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.
From the Supplement (Vol. V)
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.