No other single genus of trees dominates so vast and climatically so diverse an area as do the eucalypts in Australia. By far the greater part of the natural vegetation of the sub-continent outside the deserts and semi-deserts consists of communities of woody plants in which the eucalypts play a predominant or leading role. Only in those parts of eastern Australia and Tasmania where the rainfall is high enough to support tropical to temperate rain-forest do they recede in importance.
Some 500 species are recognised, all in Australia and Tasmania save a few found in the larger islands of the S.W. Pacific. The genus is absent from New Zealand in the wild state. In life-form the eucalypts range from tall trees – in E. regnans attaining a height of over 300 ft – to shrubs and small, stunted trees. A peculiar form – the mallee – is assumed by many species in the dry regions of southern Australia; in these the most permanent part of the plant is a large, woody root-stock bearing numerous slender stems and containing a supply of dormant buds which quickly develop if the stems are killed by fire or drought.
The bark is very varied and serves as a useful character for identification in the field. In many species it is persistent and in mature trees becomes thick, hard and deeply furrowed (as in the Ironbarks) or fibrous (as in the Stringybarks and so-called Ashes). In the Gumbarks – and here belong nearly all the species cultivated outdoors in this country – the bark is smooth and deciduous, the outer (older) layers being regularly shed in flakes or ribbons. The freshly exposed bark is white or distinctly coloured, and contrasts with the darker colouring of the older layers.
The eucalypts have the peculiarity, not uncommon among Australasian plants, of having a juvenile phase during which the leaves produced are strikingly different from those of the adult plant. In Eucalyptus these are usually shorter and broader than the adult ones, sessile or short-stalked and, at least at the base of the shoot, opposite. The duration of this phase varies with the species and usually gives way, through an intermediate phase, to the production of adult leaves, which are alternate, stalked, mostly lanceolate or sickle-shaped, with entire margins. The blades are usually identical in appearance on the two surfaces (isobilateral). In some species (e.g. E. cordata) there is little difference between the juvenile and adult leaves. The juvenile phase can be prolonged by regular pruning and there is always a reversion to it on stump shoots.
Flowers in most species are borne in axillary umbels (rarely solitary). The buds are formed in summer on the young shoots and open about a year later. The most conspicuous feature of the flower is the numerous stamens, which are creamy white in the species described here but in the beautiful but tender E. ficifolia (and other species from W. Australia) they are in some shade of red. They are inserted on the rim of the calyx-tube (receptacle) and in bud are enclosed in a cap known as the ‘operculum’, made up of the united sepals and petals, which fall off as the stamens develop. The ovary is embedded in the calyx-tube and develops into a capsule opening by valves to release the numerous small seeds; the calyx-tube itself becomes enlarged and woody, and is the most conspicuous feature of the fruit. The top of the capsule may be more or less level with the rim of the tube, with the valves exserted; or it may be sunk within the tube, with the valves wholly or partly concealed. The nectar-secreting disk of the flower may become woody and enlarged in fruit, partly covering the top of the capsule. The capsules need at least one year to ripen and may remain closed for several years. The seeds are small and usually, at least on cultivated trees, only a few are formed in each capsule, most of the ovules remaining unfertilised and turning into chaff.
Eucalyptus is, for the taxonomist, a genus full of complexities. A species of wide range may be subdivided into numerous geographical and ecological races which have in common the broader characters by which the species is defined but may differ quite considerably in minuter details. Intercrossing between species is also quite common in nature: this further increases the difficulty of identification and necessitates the proviso that some of the cultivated specimens mentioned in the following account might prove not to be true to type were they to be examined by an experienced eucalyptologist. It should also be added that there are few woody genera in which more complete material is needed if a specimen is to be accurately identified. In addition to adult foliage, flower and fruit, the sample should include juvenile foliage and a description of the bark.
These taxonomic complexities may not be of much concern to the grower, but the variability in hardiness shown by many species when brought into cultivation is of the greatest importance. All but the very hardiest species will have their more tender forms and equally the reputed tenderness of a certain species may be due simply to its having hitherto been represented in cultivation by a tender provenance. Seed from Australia, and plants raised from it, should therefore be treated with some circumspection, unless it is known that the seed was collected with frost-resistance in mind.
The eucalypts must be raised from seed, which should be sown thinly in deep containers and the seedlings potted-off, when the second pair of seed-leaves has developed, into paper, fibre, or polythene pots (or sleeves) 31⁄2 in. deep or longer. Ideally, the seed should be germinated in February-March with artificial heat, the plants grown on under cool-house conditions and put into their final positions during the summer of the same year. By autumn they will have become well established and can face the dangers of the ensuing winter with as good a chance of survival as potted plants overwintered in an unheated house or frame. Despite the obvious risks, this early planting out is to be recommended, as the eucalypts resent any restrictions at the roots and will develop quicker and need artificial support for a much shorter time if allowed to grow freely from an early age. If it is intended that the young plants should be planted out in their second year, the seed should be sown later, in late spring or early summer and potted-off by August. For further details, see the article by R. C. Barnard cited below.
Eucalypts have been so infrequently planted on the chalklands of south-eastern England that nothing can be said for certain about their suitability for such soils. But E. parvifolia lived for many years in chalk at Messrs Hilliers Winchester nursery and it has been reported that E. dalrympleana will tolerate a similar soil.
The most recent work on the taxonomy of the genus is: W. F. Blakely, A. Key to the Eucalypts, 2nd Ed., Canberra, 1955. Emendations to this, with references to recent literature, will be found in: R. D. Johnston and R. Marryatt, Taxonomy and Nomenclature of Eucalypts, Canberra, 1965. For British growers the most useful works are ‘An Introduction to some Garden Eucalypts’, by R. C. Barnard, published in Journ. R.H.S., Vol. 91 (1966), pp. 209-216, 250-261, 293-303; and the section on Eucalyptus in Dictionary of Gardening, Supplement 1969, pp. 282-288. Mr Barnard has kindly read through the following pages in proof and made some valuable suggestions.
From the Supplement (Vol. V)
Of particular interest to British growers is: M. I. H. Brooker and D. A. Kleinig, Field Guide to Eucalypts, Vol. 1 (1983). Devoted to the species of south-eastern Australia (including Tasmania), this consequently deals with virtually all the eucalypts grown outdoors in this country. There is a botanical key, colour photographs for each species showing buds, fruits, bark and habit, and a distribution map.