Under Utilized Desert Landscape Trees
Prosopis pubescens (Screwbean Mesquite)
Perhaps the first thing people notice about the Prosopis pubescens, Screwbean Mesquite is the unusual spiral of the seed pods from which it takes its name. The tree is sometime referred to as a large shrub, but in landscape settings it regularly grows to 25' tall and as wide. Unpruned it has a shrub like growth habit with fairly dense branching. Pale to very bright yellow, 2" to 3", spike blooms are produced from May to June. The foliage is pale blue-green and made up of leaves with as many as 18 tiny leaflets. The tree bark matures from a reddish-brown to a thick gray-brown, uniquely shaggy bark, while young twig tips are gray. The tree grows at a moderate rate and is deciduous in winter. Screwbean are found, at elevations below 4000 feet, in California, Nevada, Utah, Arizona, western Texas, New Mexico and northern Mexico including Baja, Sonora, Coahuila and Chihuahua. Unlike many desert natives, Screwbean cannot be naturalized to survive on rainfall alone. Supplemental summer irrigations are essential in most Southwestern desert locations. In the desert this tree is found primarily along water courses and in "bottomlands" that experience periodic flooding. The tree will tolerate lawn plantings. It is hardy to 0 degrees F (-18 C). Tornillo, which in Spanish means screw, clamp or vise, is a common name sometimes associated with Screwbean Mesquite.
Screwbean Mature Bark
With its beautiful spring flowers, it is an excellent accent tree or it can be used as a specimen in more elaborate landscape designs. It is also used in the landscape as a background tree along with flowering shrubs or as a transitional planting between landscaped areas and undisturbed native desert. Its moderate stature, at maturity, allows it to blend in with other desert tree species in more elaborate landscape designs, adding texture and variety. Its modest mature size is ideal for courtyard and patio settings.
Screwbean Fruit
Acacia schaffneri (Twisted Acacia)
Twisted Acacia is native to the Chihuahuan desert of southern Texas and northern Mexico. Along with the common name Twisted Acacia; A. schaffneri is sometimes called Wheat's Acacia acknowledging the introduction of this specie to the southwest by renowned Phoenix landscape architect Jim Wheat. Its botanical name is derived from Wilhelm Schaffner, a German dentist and botanist who settled in Mexico City in 1856. The finely divided leaves of this tree are set close together on short petioles that originate, densely all along the branches. As a result of this unique leaf habit, the gnarled and twisted branches appear fuzzy and lacy green, beginning in early spring. The dense layer of leaves/leaflets obscure small, numerous thorns that grow along the branches, making them nearly invisible. Branches overlap and interweave to create an unusual, graceful branch canopy. Care, patience and seasonal pruning are needed to produce mature specimens that exhibit the sculptural, twisted branches that give this tree its unique charm. They can be grown as single, low breaking or multiple trunk forms, with the low break and multiple being the most commonly used.
Twisted Acacia Multiple Trunk Form
It has a moderate growth rate and in spring produces bright, sulfur yellow, ball shaped flowers, borne singly or in clusters, densely packed along the branches (in a fashion similar to the leaves). Flowers are mildly fragrant. Young trees will require pruning to achieve desired form. Once the mature form is established, little additional pruning is needed. With proper water, in a sandy loam soil, trees mature to 20’ to 25' tall and as wide with trunk calipers exceeding 12" in diameter in mature specimens. Twisted Acacia tolerates a wide range of soil types, thrives in full sun and is hardy to 17 F.
Twisted Acacia Flowers
The unorthodox leaf canopy and remarkable spring flower display make A. schaffneri an ideal focal point tree placed in a prominent place within the landscape. When used near walls, its woven branches cast intricate, lacy shadows and can be dramatically lit at night. The numerous thorns also make it an effective security barrier or perimeter planting. These trees have been used in streetscape settings, in plazas and at entry monuments.
Dodonaea viscosa var. angustifolia (Hopbush)
Dodonaea viscosa var. angustifolia is one the few, evergreen, shrubs/small trees that bring a unique color and texture to desert adapted landscape designs. The droopy leaves are shiny light green above and paler green below, with thornless, light grey, finely fissured branch and trunk bark. The varnished looking leaf coating protects the plant from water loss. With a mature (unpruned) height that can reach 15 feet, it brings a lush, dense, green leaf canopy that integrates seamlessly with surrounding desert trees and shrubs. The flowers are small and yellowish green and are known to attract butterflys. These flowers are followed by this Dodonaea’s most outstanding feature; decorative fruit clusters that start out yellow then turn to a pink-reddish color and ultimately brown. Fruit also have delicate, papery wings. The common name hopbush came from these winged pods that resemble the hop pods use in brewing beer. Flowers are produced intermittently from April to August (spring to summer).
Dodonaea: Maturing Pink Seed Pod
Dodonaea grows best in full sun and well drained soils but will tolerate less ideal conditions. The growth rate is moderate to fast, given adequate water and fertilizer. Mature, established specimens require minimal water. The amount of pruning depends on the plant use and location within the landscape. It is excellent as a hedge, screen or barrier planting to hide irrigation control or other moderate sized structures. Lower branches can be pruned to develop a “small tree” forms that are ideal as individual specimens or in small groupings with deciduous trees and shrubs. This shrub is grown worldwide, as the roots have soil-binding properties which are effective for the purpose of stabilizing sand dunes and to control erosion. In North America it grows native in California, Arizona, Florida and Hawaii. In the southwest it is primarily used as an accent shrub/small tree, single specimen or as a screening planting. Cold damage can occur at 20 degrees F. on young plants. A purple leaf variety, native to Australia, is produced for retail sales but these selections are not as cold hardy.
Dodonaea: Immature White Seed Pod