Scientific: Mariosousa willardiana (Synonyms: Acacia willardiana, Mariosousa heterophylla)
Common: palo blanco
Family: Fabaceae (Leguminosae)
Origin: Rocky slopes and canyons of the southern Sonoran Desert south of Hermosillo, Sonora, Mexico.
Taxonomic confusion: The current botanical name of palo blanco is "officially" recognized as either Mariosousa willardiana or Mariosousa heteorphylla, depending on the botanical resource. I ascribe to what is recognized by ITIS as its official scientific name, in this case Mariosousa willardiana.
Pronounciation: Mar-i-o-so-U-sa wil-lar-dee-A-na
Hardiness zones
Sunset 13
USDA 9-11 (arid zones only)
Landscape Use: Specimen accent, broad, open entryways, or anywhere in desert gardens or xeric landscapes where close inspection of its exquisite trunk character can be seen.
Form & Character: Multiple or single (with training) trunk tree, open, airy, refined, intimate, cascading, fountain like.
Growth Habit: Semi-evergreen, woody, perennial narrowleaf small tree, upright, basally branching, and pendulous to 20 feet in height, slender when young but gracefully spreading with age.
Foliage/Texture: Long phyllodes, 10- to 20-inches long with occasional, ephemeral, small, pinnately compound leaves at the terminal ends of the phyllodes. The trunks and large branches and stems of palo blanco have beautiful white-colored bark that sheds like paper; fine texture.
Flowers & Fruits: Branched terminal stalks of cream-colored, powder puff, ball-like flowers; fruits are small brown pods.
Seasonal Color: Creamy white flowers in spring.
Temperature: Tolerant to 25oF.
Light: Must have full sun.
Soil: A rocky or sandy, porous, well-drained soil is best.
Watering: Little to no water needed once established, although supplemental water in summer and early fall will encourage growth and canopy density.
Pruning: Remove unwanted basal suckers. To encourage and preserve the beauty of the trunk character of palo blanco, rigorously train structurally when it is young so as to avoid having to make large unsightly pruning cuts when the tree gets older.
Propagation: Seed
Disease and Pests: None
Additional comments: Palo blanco is spindly, rangy and open when young, but will eventually (after 10 to 15 years) become a graceful and beautiful small tree (usually multi-trunk) with a pendulant (weeping) habit - be patient! This is truly a great, small tree with year-round accent for any Sonoran Desert style or water-conserving garden in lower desert elevations. Because palo blanco is such a wonderful visual accent tree, I recommend it more for use as an accent and/or focal point in the landscape, which logistically means that within the appropriate landscape context designers should specify fewer rather than more of them. In other words, use palo blanco in the landscape with restraint. Like anything in life, too much repetition of a good thing is going to dull the senses....and give one that "stuffed" feeling.
Neat Freak Warning: This unassuming Sonoran Desert native can make a real mess. The amount of litter shed from its sleek frame is actually quite shocking.
