Herb To Know: Eucalyptus

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Eucalyptus spp.
Family Myrtaceae Tree

What do you associate with the word “eucalyptus”? If you say “cough drops,” you’re not alone. Many people know this lofty herb solely from the camphorous fragrance of several species whose essential oil is an ingredient of popular cold remedies. However, the genus Eucalyptus comprises more than 500 species and perhaps 200 varieties of evergreen tress and shrubs, and the familiar medicinal smell is just one in the spectrum of fragrances found in this aromatic genus.

The generic name Eucalyptus comes from the Greek for “well covered” and refers to the cap, or operculum, of fused sepals and petals that covers the flower bud and is later cast off as the bud expands. Flowers are showy puffs with numerous protruding stamens. Most are white, but some species and varieties have yellow, pink, or red flowers. They are usually borne in clusters.

Most eucalypts in the United States are found in California. About seventy-six kinds are grown there for shade, windbreaks, street trees, honey, timber, fuel, and oil. A number of species can survive winter temperatures as low as 0°F, but even alpine species die if the ground freezes. A sheltered site, good drainage, and a heavy winter mulch may offer protection in marginal areas.

Plants range in height from a few feet to 380 feet (E. regnans–known as mountain ash–is the world’s tallest hardwood). All parts of the plant are aromatic. Fragrances include lemon, apple, honey, and peppermint, in addition to the familiar camphorous scent. The trunks are straight. The bark may be smooth or peeling, fibrous, or rough and fissured. Most species have leaves of two kinds. The juvenile leaves are rounded or heart-shaped in pairs on the stem; in some species, the stem appears to pierce a single leaf. Adult leaves are pendulous, usually alternate, lance- or sickle-shaped, bluish or gray-green, and smooth, shiny, tough, and leathery. A thick cuticle slows water loss during droughts.

Blue gum (E. globulus) is the most commonly planted eucalypt in the world. Its outer bark is constantly shedding, and this tendency along with its falling fruit caps makes it undesirable as a street or landscape tree. The smooth inner bark is gray or white. It has large white, mostly solitary flowers. Its mature height of 70 to 140 feet is much too tall for smaller gardens, but a cultivar, E. g. ‘Compacta’, is shorter and quite hardy.

Flowering species such as scarlet gum (E. ficifolia), planted as a street tree in California, tolerate drought but not frost or heavy winds. Some species become more frost-tolerant as they mature or when grown in cooler areas.

Medicinal uses

The oil of several species is obtained by distilling the fresh leaves. The principal constituent, eucalyptol (cineole), is a strong antiseptic that has been used to treat sore throats, bronchial disease and influenza in horses, and distemper in dogs. Large doses irritate human kidneys. The oil is used externally for skin ailments and to repel insects.

Blue gum is known also as fever tree because of its antiseptic qualities. It was introduced into Algeria in the 1850s in the hope that “exhalations” of its aromatic leaves would disinfect the country’s malarial swamps. The regions did become more healthful–but in a way that was not anticipated. The trees thrived, and within five years, their prodigious root systems had sucked up the moisture from the wetlands, eliminating the habitat for the mosquitoes that caused malaria. Blue gums were later introduced into many other countries, with similar results. For a decongesting vapor, add leaves or a few drops of essential oil of E. globulus to a tub or pot of hot water and inhale the steam. The leaves of E. cladocalyx are high in hydrocyanic acid, and sheep in Australia have died from eating them. Eucalypts are suspected of causing hay fever and contact dermatitis in sensitive individuals.

Other uses

Oils from several species are used in soaps, creams, and other cosmetics and for flavoring ice cream, candy, and baked goods. The oil of a few species is used in mining to separate certain chemical compounds from ores.

The juvenile leaves of many species can be used in fresh floral arrangements or preserved in glycerin for use in dried arrangements. Some possibilities: E. nicholii, a frost-resistant species widely grown for the florist trade, with purple young foliage; E. perriniana, a hardy mountain gum that grows 12 to 20 feet tall, with leaves like round disks with the stem passing through the center; and silver dollar gum (E. cinerea ‘Pendula’), with silver-blue rounded opposite leaves.

Eucalypts are bee herbs; the nectar and pollen of their flowers are the main food of Australian honeybees. Small brush-tongued parrots called lorikeets are also important as pollinators; they feed on the nectar by crushing the flower, then licking up the nectar with their fringed tongue.

Eucalypts are also koala herbs–in fact, the only koala herb. The furry marsupials eat only the foliage of about a dozen species of Eucalyptus, about 21/2 pounds of leaves a day.

Growing them

In cool climates, tender species such as the lemon-scented E. citriodora can be grown in tubs indoors year round or wintered indoors. Obviously, it will not attain its mature height of 75 feet with this treatment. Dwarf kinds are the most suitable choice for tub culture indoors, but all kinds can be pruned heavily every year to stimulate growth of new juvenile leaves.

Grow eucalypts in well-drained, fertile, sandy loam. Provide moderate water when in full growth. They are susceptible to root rot and mildew; maintaining good ventilation and drainage and avoiding overwatering will do much to keep your plant healthy.

Eucalypts are easily grown from seed. Moist-chilling (stratifying) seeds of alpine species for three to eight weeks in the refrigerator will promote germination. Sow seeds in early summer under part shade. Germination usually takes ten to twenty days. When seedlings are 2 to 3 inches high, transplant to quart or gallon cans. Plant them out the following year, or when 6 to 12 inches high.

Propagating eucalypts from cuttings is more difficult than growing them from seed, but you may have luck rooting cuttings taken in fall, winter, or early spring from juvenile stems and placed over a heating cable in cool, moist shade with good air circulation. Insects are unlikely to be a problem.


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