Tree of the Week: The Blackwood Acacia is a native of Australia

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The Blackwood Acacia -- Acacia melanoxylon

Native to Australia’s wet east and southeast coasts, the Blackwood Acacia is a fast-growing, relatively short-lived (20 to 50 years) species. It is considered invasive in many areas of the world, including Hawaii. It was used here for a quick shrub or tree cover, but the California Invasive Plant Inventory now lists it as invasive-limited, meaning either that it may be a locally significant but not a statewide problem, or too little is yet known about it.

The tree is valued in Australia for its decorative heartwood, used for timber, beer barrels and musical instruments. The species is classified in the Fabaceae or legume family, whose members fix atmospheric nitrogen for their internal use, due to rhizobia bacteria that live in nodules on the plant’s roots. Legumes are not able to convert nitrogen from the air without the bacteria.

The Blackwood Acacia grows fast and aggressively to a dense 40 feet tall by 20 wide evergreen tree, whose branches are often brittle. The bark is dark gray and furrowed. Leaves on young wood start out feathery, finely divided, with tiny leaflets, but eventually the tree switches to dark green, 2-to-4-inch-long, elliptic, flattened leaf stalks with parallel veins that look and act like leaves. Profuse litter is common. Small creamy to white puffball flowers huddled in clusters bloom in the spring. They develop into reddish brown fruit pods that contain shiny, tiny, black seeds.

The Blackwood Acacia has a wide soil and climatic tolerance, including seaside, but it prefers full sun and cooler, wetter sites. It loves the San Francisco Bay Area climate. The tree is drought tolerant and winter hardy to 15 to 20 degrees Fahrenheit. Roots are as aggressive as the species and may lift sidewalks, but the root system is shallow.

Aborigines soaked their rheumatic joints in a roasted bark infusion, which made an effective fish poison as well. Given its negative properties it should be clear that a description of this tree, or for that matter any tree of the week, does not imply an endorsement to plant. Instead it is offered as a means to learn more about the existing trees that make up the fascinating urban forest surrounding us in the Southland.

-- Pieter Severynen

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