Eucalyptus in San Diego

Eucalyptus, also called gum trees--the tallest flowering plants in the world, are one of the more obvious and widespread imported and feral trees throughout central and southern California. However, they vary so much it is hard to describe exactly what it is that makes a tree identifiable as a Eucalyptus to the casual observer.

There are over 700 species of Eucalyptus. They are native to the Australian region. Over 250 species have been cultivated in California. [Read "How the Eucalyptus Came to California" by Teisha Rowland in the Santa Barbara Independent newspaper of January 15, 2011.]

In this post I describe marked differences in the bark, leaves, and flowers of several varieties seen in the San Diego area, which I haven't even identified to species level! I've done some research on Eucalyptus, which provides additional details beyond my observations.

The tree shape, more than anything identifies the Eucalyptus at a distance. The tree shape is tall and generally thin; it doesn't have large spreading branches. All have leaves in sparse round clusters at the tops of the tree, most have some thin clumps of leaves on smaller branches hugging the trunk. The leaves and clusters hang down, slightly drooping. All forms are rather unsymmetrical, being lopsided and crooked, and there are usually long bare trunk and branch sections amid tree. The general appearance of most Eucalyptus is tall and "trunky," open, with round bubble-shaped crowns.

Leaves are generally blue-green and sickle-shaped--long, thin, curved. Some tend more toward green, blue, or gray. These are evergreen, with a constant supply of new leaves, though more leaves are shed in dry times. Leaves contain oil that have disinfectant qualities--toxic in large quantities. The oil can also be used as a solvent and insect repellant. You may find the oil in cough drops and the leaves used in floral displays. The koala is one native Australian mammal that can eat the leaves. Leaf-bearing stems are white, gray, light brown in most species, reddish in some.

Little fluffy flowers vary from reddish to pink to yellow to white. They grow in a capsule without flower petals, The capsules and seeds tend to be messy. Flowers are full of nectar and attract insects and birds (especially hummingbirds in the Americas, which are not found in Australia).

The very thin sheet-like bark on most varieties sheds annually to some degree, revealing a smooth white, pale gray, or golden-brown trunk. But not all. One common form has hard dark brown bark with deep furrowed ridges and doesn't shed ("ironbark"). Others have bark in strips ("stringybark") or scales, and don't seem to shed much.

Up close, the fluffy flowers in capsules and shedding bark of most varieties are the identifying marks.

The ground under many varieties is littered in dead leaves, bark strips, and smaller broken branches. The trees produce chemicals inhibiting other plants from growing under them, so the ground under these trees tends to be bare of understory plants. The oil in the trees is highly flammable, thus untended groves of these trees in dry southern California can be a fire hazard.

Blue Gum?

The Blue Gum (Eucalyptus globulus) is supposed to be the most widespread in California. It was imported as a failed experiment in growing trees for lumber and pulpwood in the late 1800's. I believe the following 4 photos are probably this species.

A covering of thin brown bark sheds to reveal white and tan patchy trunk.

Patchy trunk where the bark remains partially attached to the lower part of the tree.

Red Ironbark

If I have identified this common street-lining species correctly it is Red Ironbark (Eucalyptus sideroxylon). Flowers are dark pinkish, stems reddish, leaves greener rather than blue, giving the overall tree a richer green color rather than the typical blue-gray of many other forms. The next 4 photos show this tree.

The hard, furrowed bark gives rise to the name "Ironbark" shared by several species.

Silver Dollar Tree

Local tree trimmers gather fresh cuttings each spring for floral displays. I haven't determined the exact species, but it may be Eucalyptus cinerea. Juvenile leaves of many Eucalypts are round. By trimming trees each year, the rounder juvenile leaves are produced.

One of the "stringybark" forms.

Rounder juvenile leaves are blue-green.

Perhaps also the Silver Dollar Gum, but with mature sickle-shaped leaves.

Unknown species

A massive, messy, untrimmed tree. Shedding strips of bark are lodged in the crotch of the tree. This species reveals a smooth golden-brown trunk under the strips of gray-brown bark...

Truffula Tree

I don't know what these Eucalyptus are, or if they are only one species. They are wispy, tall, with smooth white-trunks. Some are so tall and slender and bent, with little puffs of leaves way at the tops, that they remind me of the Truffula trees designed by Dr. Seuss in his children's book The Lorax. [Legend has it that the original Truffula tree is a Monterey Cypress at Scripps Park in La Jolla, California, where Theodor Seuss Geisel (Dr. Seuss) lived from after World War II to his death in 1991.]