Sugar Gum (not native)

Eucalyptus cladocalyx

large tan branches intertwined
Santa Carina trailhead | September 2015

Sugar gum (Eucalyptus cladocalyx) is one of many species of eucalyptus trees native to Australia and introduced into California for wood, for fuel and for landscaping. Eucalyptus are extremely well suited to our climate and some species have escaped into natural areas where they have displaced native species. To many, the eucalypts in the Reserve are unwelcome pests.

Our eucalyptus trees are also direct links to our heritage. We have eucalyptus that were planted as a wind break for fruit trees, as a screen for a duck hunting club, and as part of the failed project to grow railroad ties for the Santa Fe Railroad. The two old sugar gums below the Santa Carina trailhead once sheltered a homestead.

Many eucalyptus trees in the Reserve have been removed, and others are on “the hit list”. The two twisted old trees at Santa Carina, however, have been given a reprieve. We hope to have a historical site in their shade, where a plaque will explain the early history of this area, and a bench will allow visitors to sit and imagine.

Other Common Names:


Description 2,198,199,200,202,205

Sugar gums are medium-large evergreen trees to 65 feet (20 m) in height, although taller trees are reported from cultivation.24,199 Trunks tend to be straight; however, the two old trees near the Santa Carina trailhead are bent and twisted, perhaps due to their exposed location. Sugar gum bark sheds in patches and strips leaving a smooth trunk pale, mottled in grays, oranges, and tans. Leathery leaves are ovate to lanceolate, often sickle-shaped, 3-6 inches (8-15 cm) long with smooth margins; they are medium green, with a waxy sheen and are somewhat paler on the underside. Numerous tiny glands are embedded in each leaf; these produce the volatile oil that gives eucalyptus its aroma.

Sugar gum flowers occur in small groups on stout peduncles from the leafless portions of branches. In this respect, sugar gum is unlike most eucalyptus species, which produce their flowers from leaf axils.196 The structures of eucalyptus flowers and fruit are unique; Johnston gives an excellent photographic description of one species.197 The base of the flower is a vase-like structure,  the hypanthium, which encloses the developing pistil and stamens. In sugar gum, the hypanthium is shaped like a barrel. Traditional petals and sepals are absent. Instead, the petals have united into a bud cap or inner operculum, that covers the end of the hypanthium, protecting the immature pistil and stamens. In sugar gum, the bud cap is hemispherical. Also, sepals are joined to form an outer operculum, which is shed early in development. Just prior to flowering, the stamens expand and lift off the bud cap. The “flower” of a eucalypt is a spray of numerous stamens, within which is a stout, green pistil. The main flowering time of sugar gum is April through July;7 the September flowers in 2015 may have resulted from the unusual rains that summer.

The fruit of a eucalyptus often called a gumnut, is a compound structure of hypanthium and ovary which becomes woody when mature. In sugar gum, the fruit is barrel-shaped, 1-1.5 cm long. Tiny seeds are released through slits in the top of the ovary. In sugar gum, the ovary is recessed well below the edge of the hypanthium. The seeds are wind-dispersed.

tall tree with multiple large branches coming from trunk

Rios trailhead | October 2015

seed pod blooming with white strings coming out

Flower with two mature buds with bud caps | Santa Carina trailhead | September 2015

green seed pods on red branch

Developing fruit | Rios trailhead | September 2015


Sugar gum is native to a few restricted localities in Southern Australia. It has been introduced to all continents and has become a serious pest in Western Australia and South Africa.202 In the late 19th and 20th centuries, millions of eucalyptus trees were planted in California, for shipbuilding, fuel, medicines, windbreaks and, in San Diego, railroad ties.204

Inevitably, eucalyptus escaped their plantations and became established in uncultivated areas. Today, naturalized populations occur primarily in coastal southern California, below 2000 feet (600 m).7 Although not on the invasive threat list of the California Invasive Plant Council, sugar gum was recently (July 2015) added to their watch list.183

In the Reserve, a small grove of sugar gum trees grows in Central Basin along the Rios trail, part of the forest of eucalyptus and acacias west of the freeway. These and the two trees near Santa Carina are the only confirmed sugar gums. However, many of the eucalyptus trees west of the freeway and near the La Orilla trailhead are either too far off the trail or too tall for us to easily reach the diagnostic flowers and fruit, so more sugar gums may be present.

Classification 2,44

Sugar gum is a dicot angiosperm in the myrtle family (Myrtaceae). This family contains as many as 3000 species in 130-150 genera, mainly in the Southern Hemisphere. They range throughout the world’s tropical and subtropical regions and make up a large part of the tree and shrub population of Australia. All species are woody and have floral parts in fours or fives. The evergreen leaves are simple and mostly opposite with entire margins. 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 eucalyptus,bottle-brush, allspice, Surinam cherry,  guavas, and cloves.

There are over 700 species of eucalyptus, most of which have evolved in Australia.196 The taxonomy is complex and not completely agreed upon. It appears that sugar gum is not closely related to any other eucalyptus species and most taxonomists place it in its own series, Microrythae in the subgenus Symphyomyrtus.202

Jepson eFlora Taxon Page
seed pods clumped together on tree branch

Mature fruit | Santa Carina trailhead | September 2015

long narrow green leaves

Santa Carina trailhead | September 2015

white flowers with tiny petals

Santa Carina trailhead | September 2015

Ecology 203

In spite of the unusual flower structure, eucalyptus are not adapted to particular types of pollinators, but are pollinated by a wide variety of insects, birds and, in Australia, marsupials. Within a sugar gum flower the pistil matures after most anthers have shed their pollen, reducing the chance of self-fertilization. The green stigma is covered with a sticky mucilage which helps to retain pollen after it is deposited.

tiny white spider-leg sized petals on flower

Santa Carina trailhead | September 2015

seed pod cut open with white pod inside

Mature bud with detached bud cap, showing expanding stamens and central stigma | Santa Carina trailhead | September 2015

multiple trees with branches coming from trunks

Rios trailhead | October 2015

Human Uses  

Sugar gum is considered a premium timber in Australia.198 When grown under good conditions, the heartwood of sugar gum is a pale yellow-brown with a uniform texture that polishes to a good finish for quality flooring, veneer, and benchtops. Excellent honey is produced from the nectar.

In California today, sugar gums are used mainly in the landscape, generally in parks and open spaces and along freeways.199,200

Perhaps the best known local sugar gums are those on the UCSD campus that inspired and surround both Robert Irwin’s installation Two Running Violet V Forms, dubbed the “running fence” by students, and Terry Allen’s Trees (the “singing tree”, the “talking tree” and the “silent tree”).

many trees in forest

Robert Irwin's installation flows through sugar gums on the UCSD campus | December 2016

tall trees with leaves at the very top

Rios trailhead | October 2015

tree branch with bark peeling off

Santa Carina trailhead | September 2015

Interesting Facts  

The area below the Santa Carina trailhead is marked by two old sugar gum trees and by a large number of non-native plants, and recently, by miniature tepees enclosed by fences. These all tell the recent history of this area.

In 1868, lima beans were introduced into California as a crop.201 Because lima beans could be dry farmed, they were well suited for our low-rainfall climate and lima bean farming grew into a large and lucrative industry. At one time as much as 2/3 of the world’s supply of lima beans were grown in southern California. By 1905 the first crop was grown in San Diego County and lima bean fields were planted in many of the coastal regions; some persisted into the 1980s.

The gentler slopes below the Santa Carina trailhead were planted in lima beans. The two old sugar gum trees sheltered a homestead on the plateau just north of the fields. Rumor has it that the owner of the homestead was an irascible fellow who feuded with the farmer on Stonebridge Plateau. Occasional rifle shots were exchanged across the water.

When the lima bean fields were abandoned, non-native plants such as fennel, mustard, and annual grasses quickly took over. Today, this disturbed habitat is being restored by volunteers under the direction of Nature Collective (The “tepees” protect the tender young plants from grazers; the fences protect them from unwary human feet.)


large tree with intertwined branches

Santa Carina trailhead | September 2015

giant tree out in the distance of a field

Santa Carina trailhead | October 2015

picture of field with two large trees in the center

Santa Carina trailhead | September 2015

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