious example is Q. canariensis, which rarely comes true when raised from domestic seed, and the same seems to be true of Q. pyrenaica and Q. frainetto.
I strongly advocate getting all oaks into their permanent places as soon as possible. If I could, I would sow all acorns in situ, for thereby the tap-root is preserved and the plant never checked, but for many reasons that is not often possible except in pure forestry. Few trees in nurseries need transplanting with greater regularity every two or three years than oaks do if their final removal is to be accomplished safely, and few suffer more through shifting if their roots have been allowed to wander at will for a longer term. Evergreen species especially are liable to die. They should never be transplanted until after they show signs of growth in late May or early June, or else in September.
Some of the deciduous oaks are infested with an extraordinary variety of gall-producing insects, the best known of which are those that produce oak-apples and flat, circular, disk-like galls, sometimes so dense on the leaf as to partially overlay each other. Although frequently a disfigurement, and inducing a premature yellowing of the leaf, the production of galls does not seem to have noticeable effects on the health of trees. There is no generally practicable means of preventing them.
Oak Wilt. – This disease, which is confined to the United States, was first recorded there in the 1940s, and a decade later was threatening some important timber-producing areas. Red oaks are particularly susceptible and may start to die within a few weeks of infection. The white oaks (to which our native species are related) are more resistant and may recover from an infection. Oak wilt is related to Dutch elm disease and is spread in the same manner, by root-grafts and insects. But the species believed to act as vectors in the USA are sap-feeders, or small bark beetles which breed only in thin branches. These are much less efficient in transmitting the disease than the much larger European bark beetle would be should it ever reach Europe. However, stringent precautions are being taken by the EEC countries to prevent this occurring.