The remarkable diversity exhibited by a number of desert region landscape tree species has been attributed to the ability of these trees to cross pollinate or hybridize. Natural hybridization and cross pollination among closely related desert tree species is quite common. One of the most popular hybrids used in the landscape today is the Thornless Mesquite. By definition, a hybrid is a offspring of two plants of different varieties, species or genera.

Newly recognized hybrids, particularly those among the Palo Verdes, have shown potential as landscape trees and have been brought into commercial production by nurseries. Each hybrid is unique, with its own combination of leaflet number and arrangement, leaf size and flowering habit. Palo Verde hybrids gaining acceptance are those that incorporate the smooth bark characteristics of Cercidium praecox with multiple leaflet leaves similar to those found on Parkinsonia aculeata.

Other characteristics of interest are extended flowering season and increased flower size and number. Hybrids exhibit growth and horticultural characteristics appear similar to the Blue and Sonoran Palo Verde in commercial production but have a more lush, denser leaf canopy that provides greater shade.

Seeds collected from these hybrid trees do not typically produce trees that have the same qualities and appearance as the hybrid. For this reason hybrid Palo Verdes are usually propagated using vegetative method (rooted cuttings, tissue culture). Difficulty in developing appropriate and highly productive propagation methods is probably the single factor limiting the number of hybrids currently available for sale. Because of this unique combination of attractive characteristics, it is likely that a number of Palo Verde "hybrids" will become part of the arid landscape tree palette.

Another addition to the "traditional" arid landscape palette has been the introduction of a number of Desert Willow varieties that offer larger, more colorful flowers. Desert Willows are among a very small group of Sonoran desert natives that produces flowers during the summer months. Most native Desert Willow have limited numbers of small, pale to off white, orchid shaped flowers. Through efforts of individual growers and University horticultural departments, an assortment of varieties have been introduced. The efforts to improve desert willow have focused on selecting trees with larger, more abundant, richly colored flowers. These new varieties offer flowers that range in color from a showy white to pale pink to deep purple, some with multiple colored patterns. Along with flowers, growers are selecting willows with more lush and uniform foliage.

Like Palo Verdes these new varieties do not produce reliably from seed. In contrast with the Palo Verdes, Deserts Willow propagate from cuttings with relative ease making it easier to bring the new selections into commercial production.

The experience with Palo Verdes and Desert Willows demonstrates that the desert landscape palette can be expanded by continuing to look for hybrids or unique selections of already popular species. Evaluation and selection is an ongoing process that shows promise for bringing greater beauty and variety to arid landscape designs.