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The Down Under Connection

AUSTRALIA and New Zealand make up only 5% of Earth’s landmass, but they’re much more important than that to San Francisco’s urban forest. The two countries have contributed a total of 20 of the city’s 50 most frequently planted trees.

Why are trees from that part of the world so common in San Francisco? It’s all about climate. Coastal regions in Australia and New Zealand have the same mild climate as San Francisco: rainy, mostly frost-free winters, and long, mild summers. Their maritime conditions also mimic those of San Francisco: many of these trees in their native regions have become used to strong winds, salt spray, and sandy soils.

Eucalyptus, in particular, has really taken off as a California import. The blue gum (Eucalyptus globulus) and other eucalyptus species have become naturalized in the California countryside, where they are so common that many people assume they must be native trees. In San Francisco, blue gums are one of three foundation trees of Golden Gate Park, along with California’s native Monterey pine and Monterey cypress. Ironically, of the three species, the blue gum is the only one that reproduces on its own in the park, since both native species require either fire or very hot, prolonged summer heat (neither of which is common in San Francisco) for their seeds to open.

California has not been entirely on the receiving end of botanical exchange with Oceania, however. One of the most important timber trees in Australia and New Zealand is California’s Monterey pine (Pinus radiata), which is known Down Under as radiata pine. Monterey pine represents almost 90% of the planted forest in New Zealand, where it is prized for its rapid growth and suitability for many uses.

Leptospermum laevigatum

AUSTRALIAN TEA TREE

LOCATION: 1278 Alemany Blvd./Silver Ave. in the Outer Mission

The most interesting feature of this tree is its “muscular-looking, twisted and gracefully curved, shaggy, gray-brown trunk,” as Sunset’s Western Garden Book so aptly describes it. Although trainable as a tree, the Australian tea is by nature a shrub, native to the coastal dunes of southeastern Australia and Tasmania. Because it tolerates sandy soils and windy conditions, the tree is an excellent windbreak near the ocean and is used extensively on the western edge of Golden Gate Park. The foliage is an attractive gray-green, with oval leaves to 1 inch in length. Sprays of small white flowers bloom along stems and branches April–June, followed by ¼ -inch woody seed capsules that form during the summer and persist year-round.

Geijera parviflora

AUSTRALIAN WILLOW

This evergreen Australian native has pendulous branches that make it look like a willow, although it is more closely related to citrus trees. Australian willows have an oval form and grow 20–30 feet tall, with a dense canopy of narrow, gray-green leaves. Flowers are inconspicuous, and the tree has smooth gray bark. This tree withstands neglect better than most, as it is very resistant to drought, disease, and pests. The Australian willow is native to the dry interior of Australia (unlike most of our other Australian natives, which are from coastal regions). In its homeland, the tree is known as the wilga, a name probably derived from local aboriginal languages, and it is a common shade tree near dusty farmhouses. The leaves have a mild analgesic effect—as a traditional folk remedy, chewed leaves were used to stop toothaches. The aborigines also used the tree as a kind of narcotic; during ceremonies they smoked the dried leaves, which reportedly induced drowsiness and drunkenness. In today’s San Francisco, Australian willow is appreciated for its aesthetic appeal rather than for any psychoactive properties, and it’s valued as an attractive ornamental that takes almost any kind of neglect.

LOCATION: 673–675 Guerrero St./22nd St. in the Mission

Persea americana

AVOCADO

The avocado is native to the Americas. Widely distributed throughout Central and South America, it ranges from eastern Mexico through Central America to the northern Andes. Human beings have appreciated the avocado for thousands of years. Avocado seeds dating to 7000 BC have been found at Mexican archaeological sites, and experts believe that the Aztecs cultivated the plant as early as 500 BC. (The seeds in archaeological deposits from this more recent time are larger, suggesting that natives were cultivating plants from seeds selected on the basis of fruit size.) The Aztecs had an interesting anthropomorphic association with this tree, as the common Spanish name for the tree is aguacate, from the Aztec ahuacatl, meaning “tree with testicles.”

Conquistador Hernán Cortés discovered the avocado in 1519, when he was the first known European to arrive in Mexico City. A few years later, the Spanish historian Oviedo wrote the first description of the avocado: “In the center of the fruit is a seed like a peeled chestnut. And between this and the rind is the part which is eaten, which is abundant, and is a paste similar to butter and of very good taste.” The Spanish soon introduced the avocado to the West Indies, the Canary Islands, and other possessions around the world.

LOCATION: 1011 S. Van Ness Ave./21st St. in the Mission; also at 438 Arkansas St. in Potrero Hill

Avocados were introduced to Florida in the 1830s, but not until the 20th century did the avocado industry really take off in the United States, in California. Carl Schmidt, a 21-year-old employee of a nursery in Altadena, California, was sent to Mexico in 1911 to search for the country’s best avocados and bring back cuttings of the trees from which they came. On his return, many cuttings refused to adapt to California’s soil and cooler climate, but one flourished; when it survived the great freeze of 1913, it was given the name fuerte, the Spanish word for “strong.” This tree is credited with starting California’s avocado industry. The famous ‘Hass’ variety (95% of the California crop) originated in 1935 when Rudolph Hass, a postman and amateur farmer in La Habra, California, discovered a superior tree in his 2-acre orchard. (The tree can still be seen in La Habra.)

Avocado flowers are cross-pollinated, meaning that flowers of one tree can be pollinated only by pollen from another tree—a botanical trick that encourages genetic diversity. Cross-pollination is actually achieved by the stigma (the female part of the flower) becoming receptive to male flowers’ pollen before the pollen is released from flowers of the same tree. (This is one reason that avocado trees rarely set fruit in San Francisco—not enough other avocados are nearby!) The large seed of the avocado is an adaptation for supplying young plants with enough food to enable them to survive in the dark forest undergrowth until they can attract sufficient light to survive.

It’s too bad that avocado trees aren’t more common on San Francisco streets, because most of the specimens in the city are beautiful and healthy trees. Even if they don’t produce much fruit in the city, avocado trees are attractive ornamentals, with handsome glossy foliage and an open branching pattern that reaches to 35 feet.

Acacia baileyana

BAILEY’S ACACIA

LOCATION: 1201 Shrader St./Grattan St. in Cole Valley; also at 236 Ashbury St./Fell St. near the Golden Gate Park panhandle

This Australian native is the harbinger of spring in San Francisco. It is the earliest tree to flower, putting out brilliant yellow blossoms in January. (As a native of northern New York, I still find it jarring to associate January with spring.) The tree is popular for its feathery, blue-gray foliage, although the ‘Purpurea’ variety has lavender new growth.

Bailey’s acacia is one of the fastest-growing San Francisco street trees, quickly reaching 20–30 feet in both height and width. Like most fastgrowing trees, however, it is short lived, typically surviving no more than 25 years. Known as Cootamundra wattle in Australia, this plant is native to a small area near the town of Cootamundra in New South Wales. It is a woody shrub in the wild, but it can be trained to grow as a tree.

Acacia melanoxylon

BLACKWOOD ACACIA

One of the largest of San Francisco’s street trees (to 40 feet in height, and much higher under ideal conditions), blackwood acacia is also one of the most common. It was planted heavily throughout the city in the 1960s and 1970s—so many large, mature specimens are now found citywide. The tree is evergreen, with dark brown bark and dense gray-green “leaves” 3–5 inches long, though they actually are not leaves but enlarged leaf stalks called phyllodes. (Botanists believe that phyllodes are a moisture-preserving adaptation to a dry climate.) In February and March, the tree produces an abundance of globular, pale yellow flowers that put out a great deal of pollen.

LOCATION: 1 Northwood Dr./Montecito Ave. in Westwood Park; also at 740 Masonic Ave./Hayes St.

Blackwood acacia is well adapted to San Francisco’s coastal climate and will grow (rapidly) almost anywhere on city streets, as it is not afraid of sidewalks. In fact, just the opposite is true: this tree’s aggressive roots will crack and lift sidewalks, which helps explain why in recent years it has been planted less frequently. Nevertheless, this is a good choice where a large, fast-growing tree is desired.

Blackwood acacia is native to the forests of Tasmania and southeastern Australia. It has always been an important timber tree in its native zone, and the tree’s hard wood made strong boomerangs, clubs, and shields for Australia’s aboriginal people.