Cercidium Hybrids: Desert Museum and 'AZT'

Nothing heralds the arrival of the desert spring (and in some years 100 degree temperatures) as brilliantly and conspicuously as the blooming of the Palo Verdes. With a succession that includes Blue (Cercidium floridum), Foothill, Mexican and Sonoran, Palo Verdes color the desert a brilliant yellow from spring to early summer. Other than Mexican Palo Verde (Parkinsonia aculeata) all the Palo Verdes are popular and widely used landscape trees. All these species are so closely related botanically that hybridization between species is quite common in native and nursery environments. In the past this hybridization went largely unnoticed or was mistaken for genetic seed variation within a single specie. The potential for hybrids to offer advantages like increased vigor, greater cold hardiness, enhanced flowering and absence of thorns eventually brought them to the attention of horticultural professionals. 

In the late 1970's Mark Dimmitt with the Arizona Sonora Desert Museum (ASDM) began noticing Blue Palo Verdes that exhibited characteristics suggesting they were hybrids of other Palo Verde species. He collected and planted seeds from the assorted trees he had observed and began evaluating them. By 1981 he had identified a thornless seedling as clearly superior to the others collected. Careful evaluation of the genetic composition of this hybrid, named 'Desert Museum', revealed it to be a complex hybrid having genetic characteristics from Mexican, Blue and Foothill Palo Verde. Dimmitt suspects that 'Desert Museum' gets it vigorous growth, sturdy, upright branching habit and bright flowers from P. aculeate, and its small delicate leaves from the Cercidium species. Trees have tolerated temperatures of 15 degrees without damage in Tucson. The most remarkable and unique feature of this hybrid is the absence of thorns. Flowers are slightly larger than those of P. aculeata and other Cercidiums and trees have been observed to flower abundantly as early as mid-March in southern Arizona with intense, full bloom lasting into late spring and early summer. Intermittent flowering can continue into the mid to late fall. Pruning and staking is best done during the growing season when the trees limbs are more flexible. The v-shapes branching, thornless growth and ample shade make Desert Museum a pedestrian friendly desert adapted tree that could be used in a wide array of landscape applications. 

The introduction of the Desert Museum to the landscape market in 1987 encouraged many other landscape professionals to look for other Cercidium hybrids that exhibited phenotypes that would expand the desert landscape palette. Cercidium hybrid 'AZT', a thornless Palo Verde hybrid, exhibits qualities found in Palo Brea, Blue and Mexican Palo Verde. The canopy provides ample shade with lush, green leaves. Trees are thornless and produce brilliant yellow flowers abundantly in spring and intermittently during the summer months and occasionally in the fall. Similar to the Palo Brea, the branches and trunks of variety 'AZT' remain a smooth lime green as they mature. Branches are arranged in a graceful candelabra form. The orientation and angles of branches are also a bit reminiscent of Palo Brea making the trees excellent single specimens, theme trees, street and perimeter planting and can be dramatically up-lit at night. 

Both trees have what are called twice compound leaves. Each leaf petiole is subsequently subdivided into pinnae with leaflets attached to these pinnea. 'Desert Museum' has one to two pairs of pinnea per petiole with 10 to 16 pairs of leaflets per pinnae. 'AZT' has 4 to 6 pairs of pinnae with 15 to 24 pairs of leaflets per pinnae. The tiny leaflets generate minimal leaf litter and easily vanish into most rock surface mulches. These highly divided leaves produce a canopy that it lush yet airy and provides filtered shade that promotes flowering of under-story shrubs and groundcovers. Both 'Desert Museum' and 'AZT' are effective and dramatic accent or "focal point" specimen trees, ideal for streetscape and parking lot plantings or anywhere a desert look and filtered shade are needed.

'Desert Museum' and 'AZT' are propagated from branching cuttings (cloned) as they do not produce true to type from seed collected from the tree. In recent years cloning techniques have been developed that facilitate large-scale production of the tree but, between the high demand for these trees and the nature of the propagation method, the supply of these trees is limited. Several wholesale growers in Arizona currently propagate the 'Desert Museum' with the guidance and permission of the ASDM. 'AZT' is propagated and available exclusively from Arid Zone Trees. 

Given the limited number of these trees planted in landscapes, additional study and observation will to needed to determine the appropriateness of either tree for planting in turf areas and for assessing how deciduous they are. Preliminary observations suggest that the "Desert Museum' is 80 to 100% winter deciduous in the colder areas of metropolitan Phoenix and at the Boyce Thompson Arboretum in Superior, AZ. 'AZT' retains anywhere from 30 to 60% of its leaves in winter depending on the heat retention of the site and the severity and duration of low temperatures. It has been suggested, based on the adaptability of P. aculeate, that the growth of these Cercidium x Parkinsonia hybrids would not be adversely affected by planting in turf areas. Initial observation of trees planted in turf are encouraging but more time will be needed to fully evaluate the advisability of planting these trees in turf.