From the legumes of arizona project:
an illustration of acacia salicina, Australia's willow acacia

Legumes of Arizona: An Illustrated Flora and Reference – Publication expected in late 2019.

One of the non-native plants included in Legumes of Arizona is Acacia salicina, Australia’s willow acacia. Growing to 40’, this tall, graceful tree with a willow-like canopy has been planted in many arid regions of the world for its value as a shade tree, screen, windbreak, and soil stabilizer. In our desert, it tolerates heat, poor soil, and dry conditions while providing abundant grey-green foliage that needs little tending—as long as it’s allowed to keep its lovely pendulous form. The main landscaping concern is the tree’s tendency to blow over if given too much water.

Acacia salicina was first named and described by the British orchidologist John Lindley (1799–1865) in Three Expeditions into the Interior of Eastern Australia by Thomas Livingstone Mitchell. This fascinating account of Mitchell’s travels in Australia from 1831 to 1836 includes Lindley’s botanical descriptions of 77 new species, including the willow acacia.

The name of the genus Acacia is taken from the Greek word akis, or “sharp point”, a reference to the many acacia species bearing thorns. It was first used, in 1754, by the Scottish botanist Philip Miller and later applied by Linnaeus to the African gum arabic tree, Acacia nilotica. The species name salicina is based on the Latin word salix, or willow.

Willow acacia is now classified in the legume subfamily Caesalpiniodeae, the peacock flower family. Not too long ago, it was in the subfamily Mimosoideae, and its flowers resemble those of the mimosa and fairy duster, with very prominent, long stamens.

The illustration here of Acacia salicina is by Chris Bondante, whose drawing of Dalbergia sissoo, another non-native legume tree, was included in last month’s Desert Breeze. The top half of the drawing shows the habit of the plant, a branch with both seed pods and flowers. The flat leaf-like structures on the branch are called phyllodes, which are expanded and flattened petioles that function as leaves. As a seedling, willow acacia has true leaves growing from the petiole, but these soon drop off, leaving the phyllode to serve as a leaf. Phyllodes are common among the Australian acacias.

In the bottom half of Chris’s drawing are three floral structures: an inflorescence with several globular flower heads, a single flower head containing 15-25 pale yellow to white flowers, and, at far right, a greatly enlarged mimosa-type flower with numerous stamens that extend beyond the petals, or corolla.

At the far left of the illustration is an open seed pod that contains a very interesting structure. Note that the black seeds are surrounded by folded fleshy outgrowths. This is an aril, a sort of umbilical-like appendage that attaches the seed to the pod and helps in seed dispersal. In the case of willow acacia, the aril is a brilliant scarlet color that is attractive to birds who in turn eat the seed. Chris’s photo of the seed shows the beautiful red color of the aril.

If you’re wondering why Acacia salicina remains in the genus Acacia while our marvelous catclaw acacia must now be called Senegalia greggii and our whitethorn acacia has become Vachellia constricta, it’s because, in 2011, the Australian acacias emerged victorious from the Acacia Wars that roiled the world of botanical taxonomy for several years. The acacias in Australia got to keep their name, and acacias elsewhere in the world got new ones. — Cindy Hartwell



This article appeared in the March 2019 issue of Desert Breeze, the monthly newsletter of the Tucson Cactus and Succulent Society.