Looking like a frozen column of smoke rising improbably from an austere desert wash, it is obvious that Smoke Trees are aptly named. With the unique arrangement of dense, ash gray branches and short-lived gray leaves the tree has an unmistakable billowy appearance that draws the eye to its unusual form and color. Smoke Tree (Psorothamnus spinosus) is native to dry, sandy washes in southwestern Arizona and southeastern California at elevations below 400 to 500 feet. In these desert conditions it matures to a height of anywhere from 12' to 20' with a spread approaching 8' to 12'.

Smoke Tree, also called Smoke Thorn Dalea and Corona-de-Cristo, is highly drought tolerant and best adapted to decomposed granite or sandy soils that are well-drained and very low in organic matter. It tolerates full sun, grows slowly even under ideal conditions and is hardy to 20 to 25 degrees F. Small gray leaves, 1/4 to ½ inch long, are short lived and generally appear for a few weeks in response to fall and winter rains. The form and texture of the canopy are created by the repeated, forked branching of the tree as it grows. Branches and branchlets are sharp tipped and stiff. The vast majority of photosynthesis is carried out by these branchlets with the chlorophyll found just below a surface layer of dense gray hairs.

In stark contrast to the generally subdued appearance of this tree, it produces the most remarkable flowers. Small, dark purple to deep blue pea-like flowers, up to ½" long, appears in clusters at or near branch ends. Emerging from the purple flower petals are bright orange anthers. Flowers can lend a brief, brilliant blue cast to the appearance of the tree. Flowers appear from June to July with buds developing in April and May. Flowers are followed by single seeded pods. Smoke Trees bring a unique and delicate texture and remarkable flowers into the landscape.

It works well as a single accent or specimen tree, in groupings, as a transitional tree back to native desert on the perimeters of the landscape or to cast elaborate shadows against structures or walls. It offers strong contrast to dense, green, canopied, desert tree species and blends well with cacti, boulders and hardscape elements. Locate Smoke Trees within the landscape with care and appreciate that they require little or no supplemental water (one study reported that under green house conditions Smoke Trees survived on four irrigations in 2 ½ years). Understory planting or adjacent trees and shrubs that require regular irrigation will be incompatible. Prune trees for shape or to enhance natural form and texture.

Geoffroea decorticans has two common names, Chanar and Chilean Palo Verde. Chilean Palo Verde possibly comes from the mottled green color of the trunks but does not seriously resemble Cercidium. It is found in a number of semi-arid regions of Bolivia, Brazil, Patagonia and Argentina. Chanar is found growing naturally from sea level to over 7,000 feet where it may grow as a shrub or to a tree nearly 30 feet tall. Trees tend to be quite upright with a spreading canopy with both straight and mildly curving trunks. Leaves are cold and drought deciduous. It tends to occur in areas with high water tables or near permanent sources of water.

As trees mature the trunks and branches take on a sculptural quality with long longitudinal, irregular ridges and valleys. Along with this undulating trunk, large flakes of the bark peel off or decorticate (hence the species name decorticans). The peeling tan to brown bark is eventually shed revealing the dark green, "immature" trunk beneath. The contrasting colors and textures created by this puzzle-piece pattern make the tree visually fascinating.

Small, papery, pale yellow flowers are produced in spring, either singly or in clusters. Geoffroea is unique among legumes in that it produces fleshy, oval pods that hold a single seed. Fruit are initially green but turn orange brown as they mature. Seeds and fruit are valued as human and animal feed and the yellow wood is suitable for carpentry and furniture making.

Cold hardy to at least 15 degrees, the tree has grown successfully for many years at the Boyce Thompson Arboretum (Superior, AZ) and at the Desert Legume Program in Tucson, AZ. Leaves are small and dispersed and pale gray green in color. Twigs are rigid, gray green and end in a sharp thorn. Trees can spread by root sprouts to form thickets. In wide-open areas or on the landscape perimeter this may be a desirable quality. In more confined settings sprout growth can be eliminated by pruning and by avoiding over irrigating trees.