Shoestring Acacia

Young plants can look thin and awkward, with sparse foliage and slender, whippy branches that give little hint of the mature tree. Growth is quick in the early years, sometimes approaching three feet in a season under favorable conditions, then slows as the trunk thickens. Branches begin low, often around three feet from the ground, and a tree left unpruned will typically carry several trunks rather than one. As the crown fills in, the branch tips arch outward and hang down parallel to the trunk, which is what produces the tall, narrow, weeping silhouette. The phyllodes themselves are 6 to 16 inches long and rarely more than a third of an inch wide, marked by many fine parallel veins and slightly rough to the touch. Bark on new wood is smooth and maroon-tinged; on older trunks it becomes rough, deeply furrowed, and dark gray-brown to nearly black, a strong contrast against the pale gray-green foliage above it.

In Australia the species grows along drainage lines, river flats, and seasonal swamps, on heavy alkaline clays that may be salty in the lower soil layers, and it withstands weeks of standing floodwater. Its reproduction is tied to those floods: the beaded pods and seeds float, and moving water carries them downstream to fresh sediment where they germinate. The same tolerances that let it hold a floodplain also make it adaptable in Southwest gardens, where it accepts clay, sand, caliche-influenced alkaline soils, salty runoff, reflected heat off walls and paving, and dry desert air. Like most legumes, it forms root nodules that host bacteria capable of converting nitrogen gas from the air into a usable form, so it rarely needs fertilizer. It is worth being honest about the limits, though. It develops good drought tolerance once established but is not a plant that lives on rainfall alone in low-desert gardens, and it needs periodic deep watering. It also resents soil that stays wet without draining, and root rot is the usual result.

The species name comes from the Greek stenos, meaning narrow, and phyllon, meaning leaf, and it points to the tree's defining feature. Seedlings begin life with divided, ferny leaves, but the adult tree abandons them for phyllodes, flattened leaf stalks that take over the work of photosynthesis while presenting far less surface area to the sun. In inland Australia, the tree's floodplain groves provide nesting sites for a range of birds, and Aboriginal peoples of those regions use its seeds as food and its hard, dense wood for tools and implements. In North American landscapes it functions mainly as a fast, evergreen source of filtered shade for patios, narrow side yards, and screening, but gardeners should expect steady litter from spent phyllodes, flowers, and pods, and suckers where roots are shallow, exposed, or disturbed. Structural pruning while the tree is young pays for itself later, since a well-trained trunk stands up to wind far better than a crowded cluster of weak, co-dominant stems.