Blue Gum (not native)

Blue gum (Eucalyptus globulus) is one of many species of eucalyptus, or eucalypts, that are native to Australia and that were introduced into California for wood, shelter, landscaping and for their presumed curative powers. They are extremely well suited to our climate and some species have escaped into natural areas where they not only displace our native species but also present a fire hazard.

While the three Eucalyptus species in the Reserve are unwelcome pests, they are also direct links to our heritage. We have eucalyptus trees that were planted to shelter an old homestead, and as landscaping for the lodge of a duck hunting club. The old blue gum trees midway up Holmwood Canyon, provided a windbreak for fruit trees planted above them.

Description ^{2,4,199,200,202,229}

Blue gums are large evergreen trees which, under good conditions, can reach more than 100 feet (30 m) tall and 50 feet (15 m) wide; under poor conditions, they are stunted shrubs. Trunks become massive with age with a large amount of gray/brown bark, that is persistent near base of tree but peels in long strips higher up to reveal smooth, pale inner bark. Tough, leathery leaves have flattened petioles and are alternate on stems, often hanging vertically. Leaves are lanceolate to sickle-shaped, usually less than 10 inches (25 cm) long with smooth margins and numerous oil glands. Leaves are green to dark blue-green in color. Like many species of Eucalyptus, the juvenile foliage is unlike mature foliage. Juvenile leaves are sessile and opposite on the stem, shorter and wider, with a waxy, blue gray surface.

Blue gum flowers are sessile or on short pedicels and occur singly or rarely in clusters of three in leaf axils of young shoots. The structure of the blue gum flower (like that of all Eucalyptus species) is unique; an excellent photographic description of the related swamp mahogany (E. robusta) is given by Johnston.¹⁹⁷ The base of the flower is a vase like structure, the hypanthium, which is derived from the pedicel and encloses the pistil and stamens. In blue gum, the hypanthium is shaped like a broad cone, often four-angled and strongly ridged. Traditional petals and sepals are absent. Instead the sepals and petals (or the petals alone²⁰³) have united into a thick bud cap that covers the end of the hypanthium, enclosing the developing pistil and stamens. In blue gum this cap is a thick  shallow dome, with a central cone, heavily ridged and warty, with many blister-like oil glands. The entire bud is a distinctive pale blue color, making the buds more conspicuous than the flowers.

Just prior to flowering, the expanding stamens push off the bud cap. The “flower” of a eucalyptus is a dense mass of hundreds of white stamens with pale yellow anthers. Anthers mature before the pistil, reducing self-fertilization.^183,203 The pistil has an inferior ovary that occupies the entire hypanthium; the top is flattened but slightly capped around style with shallow indents that retain copious nectar. The style is stout, topped with a green or rosy stigma.  The main flowering time is October through March, making blue gum our only winter-blooming eucalypt.⁷

The fruit of a eucalyptus, often called a gumnut, is a compound structure of hypanthium and ovary which becomes woody when mature. In blue gum the fruit is an angled or rounded flat-topped cone ½ – 1 inch wide  (1.8 – 2.5 cm) long. Tiny seeds are released through slits between four valves in the top of the ovary. In blue gum, valves rarely extend far beyond the edge of the hypanthium.

Classification ^2,44

Blue gum is a dicot angiosperm in the myrtle family (Myrtaceae).The myrtle family contains as many as 3000 species in 130-150 genera, mainly in the Southern Hemisphere. All species are woody and have floral parts in fours or fives. They range throughout the world’s tropical and subtropical regions and make up a large part of the tree and shrub population of Australia. The evergreen leaves are simple, and mostly opposite with entire margins. Juvenile leaves are often distinct from mature leaves. Most species have numerous brightly colored stamens, leaves with strongly-scented oils and fruits that are either capsules or fleshy berries. Some of the more well-known members of the family are the eucalyptus and bottle-brush species, Surinam cherry, guavas, allspice, and cloves.

There are more than 700 species within the Eucalyptus genus and most, if not all of them evolved in Australia;¹⁹⁶ eighteen species have been reported outside cultivation in California.⁷ The taxonomy is complex and not completely agreed upon. Blue  gum is usually placed in the subgenus Symphyomyrtus,²⁰² and is distinguished from other species of Eucalyptus by the size and structure of the gumnut.²⁰²

Ecology  

The success of blue gum in Mediterranean climates is partially due to its extensive root system which can extract water from soils with lower moisture content than most of our native plants. In addition, the pendulous leaves are little fog-catchers. In San Francisco area it was shown that fog condensation by blue gum can produce 17 inches (43 cm) of rainfall a year; that is as much moisture as the annual rainfall and helps explain the success of blue gum in California’s coastal fog belt.¹⁸³

The lack of undergrowth beneath mature blue gums is variously attributed to the ability of eucalyptus to compete for water, to the high volume of debris produced by the tree and by toxic chemicals in the leaves that are leached from living and/or fallen leaves by rain and by fog and inhibit surrounding growth.^202,232

Human Uses  

Although for many years blue gums were widely used as crops for wood products, windbreaks, and landscape features, their use is declining. A recent survey suggests that blue gum has all but disappeared from the nursery trade.¹⁸³

Blue gum is the primary source of eucalyptus oil, which is used for everything from medicine to insecticide, fragrance, flavoring, and fuel. The medicinal properties of eucalypts were first recognized by the Australian Aboriginals.⁴¹ Eucalyptus oil is antiseptic, and the vapor is inhaled as a decongestant and to treat bronchitis;^41,482 it is also a key ingredient in liniments used for bruises and muscular pains.⁴⁸²

Interesting Facts ^{204,230,231,366}

Early settlers did not realize that malaria was transmitted by mosquitoes. They saw that when eucalyptus, especially blue gums, were planted in low wet areas, not only was excess groundwater controlled, but malaria disappeared, and they assumed direct cause and effect.

Soon blue gum was attributed with almost supernatural healing powers, far beyond medicinal properties recognized today. Extensive planting of eucalyptus along San Diego sidewalks began in the 1880s. Promoters claimed that although “shade and moisture would cause malaria, … it is well-known that the blue gum tree (eucalyptus globulus) instead of creating malaria actually destroys it.”²⁰⁴

Strings of blue gum seedpods, sometimes interspersed with colored beds, were hung across doorways in homes. One early San Diego resident remembers “ … those eucalyptus pod portieres. They were used all over town. People thought they had medicinal values. We had friends from Connecticut who used to visit us in those years, and they always took home a suitcase packed full of eucalyptus leaves to burn in their own fireplace in the winter, to purify the air and exude health-giving qualities.”²³¹