I’ve never seen an Acacia willardiana that wasn’t beautiful, no matter its age.  A fine old specimen by the Earle Herbarium at Desert Botanical Garden casts its delicate shade over a bed of aloes.  The chalky-white color of the trunk and branches is acknowledged by both the English and Spanish common names: Whitebark Acacia and Palo Blanco (white stick).   Even the young tree that you pass after you pay admission to the DBG displays the striking white bark, already starting to peel off in the characteristic papery flakes.  Everything about Palo Blanco is graceful, including the fluffy, elongated flower clusters of a cream color, which bloom in April and May.  The tree has a narrow form, reaching a height of 20 feet and a spread of 10 feet, making it a good choice for planting in narrow spaces.  I’ve done just that in my home landscape, putting a mini-grove of three trees in the strip between my driveway and the neighbor’s yard.  I’m glad to see Palo Blanco being used in roadway medians because it won’t outgrow its space and suffer the fate of other trees whose canopies reach into the street, only to be “UPS-pruned.”

Another tree of the same genus - Acacia stenophylla - shares some of the characteristics of the Palo Blanco, though it hails from half-way around the world in Australia.  Shoestring Acacia has a tall, slender form and its wispy foliage is set a-swaying by even the lightest breeze.  The common name refers to the long, slender leaves which are evergreen.  Cream-colored, ball-shaped flowers appear in spring and occasionally fall.  The pods are sharply constricted between individual seeds, looking much like a string of beads.   Some beautiful specimens, which are near their mature height of 30 feet and spread of 15 feet, can be seen in the medians near the intersection of Scottsdale and McDowell Roads.  In contrast to those Scottsdale median trees which have been allowed to grow naturally, is the ultimate in tree torture inflicted on Shoestring Acacias just a stone’s throw away in Papago Plaza.  I can only guess that the property manager of this shopping center ordered all the trees to be flat-topped so the store signs would be more visible.  The height of Shoestring Acacias (un-pruned ones, that is) makes them great trees for planting around multi-story buildings.  They also combine well with trees such as Palo Brea, where the contrast between vertical and horizontal canopy forms creates interest.

A third species of Acacia, this one from the Chihuahuan Desert of north-eastern Mexico, has more than enough character to be used as an accent tree in the landscape.  Twisted Acacia (Acacia schaffneri) is particularly striking when its twisted, angular branches can be silhouetted against a plain background.  A nice example of this effect can be seen on the east side of the Gentle Strength Coop building in Tempe, at University and Ash.  The lacy, medium green foliage is clustered tightly around the twisted branches, giving the effect of giant green pipe cleaners that someone mangled into a variety of shapes.  Golden ball-shaped flowers appear about the same time the deciduous foliage reappears in spring.  I like the soft fuzziness of the young seed pods, which eventually turn brown and woody.  This medium-sized tree typically reaches a height of 15 feet with a spread of 20 feet.  It may be that the thorny branches offer some protection, or maybe the fluffy foliage is inviting, but it seems like the Twisted Acacia in my back yard is the favorite of the birds.  Most recently a Verdin constructed its shaggy nest near the end of a twisted branch.

I’ll admit to never having used Geoffroea decorticans in a design, but this distinctive tree deserves greater exposure so I’m going to do my part and spec it into projects whenever appropriate.  At a glance, Chilean Palo Verde looks like a Blue Palo Verde or Palo Brea, its overall shape and size similar to those two kin in the bean family.  A closer look reveals some differences: the trunk of Chilean Palo Verde develops deep longitudinal furrows with age and the tan to brown bark flakes off in irregular patches to reveal green sections beneath - hence the “Palo Verde” part of the name.   During the flowering season in spring, a whiff of the fragrant blossoms tells you that this is no North American Palo Verde.   Instead of a seed pod, Chilean Palo Verde has a fleshy one-seeded fruit which ripens to a brown color and reportedly is edible.   The small, pale green leaves drop in cold and extreme drought, revealing short spines along the branches.   Because Chilean Palo Verde’s distinctive features might not be noticed at first glance, I would use it where people could appreciate its character up close.  The tree could become the anchor to a patio landscape.  Elevating the tree to a raised planter would place at eye level its best feature - the trunk - while raising the canopy off the ground enough to mostly eliminate the need for pruning to keep the branches out of pedestrians’ way.  Geoffroea may, in some locations, produce sprouts or suckers from the roots.  If there is room in the landscape, these young trees can be allowed to develop into a thicket (great barrier planting!) or you can cut back the sprouts.  Boyce Thompson Arboretum(Superior, AZ) and Desert Botanical Garden (Phoenix, AZ) both have mature specimens of Chilean Palo Verde.

The Sonoran Desert’s most majestic tree (in my opinion) is Ironwood.  It doesn’t have the flashy bloom of Palo Verdes or the lush green-ness of Velvet Mesquite, but its branches show the character gained through years - decades, really - of deliberate growth, and the pastel pink-lavender flowers offset beautifully by gray-green foliage create a tree that surely commands attention.  Ironwood tree doesn’t grow quickly, neither does it just sit there.  Compared to similarly-sized Blue Palo Verdes planted in my front yard four years ago, the Ironwood tree has grown about 2/3 as fast.  When mature, it should reach a height and spread of about 25 feet.  Like many other desert trees, Ironwood is typically multi-trunked with branches low to the ground.  It can be allowed to develop naturally, or be pruned up into a more traditional tree form, which is no simple task thanks to an abundance of stout, ½ inch spines.  Ironwood drops a relatively small amount of litter, making it a good choice to shade patios, even at poolside.  As an evergreen tree, Ironwood serves well to anchor the landscape through all seasons and its dramatic character can create a focal point in entries and other high-visibility areas.

Texas Ebony’s tropical appearance belies a tough, drought tolerant constitution, which suits it for a role in freeway and streetside landscaping as well as residential and commercial landscape settings.  The beautiful specimens you see along the Superstition Freeway have been allowed to grow naturally, their branches brushing the ground.  With some pruning Texas Ebony makes a fine patio tree.  Its mature size of 15 feet high and as wide fits easily into even relatively small residential spaces.  Just be sure that the thorny branches won’t cause a hazard.  Fuzzy, cream-colored flowers bloom in spring and are followed by ornamental woody pods an inch wide and several inches long.  Texas Ebony’s botanical name, Pithecellobium flexicaule, is so much fun to say (once you’ve mastered the pronunciation).

There are occasions when a deciduous tree is called for, and Honey Mesquite (Prosopis glandulosa) is one of the most attractive choices.  The bright green, lacy foliage casts a nice shade and is visually cooling during summer’s heat;  in winter, when the sun’s warmth is welcome, the bare branches allow the rays to shine through.  Spring-flowering annuals and perennials such as Penstemon, Betony and Salvia thrive under the conditions created by Honey Mesquite: plenty of sunlight during the growing season and filtered shade to help the plants endure summer.  Several magnificent Honey Mesquite highlight the entry of an apartment complex called Acacia Creek on the west side of Scottsdale road, a few blocks south of Shea. Arid Zone Trees is growing a thornless selection which will be especially useful in places such as parks and playgrounds where people and plants mingle.

I think one of the best ways to get people to notice new landscape plants is to put them along the freeway.  A grouping of Cascalote (Caesalpinia cacalaco) trees on the 202 just east of Sky Harbor Airport has certainly caught my eye.  And the plants could almost stop traffic when they’re in bloom, not only because of the showy yellow flower spikes standing above deep green   foliage, but also the timing of the bloom (winter) when the rest of the landscape is relatively quiet.  Of course, at 60+ miles per hour you can’t see the rose-like thorns that arm the branches, but these shouldn’t deter people from using Cascalote as a specimen patio tree or as a backdrop and companion plant to spring and summer-blooming shrubs.  You don’t have to dodge speeding cars to get a close-up look at Cascalote.  Just visit the Mesa Community College Xeriscape Demonstration Garden where several specimens are nearing their mature size of 10 feet high and 15 feet wide.  Another good place to check out Cascalote trees and many others growing in a landscape setting is AZT’s nursery, where the demonstration garden designed by Steve Martino gets better each year.

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