Eucalyptus belongs to the family Myrtaceae that has more than 700 species of flowering trees and shrubs. They are commonly known as Eucalypts. Most of the species of Eucalyptus are native to Australia. Most of the trees in Australian forests are Eucalypts. Approximately three-quarter of forests in Australia is Eucalypt forests.
Australian landscapes observe wildfires often. Eucalyptus species are adapted to fire which lets them re-sprout after a fire. Their seeds are resistant to fire which makes Eucalyptus species the dominant plant type in Australian forests.
Some species of Eucalyptus are also found in other countries. They have been grown in numerous countries because they grow fast and provide valuable timber. They are used for plywood and production of honey and essential oils. However, they are flammable which has resulted in their removal from some countries.
Eucalypts have can have a smooth, stringy, or a fibrous hard bark. Their leaves have oil glands, and their petals and sepals are fused together to form a ‘cap’ over the stamens, the fruit of plants in this species is a woody capsule which is commonly known as a gumnut.
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History of Eucalyptus
Eucalyptus fossils indicate that their evolutionary roots lie in Gondwana back in time when Australia was connected to Antarctica. They originated from South America, where they are surprisingly no longer endemic. Fossil leaves were also found in New Zealand as well, where they are not present today. The DNA sequencing of Eucalyptus fossils indicates that they are as old as 52 million years.
Description of Eucalyptus Plants
Size and Habit of Eucalypts
Eucalypts range from shrubs to trees. Eucalypt trees usually have a single trunk but some species, like Mallees, are multi-stemmed and are short, rarely taller than 10 meters or 33 feet. A Eucalypt shrub is less than 1m or 3 feet in height and can grow in extreme conditions.
Bark of Eucalypts
The uppermost layer of Eucalypt bark is replaced every year. in most species, the dead bark is shed as ribbons, small flakes, or as large slabs, exposing a new layer of bark underneath. These species of Eucalypts are called ‘smooth barks’.
Some species are known as ‘half-barks’ in which the dead bark is retained at the lower half of the trunk. It forms a thick accumulation at the base.
The common types of barks that have been registered are;
Stringybark has long fibers which can be pulled off as long piece. These barks are thick and have a spongy texture.
Ironbark is thick, hard, and deeply furrowed. It appears dark red or black because it is impregnated with sap that the tree gives out (dried kino).
- Tessellated Bark
Tessellated bark has cork and they can flake off as it is broken into many flakes.
Box barks have short fibers.
- Ribbon Bark
Ribbon bark is the one that comes off as thin and long pieces but loosely attached at some points. They can be twisted curls, long ribbons, or firmer strips.
Leaves of Eucalypts
Most of the Eucalypts are evergreen but those species which are not losing their leaves at the end of the dry season. The leaves are covered with oil glands, which is a characteristic of Myrtaceae family. The copious oils that these glands produce are an important feature of plants of this genus. The leaves of Eucalypt trees hang downwards.
The leaves of a mature plant in this species are petiolate, lanceolate, apparently alternate, glossy or waxy green. The leaves of the seedling, however, are quite the opposite. They are glaucous and sessile.
The four-leaf phases identified in Eucalyptus plants are a seedling phase, juvenile phase, intermediate phase, and adult phase. The intermediate phase has the largest leaves.
In the majority of species, leaves form in pairs that appear on opposite sides of a square stem. Consecutive pairs of leaves are present at right angles to each other.
Flowers of Eucalypts
Flowers and fruits are the most readily recognizable feature of Eucalyptus plants. Flowers have many fluffy stamens that are white, yellow, pink, cream, or red. The stamens are enclosed in a cap that is called the operculum. Operculum consists of fused sepals, petals, or both. Thus, these flowers do not have petals. They decorate themselves with the showy stamens.
Types of Eucalyptus
There are numerous varieties of Eucalyptus. Many varieties, ranging from 120 feet tall trees and 4 feet tall shrubs have been widely planted in the U.S.
1. Mallet Eucalyptus
Mallet Eucalyptus is also known as open-branched forms. They have a single trunk. The branches are angled steeply upwards and they have open space between them. They lack lignotuber and epicormic buds as in Eucalyptus astringens. They fall in the USDA plant hardiness zones 9 through 10.
It is commonly known as the Brown Mallet. It is native to Western Australia’s South West region. It is commonly found on ridges, rocky outcrops, hills, breakaways, and valley floors in Great Southern and South West Goldfields-Esperance and Southern Whaetball regions of Western Australia.
It is found growing in brown clayey sand, red-brown gravelly sand, sandy loam, and laterite, spongolite and sandstone based soils.
Eucalyptus astringens trees are usually 5 to 49 feet tall. The bark of Eucalyptus astringens trunk and stems are grey-brown colored, smooth, and shiny. The leaves are lance-shaped. Flower buds are pendulous and are arranged in clusters of seven. They are cream or pale-lemon colored flowers. The fruit is bell-shaped, cup-shaped, or conical.
The wood from these trees is used for mining timbers, construction, and for making tool handles. It is also suitable to be sued for firewood. The bark is rich in tannin (40%) which makes it suitable to be used for tanning of leathers and for adhesives’ production.
It is commonly known as the Sugar Gum. It is native to South Australia. It is found in three distinct regions which are Eyre Peninsula, Flinders Ranges, and Kangaroo Island.
It is a fast-growing species. It grows best in sandy, loamy, or clayey soil.
Sugar Gums growing in each region has different features. Sugar gum that grows in Flinders Ranges is usually 115 feet in height. They have a classic gum habit having a straight trunk. The trunk has a diameter of 3.3 to 4.11 feet. Their branches are steep and occur halfway up. Each main branch of Sugar gum ends with a little canopy. They are planted for timber and are cultivated as windbreaks for farms.
Sugar Gum growing in the Kangaroo Island and the Eyre Peninsula are shorter as compared to those growing in Flinders Ranges. They are about 26 to 49 feet tall. Their trunks are crooked and have a diameter of 1.4 feet. The habit of the crown is open spreading. They typically spread from 39 to 49 feet.
The leaves are glossy and discolorous. Flowers are white, cream, or yellow colored. The fruits are barrel-shaped or urceolate and are longitudinally ribbed.
They are planted across Southern Australia to be used as shelterbelt or windbreak, for timber and firewood. The wood from these trees is used in making furniture, posts, flooring, construction timber, and railway sleepers because it is termite resistant.
It is commonly known as the Brittle Gum. It is native to Eastern Australia.
It grows best in soils that contain large amounts of clay, in rocky and shallow soils.
Brittle Gum grows as tall as 20m and spread to about 13m. They have smooth, white colored trunk having grey patches. These grey patches become pink in late spring or in summer. The bark turns into a red color before it sheds. They have an open spreading habit. The limbs are contorted.
The leaves are narrow and are in dull green in color. The flowers are creamy white and grow in clusters during the summer through autumn.
They are grown in gardens, parks, and other areas with open space. They can be used for wider streetscapes. They form a great habitat for native insects and birds.
2. Eucalyptus Marlock
Eucalyptus marlock is densely branched trees. The branches are densely leaved. The leaves of Eucalyptus marlock come all the way down to the ground. They fall in USDA hardiness zones 7 through 8.
It is commonly known as round-leaved moort or simply moort. This small tree is native to Western Australia; it’s found growing in the area between Albany and Esperance.
Moort grows best in loamy, in sandy or clayey soils often found around laterite.
These trees grow from 5 to 33 feet in height. They are 16 to 33 feet in width. Trees have a dense canopy. The leaves are a distinctive glossy, grey-green or green in color, concolorous and coriaceous and are elliptical to orbicular in shape. Moort have smooth bark that is grey-brown, light brown, or copper in color which turns grey as the tree ages. They are single-stemmed and form thickets. They may have a poorly developed lignotuber.
Elongated and flat peduncles are characteristic features of Moort. The buds are stalk-less having long, conically shaped caps that are clustered in groups of seven. The flowers are greenish-yellow or creamy-white flowers. The flowers are rarely red. In spring and summers, the flowers are obscured by dense foliage.
Eucalyptus platypus plants are usually used for ornamental purposes.
It is commonly known as Bald Island marlock or bushy yate. It is native to the south coast of Western Australia.
Bald Island marlock grows in sandy-loamy soil or in soils over quartzite or granite.
Bald Island marlock grows to a height of 7 to 26 feet. The bark is smooth and whitish-grey or grey in color. they grow in a bushy habit. Adult leaves are about 3.5 inches in length are 0.98 inches in width. They are elliptical, concolorous, glossy and light green in color. The flowers are yellow-green. Each axillary inflorescence has about 7 to 20 flowers. The sessile buds have fused hypanthia. Fruits of Bald Island marlock have an ascending disc along with three valves that are strongly exserted.
This plant is used for ornamental purposes. It is also used for erosion control and as a windbreak.
3. Eucalyptus Mallee
Mallee are low-growing shrubs. They look more like shrubs than they look like trees. They are multi-stemmed. They typically grow to a height of 10 feet but if the conditions are favorable, they can grow to a height of 25 feet. They fall under USDA hardiness zones 7a through 10b.
They are commonly known as sand mallee, tall sand mallee and Easter Goldfields homed mallee. They are native to Western Australia.
Sand mallee grows best in loamy, sandy and clayey soil.
Sand mallee has multi-stems that grow from a lignotuber. Single stem mallee is also found. It grows 7 to 26 feet tall. It is 13 to 26 feet wide. The habit of these plants is dense, upright, spreading, and open canopy. The bark of these plants is polished and smooth. The new bark that is exposed after shedding of dead bark is yellowish-brown in a color that becomes brownish-grey after maturation.
The leaves are grey-green in color. They are thick, stiff, and concolorous, having a distinctive arrangement. The leaf blade is elliptic shaped and has a narrow lanceolate. It is tapered at the base. Narrow-flattened or channeled petioles support the leaves. The flowers are cream-yellow-pink. A single axillary conflorescence has 3 to 7 flowered peduncles. Fruits are cylindrical or pear-shaped (pyriform) having a depressed disc and valves that are exerted. Fruits and flowers usually have a long tapering bud cap.
Eucalyptus eremophila are used for ornamental purposes. They make good, contrasting background plants in wide verges, parks, nature strips, and reserves. They are also used as a windbreak and for controlling wind erosion. They also attract insects and birds for nectar.
Eucalyptus erythronema is commonly known as red-flowered mallee, white mallee, white-barked mallee, or Lindsay gum. It is native to Western Australia.
White mallee grows best in sandy and clay soils with lateritic gravel.
White Mallee tree grows to a height of 7 to 20 feet. they can be single-stemmed or multi-stemmed, forming from a lignotuber. The bark is white colored. A new pale green bark is covered with a talc-like powder which is pinkish in color. the bark may be purplish, white, light pink, or salmon colored.
The adult leaves are thin and concolorous, having a distinctive arrangement. The leaf blade is in the shape of narrow lanceolate and tapered at the base. Petioles that are narrowly flattened or channeled support the petioles. Flowers are usually red but they can be creamy white, yellow, or pink. Each axillary conflorescence has 3 to 7 flowered umbellasters supported by peduncles.
They form great ornamental plantings. They are also used for the production of honey, products that are rich in tannin, and as a screening plant.
4. Decorative Eucalyptus
Some eucalyptus varieties are decorative and iconic. Eucalyptus pulverulenta, the silver mountain gum is loved by the florists. They provide interesting color to flower arrangements. These varieties are majorly used for ornamental purposes because of the shapes and colors of their flowers!
Problems with Eucalyptus Tree Varieties
Eucalyptus tree varieties are associated with certain problems which can be quite troublesome.
- Some varieties of Eucalyptus trees are invasive. This means that they grow out of their cultivation zone, into the wild. This shades out the native plants. an example of an invasive eucalyptus tree is Eucalyptus globules.
- Eucalyptus trees are rich in pungent oils. This makes them fire hazards when they are plated in groups.
Eucalyptus trees are extremely useful to humans. Essential oils extracted from them are used in medicines. Eucalyptus is said to hold significant health benefits like in cough, flu, insect bites, muscle and joint pains, and respiratory illnesses. Moreover, Eucalyptus has numerous household uses like as a component of cleaners, soaps, stain removers, carpet cleaners, etc. They are also an important source of timber in Australia. Different types of Eucalyptus offer different uses making this genus of plants extremely important to humans.