While the feathery, fine-textured leaves of acacia plants (Acacia spp.) look pleasant by themselves, splendor occurs in winter and spring with brilliant displays of tiny yellow flower clusters. They grow in mild climates where only light frosts occur in the winter (USDA hardiness zones 9 and higher). Specific growing tolerances and winter cold hardiness varies among species.
At least 1,100 different species of acacia plants exist. These deciduous or evergreen trees and shrubs hail from Central and South America, eastern and southern Africa, Polynesia and Australia.
Acacias belong to the large pea family, Fabaceae (Leguminosae). Within this relation, there exists three subfamilies that distinguish plants according to the shape of flowers. Acacia flowers display radial symmetry--exact symmetry when viewed in all directions. Thus, they are members of the subfamily Mimosoideae. In addition, their tiny flowers comprise tiny petals and sepals that are hidden by numerous and showy stamens.
In Africa, acacia plants are called "thorn-trees" because their branches and twigs bear small spines. They possess other interesting adaptations according to "Tropical Flowering Plants" by Kirsten Albrecht Llamas. The leaves are bipinnate--each leaf has two-branched ranks holding many small leaflets. In arid regions, they lack true leaves and grow blade-like phyllodes to conserve water. These phyllodes are flattened petioles, or leaf stems. The sap of acacias contains bitter-tasting alkaloids, discouraging animals from foraging. Some acacia species lack this poisonous sap and rely upon stinging ants to make their residences and lay eggs in the hollow thorns; the ants attack any animals looking to eat the plant.
While the leaves or phyllodes create a feathery, soft-texture for a garden setting and potentially cast shade, the crowning glory of any acacia remains its flowers. Occurring in winter or spring depending on species, the tiny pompom-like creamy white, golden or yellow flowers appear in clusters. The clusters intensify the visual display and can heighten the faint fragrance in their finger-like or even broom-like drooping forms. Not all species bear fragrant blossoms, but all yield bean pod-like fruits afterward.
Basic growing requirements for all acacia species include planting in a non-alkaline soil that has some fertility. It is best to grow them where winds do not thrash their branches and in a full sun setting, receiving over eight hours of direct sun daily for best plant shape and flowering. Most species do not respond well to hard pruning; conduct occasional errant branch removal on deciduous species to maintain a nice structure. For evergreen acacia plants, consider tip pruning branches after their flower clusters wane in late spring only.
Use gloves, appropriate protective arm and leg clothing when working around acacia plants to avoid thorn puncture wounds. Also use eye protection.
In some regions, non-native acacia species first introduced for ornamental gardens have now escaped cultivation and are ecologically invasive. In the United States, both Hawaii and Florida suffer habitat disruption from a few non-native acacia species.