Two new Acacia species being introduced into southwestern landscapes are Acacia gerrardii (Gray Haired Acacia) and Acacia erioloba (Camel Thorn or Giraffe Thorn). Both are native to the desert regions of southern Africa. As an evergreen, Giraffe Thorn offers much needed shade year round while providing food and shelter for wildlife. Selections growing in Arizona appear to be cold temperature deciduous. Young trees have a generally upright growth habit but as they mature the canopy spreads to form a wide canopy that provides substantial shade. It is the dominant tree in the natural landscape of Namibia and Botswana, growing primarily along the banks and in the middle of river beds. Thorns occur in pairs along the trunk and numerous ball shaped yellow flowers appear in the spring. Trees are reported to be hardy to 20 degrees F. Most of the more popular desert adapted landscape species (Mesquites, Palo Verdes, Sweet Acacias) tend to mature at about 25' to 35' tall and wide. Giraffe Thorn probably takes its name from its large stature. It typically grows to a mature height and width of 30 to 60 feet, leading one south African botanist to call it, "the king of trees in the arid regions of southern Africa." A. erioloba presents an opportunity to introduce a truly large stature tree into the southwest landscape palette that has a form and appearance that compliments our native desert species. Mature specimens could be used as a screen planting around the margins of a landscape, as a visual focal point or entry marker. The ultimate size of these trees should be taken into consideration when placing them in the landscape design. In natural setting the mature size of the tree is heavily influenced by the annual rainfall in the area. These observations suggest that the mature size of Giraffe Thorn trees used in landscapes settings could be controlled by effective irrigation management.

Acacia gerrardii can grow to a mature height of 30' to 35' feet and as wide. In natural setting it is found in arid river valleys or shrubby uplands. Its growth rate and overall appearance is similar to that of Sweet Acacia (Acacia smallii). Like the Sweet Acacia, it also produces a scattered shade but has leaves that are slightly larger. Cream colored 1/4 to 1/2 inch diameter flowers appear in spring that develop into curved, gray, velvety seed pods. On young trees thorns are numerous, found in pairs and are bright white making them easily visible. These thorns are very superficial and as the branch matures the thorns are sloughed off as layers of papery brown to tan bark peels off. Mature trunks and branches are almost thorn free. The leaf canopy is broad and may be either flat topped or dome shaped. Gray Haired Acacia is temperature deciduous and in colder locations will lose all its leaves in winter. It is hardy to at least 20 degrees F and sustained only minor damage from temperatures as low as 15 F. Its unusual papery bark, ample shade and spring flowers make Acacia gerrardii an excellent tree to mix with more traditional arid landscape species in both formal and more naturalistic desert landscape designs.

The seed for both the Acacia species discussed here came from the Desert Legume Program (DELEP). DELEP is associated with the University of Arizona and The Boyce Thompson Southwest Arboretum. Its mission is to establish and maintain a collection of plant material from worldwide sources for the purpose of seed exchange and research. The emphasis on Legumes (plants in the Pea Family) comes from the fact that species in this family represent important sources of food, feed, medicines, fuel wood, shade plants, ground covers and ornamental the world over. DELEP conducts research, maintains an extensive seed collection and publishes an informative monthly newsletter, ARIDUS. For more information contact DELEP at 2120 East Allen Road, Tucson, AZ 85719.