February Tree of the Month: Black Acacia

By David Gress

Black Acacia is a stately broad-leafed evergreen tree, which is not only attractive but quite useful in the landscape.  It can planted as a single specimen tree, to provide needed shade – or can be planted in groups or in a formidable hedge, to provide screening and privacy.

It is fast-growing – rising upwards several feet per year – to a height of 40 to 50 feet with a spread of 20 to 40 feet.  When young, it has a distinctively pyramidal shape with a straight single trunk bearing evenly spaced branches.  As it matures, its crown will broaden, and several additional trunks can develop.

The canopy has dense foliage comprised of lance-shaped leaves (2- to 5-inches long and ½ to 1- inch wide) that are curved or sickle-shaped and have 3 to 5 prominent longitudinal veins; these are a shiny light-green when young and turn a dark flat-green with age.  The “leaves” are actually modified stems, called phyllodes or petioles, that function like leaves for photosynthesis.  The juvenile phyllodes of seedlings and of very young trees can be quite different from adult foliage, in that they have very finely divided and fern-like bipinnate leaves.

The flower-bearing stems, called inflorescences, develop from the bases of the phyllode stems.

From February through April, creamy-white flowers cover the crown prolifically, blooming in groups of 2 to 8 on axillary racemes.  The flowers are “perfect”, meaning both male and female reproductive parts are found in each flower.  Surprisingly, what appears to be a tiny spherical flower (¼  to ½ inch in diameter) is actually a densely-packed cluster containing 30 to 56 even tinier individual flowers!  The hairy texture of each sphere is the result of the hundreds of male stamens that protrude from these diminutive flowers.  The flowers are happily pollinated by both our native bees and by domesticated honeybees, which are attracted to and feed on the sweet-smelling pollen and nectar.

After pollination, the flowers produce smooth green seed pods (2- to 5-inches long and ¼ to ½ inch wide), which, as they age, twist and coil and turn reddish-brown.  Each seed pod is constricted around the seeds that are arranged in a single row within the pod.  The small black seeds (1/8 inch long) are encircled with a pink to red wormlike structure that attaches to it.  The tasty seeds are eaten by animals and birds or gathered in garden waste, and are thereby easily disseminated.  Seeds will germinate quickly but only after scarification – by being exposed to fire or through hot water soaking.

On young trees, the bark is light gray and finely fissured;  on mature trees, it turns dark gray to black and is deeply fissured vertically and often scaly.  Younger branches are smooth and greenish colored.

Black Acacia is endemic to Australia, in a relatively narrow band stretching along the east and southeast coastal plains – and to Tasmania, along the east coast from sea level to 3,500 feet.

It is probably the best and most popular of the tree Acacias.  Certainly, it was one of the first Australian trees to be introduced to California and has been planted in the Santa Barbara area since the late 1800s.

Its botanical name is Acacia melanoxylon.  The genus name, Acacia, is Latin but was derived from the Greek word, “akakia”, meaning “Egyptian thorn acacia”.  The specific epithet, melanoxylon, comes from the Greek words “melas”, meaning “black”, and “xylon” meaning “wood”, and refers to its remarkably dark wood.

In the United States, it is most commonly known as Black Acacia.  In its homelands, it has many other common names, including Australian Blackwood, Blackwood, Hickory, Mudgerabah, Tasmanian Blackwood, and Blackwood Acacia.

In our area, Black Acacia is planted primarily as a medium to large landscape tree and to form thick privacy screens.  It is an excellent background planting, since it blends in well with more decorative ornamentals and with our native plants.  Importantly, it grows well beneath our non-native Eucalyptus trees.  It should be noted that it is a medium- to short-lived tree;  it can live to 80 years of age but will begin to show signs of decline after 50 to 60 years.

It should be planted in full sun, in a space that will accommodate its ultimate size, and with good drainage.  It can tolerate a variety of soils.  When established, it is quite drought resistant and is mostly pest- and disease-free.  It requires very little maintenance; however, some pruning is necessary to attain and maintain a desired size and shape.  It is cold hardy to 15 degrees F. and does well in both high heat and coastal conditions.

Unfortunately, it does have an aggressive surface root system that can push up hardscape.  Root sprouts that can occur some distance from the tree can be easily pulled up.  The problem of invasiveness that has been reported in wetter areas does not seem to be an issue in our drier Mediterranean climate.

The Black Acacia is prized for its fine-grained dark wood, which is very similar to black walnut (Juglans nigra).  It is used in building fine cabinetry, boats, and musical instruments – it has been found to be a good substitute for koa (Acacia koa) for ukuleles.  It is popular with our local woodworking artists who create from it beautiful carved sculptures and decorative turned bowls.

Mature Black Acacias can be seen in many locations in our community: along Por La Mar Drive adjacent to Pershing Park; at the southwest corner of Nopal and Cota Streets; at the southeast corner of Nopal and Canon Perdido Streets; and, in yards and gardens all around town.

Tree-of-the-Month articles are sponsored by Santa Barbara Beautiful, whose many missions include the increase of public awareness and appreciation of Santa Barbara’s many outstanding trees and, in a long-time partnership with the City Parks & Recreation Department, the funding and planting of trees along the City’s streets.

Those who wish to honor a special someone can do so with an attractive commemorative marker that will be installed at the base of an existing street tree in the City of Santa Barbara.  Because Santa Barbara Beautiful has participated in the planting to date of over 13,000 street trees, there are plenty of trees from which to choose!  Application forms are available on the Santa Barbara Beautiful website, www.sbbeautiful.org.

Article and Photos by David Gress