Eucalyptus

Eucalyptus is a versatile plant in the home and garden. Houseplant enthusiasts adore the fragrance it brings to the home. Eucalyptus makes a striking accent in landscapes with gorgeous foliage and colorful bark. Eucalyptus plants are easy to care for and can be grown in containers, maintained as a shrub, or allowed to mature into a tree. Plant in spring after all danger of frost has passed. Eucalyptus is a fast-growing plant that can grow 6 to 8 feet in a season, though it tends to stay smaller in cooler climates. It is often trained as a standard.

I. Appearance and Characteristics

Eucalyptus () is a genus of more than 700 species of flowering plants in the family Myrtaceae. Most species of Eucalyptus are trees, often mallees, and a few are shrubs. Along with several other genera in the tribe Eucalypteae, including Corymbia and Angophora, they are commonly known as eucalyptus or “gum trees’. Plants in the genus Eucalyptus have bark that is either smooth, fibrous, hard, or stringy, the leaves have oil glands, and the sepals and petals are fused to form a “cap” or operculum over the stamens. The fruit is a woody capsule commonly referred to as a “gumnut”.

Most species of Eucalyptus are native to Australia, and every state and territory has representative species. About three-quarters of Australian forests are eucalypt forests. Many eucalypt species have adapted to wildfire, are able to resprout after fire, or have seeds that survive fire.

A few species are native to islands north of Australia, and a smaller number are only found outside the continent. Eucalypts have been grown in plantations in many other countries because they are fast growing, have valuable timber, or can be used for pulpwood, honey production, or essential oils.

The genus Eucalyptus was first formally described in 1789 by Charles Louis L’Héritier de Brutelle who published the description in his book Sertum Anglicum, seu, Plantae rariores quae in hortis juxta Londinum along with a description of the type species, Eucalyptus obliqua. The name Eucalyptus is derived from the Ancient Greek words eu meaning ‘good’ and kalypto meaning ‘(I) cover, conceal, hide’ referring to the operculum covering the flower buds.

The type specimen was collected in 1777 by David Nelson, the gardener-botanist on Cook’s third voyage. He collected the specimen on Bruny Island and sent it to de Brutelle who was working in London at that time.

Size and habit

Eucalypts vary in size and habit from shrubs to tall trees. Trees usually have a single main stem or trunk but many eucalypts are mallees that are multistemmed from ground level and rarely taller than 10 metres (33 feet). There is no clear distinction between a mallee and a shrub but in eucalypts, a shrub is a mature plant less than 1 m (3 ft 3 in) tall and growing in an extreme environment. Eucalyptus vernicosa in the Tasmanian highlands, E. yalatensis on the Nullarbor and E. surgens growing on coastal cliffs in Western Australia are examples of eucalypt shrubs.

The terms “mallet” and “marlock” are only applied to Western Australian eucalypts. A mallet is a tree with a single thin trunk with a steeply branching habit but lacks both a lignotuber and epicormic buds. Eucalyptus astringens is an example of a mallet. A marlock is a shrub or small tree with a single, short trunk, that lacks a lignotuber and has spreading, densely leafy branches that often reach almost to the ground. Eucalyptus platypus is an example of a marlock.

Eucalyptus trees, including mallets and marlocks, are single-stemmed and include Eucalyptus regnans, the tallest known flowering plant on Earth. The tallest reliably measured tree in Europe, Karri Knight, can be found in Coimbra, Portugal in Vale de Canas. It is an Eucalyptus diversicolor of 72.9 meters height and of 5.71 meters girth.

Tree sizes follow the convention of:

• Small: to 10 m (33 ft) in height
• Medium-sized: 10–30 m (33–98 ft)
• Tall: 30–60 m (98–197 ft)
• Very tall: over 60 m (200 ft)

Bark

All eucalypts add a layer of bark every year and the outermost layer dies. In about half of the species, the dead bark is shed exposing a new layer of fresh, living bark. The dead bark may be shed in large slabs, in ribbons or in small flakes. These species are known as “smooth barks” and include E. sheathiana, E. diversicolor, E. cosmophylla and E. cladocalyx. The remaining species retain the dead bark which dries out and accumulates. In some of these species, the fibres in the bark are loosely intertwined (in stringybarks such as E. macrorhyncha or peppermints such as E. radiata) or more tightly adherent (as in the “boxes” such as E. leptophleba). In some species (the “ironbarks” such as E. crebra and E. jensenii) the rough bark is infused with gum resin.

Many species are ‘half-barks’ or ‘blackbutts’ in which the dead bark is retained in the lower half of the trunks or stems—for example, E. brachycalyx, E. ochrophloia, and E. occidentalis—or only in a thick, black accumulation at the base, as in E. clelandii. In some species in this category, for example E. youngiana and E. viminalis, the rough basal bark is very ribbony at the top, where it gives way to the smooth upper stems. The smooth upper bark of the half-barks and that of the completely smooth-barked trees and mallees can produce remarkable colour and interest, for example E. deglupta.

E. globulus bark cells are able to photosynthesize in the absence of foliage, conferring an “increased capacity to re-fix internal CO2 following partial defoliation”. This allows the tree to grow in less-than-ideal climates, in addition to providing a better chance of recovery from damage sustained to its leaves in an event such as a fire.

Different commonly recognised types of bark include:

• Stringybark—consists of long fibres and can be pulled off in long pieces. It is usually thick with a spongy texture.
• Ironbark—is hard, rough, and deeply furrowed. It is impregnated with dried kino (a sap exuded by the tree) which gives a dark red or even black colour.
• Tessellated—bark is broken up into many distinct flakes. They are corkish and can flake off.
• Box—has short fibres. Some also show tessellation.
• Ribbon—has the bark coming off in long, thin pieces, but is still loosely attached in some places. They can be long ribbons, firmer strips, or twisted curls.

Leaves

Nearly all Eucalyptus are evergreen, but some tropical species lose their leaves at the end of the dry season. As in other members of the myrtle family, Eucalyptus leaves are covered with oil glands. The copious oils produced are an important feature of the genus. Although mature Eucalyptus trees may be towering and fully leafed, their shade is characteristically patchy because the leaves usually hang downwards.

The leaves on a mature Eucalyptus plant are commonly lanceolate, petiolate, apparently alternate and waxy or glossy green. In contrast, the leaves of seedlings are often opposite, sessile and glaucous. But many exceptions to this pattern exist. Many species such as E. melanophloia and E. setosa retain the juvenile leaf form even when the plant is reproductively mature. Some species, such as E. macrocarpa, E. rhodantha, and E. crucis, are sought-after ornamentals due to this lifelong juvenile leaf form. A few species, such as E. petraea, E. dundasii, and E. lansdowneana, have shiny green leaves throughout their life cycle. Eucalyptus caesia exhibits the opposite pattern of leaf development to most Eucalyptus, with shiny green leaves in the seedling stage and dull, glaucous leaves in mature crowns. The contrast between juvenile and adult leaf phases is valuable in field identification.

Four leaf phases are recognised in the development of a Eucalyptus plant: the ‘seedling’, ‘juvenile’, ‘intermediate’, and ‘adult’ phases. However, no definite transitional point occurs between the phases. The intermediate phase, when the largest leaves are often formed, links the juvenile and adult phases.

In all except a few species, the leaves form in pairs on opposite sides of a square stem, consecutive pairs being at right angles to each other (decussate). In some narrow-leaved species, for example E. oleosa, the seedling leaves after the second leaf pair are often clustered in a detectable spiral arrangement about a five-sided stem. After the spiral phase, which may last from several to many nodes, the arrangement reverts to decussate by the absorption of some of the leaf-bearing faces of the stem. In those species with opposite adult foliage the leaf pairs, which have been formed opposite at the stem apex, become separated at their bases by unequal elongation of the stem to produce the apparently alternate adult leaves.

Flowers and fruits

The most readily recognisable characteristics of Eucalyptus species are the distinctive flowers and fruit (capsules or “gumnuts”). Flowers have numerous fluffy stamens which may be white, cream, yellow, pink, or red; in bud, the stamens are enclosed in a cap known as an operculum which is composed of the fused sepals or petals, or both. Thus, flowers have no petals, but instead decorate themselves with the many showy stamens. As the stamens expand, the operculum is forced off, splitting away from the cup-like base of the flower; this is one of the features that unites the genus. The woody fruits or capsules are roughly cone-shaped and have valves at the end which open to release the seeds, which are waxy, rod-shaped, about 1 mm in length, and yellow-brown in colour. Most species do not flower until adult foliage starts to appear; E. cinerea and E. perriniana are notable exceptions.

II. How to Grow and Care

Sunlight

Eucalypts like full sun. It takes at least 10 hours of daily sunlight for most varieties to grow sturdy trunks. Sunlight also ensures the trees have a good shape and healthy leaves.

Temperature and Humidity

Eucalyptus do well in moderate temperatures, between 65ºF and 75ºF. If the temperatures dip below 50ºF, they suffer, so if you’re growing them in pots, bring them indoors if they’re outside. A bit of humidity is good for eucalyptus plants.

Watering

As mentioned before, some of these trees like lots of water. Others resist drought when mature. Young trees especially will need watering deeply at least once a week when it’s not raining. Once established, the trees will go a while without water, but if the leaves start drooping madly and falling off, up the water schedule. Adding a layer of mulch around the base of the plants will preserve water.

Soil

Well-draining soil is essential. Provide slightly acidic soil with a pH of 5.5 – 6.5. If your soil is alkaline or retains a lot of water, like heavy clay, you can amend it to increase drainage and add acidity. Agricultural sand and perlite break up and aerate heavy soils. Compost assists with better nutrient exchange and retention in sandier soils.

Fertilizing

Outdoor plants need no extra feeding. They are highly adaptable to nutrient-poor soils and don’t require fertilization to grow. You don’t want to give them extra nutrients and have either over or under-performance as a result.

Pruning

There are several methods for pruning eucalyptus depending on your needs and which species you are growing.

One method—coppicing—involves cutting the tree or shrub back to almost ground level occasionally to stimulate growth. This method, which is particularly effective for E. gunnii and E. globulus cultivars, keeps the foliage in the juvenile stage, encourages new stems each year, and helps manage the size of the plant. To coppice your eucalyptus, cut back all the stems (to about 6 to 12 inches from the ground) using angled cuts in late winter or early spring. In the early stages, it may take more than one season for new growth to return, but after years of regular coppicing, vibrant new growth should return each season. The cuttings from coppiced eucalyptus plants are excellent for drying or using in floral arrangements.

If you are pruning your eucalyptus to form a shrub or hedge, cut back about one-third of the plant’s height at the end of its second growing season. After each subsequent growing season, you will only need to remove about a quarter of the plant’s height to maintain a reasonable size and shape.

If you are growing eucalyptus as a tree, it won’t need much pruning beyond the occasional removal of dead leaves and branches. You can, however, begin pruning some of the lower branches (after the tree is at least 2 years old) if you would like to create a more defined canopy when the tree reaches maturity. You can also cut eucalyptus branches from your tree to use in floral arrangements or for their aromatic properties.

For potted eucalyptus, you can snip off unruly or overgrown ends as necessary, but avoid doing any major pruning until spring. Come spring, you can prune your plant to direct its shape and remove any dead or broken branches.

Propagation

Propagation can be done with cuttings taken during late summer from trees between two and 12 months after planting. Cut a 5-inch branch and dip it in rooting hormone. Add the branch to a pot with a growing medium. Set the pot in indirect sunlight to keep the plant at around 70ºF. In about a month, roots should form.

Planting eucalyptus from garden plants or seeds is the easiest way to grow them, but many species of eucalyptus seeds must be stratified in the refrigerator for at least two months before planting. This will simulate winter and promote germination. Toss the seeds in a plastic bag with some vermiculite, perlite, or sand and spritz the contents to moisten the mix.

Date the bag and place it in the refrigerator for 6 to 8 weeks. When you are ready to plant—preferably two months before the last frost, fill a container (if growing indoors), starting tray, or growing pot with a well-draining potting mix and place the seeds on top, lightly covering them with soil. Place the container in a warm place with lots of indirect light and spritz the seeds daily as they germinate (which may take 3 to 4 weeks).

Potting and Repotting

Eucalyptus are fast growers; their roots can quickly fill a small container. Their roots are quite sensitive, so it’s best to start with a larger pot to avoid the need to transplant them later on. If you’re growing eucalyptus as a houseplant, or overwintering one, give it as much sun as possible in the home. Usually, a bright southern exposure will work best.

Pests and Diseases

The longhorned borer beetle attacks eucalyptus in California. They create holes in the bark where liquid will ooze out. Unfortunately, there’s no effective pesticide for these pests, so tree management is essential to keep these insects from damaging trees.7 Otherwise, there are few pests or problems that can harm eucalyptus trees.

III. Uses and Benefits

• Ornamental uses

In landscaping, eucalyptus is often used as a shade tree or as a windbreak. It can also be grown as a specimen tree or in a mixed border. It is also widely planted for erosion control and reforestation.

• Pulpwood

Eucalyptus is the most common short fibre source for pulpwood to make pulp. The types most often used in papermaking are Eucalyptus globulus (in temperate areas) and the Eucalyptus urophylla x Eucalyptus grandis hybrid (in the tropics). The fibre length of Eucalyptus is relatively short and uniform with low coarseness compared with other hardwoods commonly used as pulpwood. The fibres are slender, yet relatively thick walled. This gives uniform paper formation and high opacity that are important for all types of fine papers. The low coarseness is important for high quality coated papers. Eucalyptus is suitable for many tissue papers as the short and slender fibres gives a high number of fibres per gram and low coarseness contributes to softness.

• Eucalyptus oil

Eucalyptus oil is readily steam distilled from the leaves and can be used for cleaning and as an industrial solvent, as an antiseptic, for deodorizing, and in very small quantities in food supplements, especially sweets, cough drops, toothpaste and decongestants. It has insect-repellent properties, and serves as an active ingredient in some commercial mosquito-repellents. Aromatherapists have adopted Eucalyptus oils for a wide range of purposes. Eucalyptus globulus is the principal source of Eucalyptus oil worldwide.

• Musical instruments

Eucalypt wood is also commonly used to make didgeridoos, a traditional Australian Aboriginal wind instrument. The trunk of the tree is hollowed out by termites, and then cut down if the bore is of the correct size and shape.

Eucalypt wood is also being used as a tonewood and a fingerboard material for acoustic guitars, notably by the California-based Taylor company.

• Dyes

All parts of Eucalyptus may be used to make dyes that are substantive on protein fibers (such as silk and wool), simply by processing the plant part with water. Colors to be achieved range from yellow and orange through green, tan, chocolate and deep rust red. The material remaining after processing can be safely used as mulch or fertilizer.

• Prospecting

Eucalyptus trees in the Australian outback draw up gold from tens of meters underground through their root system and deposit it as particles in their leaves and branches. A Maia detector for x-ray elemental imaging at the Australian Synchrotron clearly showed deposits of gold and other metals in the structure of Eucalyptus leaves from the Kalgoorlie region of Western Australia that would have been untraceable using other methods. The microscopic leaf-bound “nuggets”, about 8 micrometers wide on average, are not worth collecting themselves, but may provide an environmentally benign way of locating subsurface mineral deposits.