Eucalyptus is an enormous and fascinating genus. It comprises over 700-800 species, and accounts for more than two-thirds of Australia’s vegetation. They populate almost every habitat in Australia, from high snowy mountains to arid deserts to tropical rainforests.
Nearly all species of Eucalyptus go through a change (which may be abrupt or gradual) from juvenile foliage, which is often round and stem-clasping, to distinctly different mature foliage, which is usually long and willowy. This often comes as a surprise to the gardener who is not expecting this change. But it does explain something that can be initially confusing: Some people will think of Eucalyptus as those large trees in Australia with willowy leaves that Koalas eat, and other people will think of Eucalyptus as those cute little silvery stems they use in floral wreaths and boquets. Indeed, they are both Eucalyptus. In general, the small cute stems represent the foliage in the juvenile phase of the plant's life, and the long willowy leaves are characteristic of mature trees.
Not all euc species go through this foliage change in the same time frame or manner, and several different species planted together can make for quite a variety of foliage when the plants are young. Often they will continue to grow juvenile foliage near the base of the tree long after mature foliage has been growing higher up. In many species the change manifests itself when the tree is about 1 - 2 years old, and in some only the first few leaves of the tree will be juvenile before the change is complete. Some exceptions include E. cordata, whose foliage usually doesn't change much at all, and E. kitsoniana, which tends to have more leaves that are "intermediate" rather than clearly juvenile or mature.
Many eucs have showy flowers, but the majority of these are the more cold-tender species from desert or subtropical regions. Unless otherwise mentioned, all the species listed in the species index have white or cream flowers that are not important in adding ornamental value to the tree. The flowers of E. pauciflora and its subspecies, E. delegatensis and E. regnans, to name a few, are a bit larger and more obvious, but still not really showy. Most species flower after about three to six years of growth. For example, E. gunnii can flower in three years and E. perriniana has been known to flower in as little as two years. Though some species flower unpredictably at various seasons, some will predictably flower at certain times of the year. This varies for each species, but quite a few of them bloom in the spring. Some of them even flower in the winter.
It is unfortunate that eucs are commonly regarded as not being reliably hardy outside of the subtropics, because it discourages people from planting them in cooler areas. There are many reasons to plant a euc - to begin with, they help to control aphids and other insects, most of them will grow rapidly even in poor soil (12' per year is not out of the question), and most of them are very attractive--eucs have a unique beauty all their own. And eucalyptus foliage, which shows incredible variety, is excellent for cut foliage in floral arrangements.
Eucayptus classification and taxonomy
Eucalyptus classification is quite complicated and somewhat confused. To begin with, many taxonomists have now agreed upon the division of the genus Eucalyptus into several new genera, including Symphiomyrtus, Corymbia, Eudesmia and Monocalyptus. I suspect it will be a while before this reclassification takes hold in cultivation - it will be a long time before we get used to calling Dwarf Yellow Bloodwood "Corymbia eximia 'nana'" or Spinning Gum "Symphiomyrtus perriniana." As long as this does not make a big difference in the knowledge of their cultivation, I am not going to fuss over it.
Perhaps a more useful division, and one that probably corresponds somewhat with the afoermentioned classification, is to think of eucs as being in different groups based on their appearance. For example, we have the "blue gum group" (i.e. E. nitens, E. cypellocarpa, E. globulus, etc.), the "swamp gum group" (i.e. E. camphora, E. ovata, E. yarraensis, etc.), the "peppermint group" (i.e. E. tenuiramis, E. pulchella, etc.), "stringybarks," "ashes," and so forth. Often a clue to what group a species falls in can be found in its common name. I hope to do some more research on the characteristics that distinguish these groups and make it easier to tell which species belong to each. In some instances there does seem to be some overlap in these groups.
An additional complication is that the demarcation between some species, and the subspecies of some species, is not clear or resolved. To make matters even more complex, some species hybridize with each other readily. The swamp gums, for example, seem to be a very confusing group, with various populations becoming less distinct there their ranges merge into each other. Flower structure is a major consideration in Eucalyptus classification, and I have not taken any position on classification at all, since I have observed so few eucs flowering in cultivation. I have tried to retain the most familiar classifications where possible. The E. pauciflora group is one example of this--in the species index I have listed them as subspecies of E. pauciflora, the old classification of plants in this group, even though E. niphophila, E. debeuzevillei, etc. are now regarded as separate species by the majority of authorities.
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