Update on our palo blanco tree

A reader recently asked how the palo blanco tree (Mariosousa heterophylla) in our front yard was doing. Time for an update!

First of all, in spite of the similar name, the palo blanco is not related to the palo verde. “Palo” simply means “stick” in Spanish, so one is “white stick,” the other “green stick.” The palo blanco (Mariosousa heterophylla) is a slow-growing acacia native to the Mexican state of Sonora where it grows on hot rocky slopes. The palo verde (Parkinsonsia sp.) is a group of fast-growing trees in the pea family native to semi-desert regions from Arizona south to Argentina; the ‘Desert Museum’ hybrid has become a popular street tree, not only in Arizona and Southern California, but increasingly up here in Northern California.

I fell in love with the palo blanco years ago when I first saw it at the Desert Botanical Garden in Phoenix, Arizona. After I became aware of its existence, I started to notice it all over the place, both in the Phoenix and the Tucson areas. 

In early 2016, we removed the front lawn and replaced it with a waterwise landscape heavy on succulents and other dryland plants. To qualify for a rebate from the State of California, we had to plant a tree. And I knew exactly what that would be: a palo blanco. With its wispy presence and fairly narrow growth habit, it wouldn’t overpower the space the way a palo verde would have.

Our palo blanco was grown by Mountain States Wholesale Nursery in Arizona and purchased from the Ruth Bancroft Garden Nursery. It came in a 15-gallon container and was about 6 ft. tall when I brought it home:

November 2015

When we planted it, it was so diaphanous, you could barely see it:

February 2016

Fast forward to now, July 2023:

Our palo blanco is a good 12 ft. tall (its mature size is expected to be 20 ft.) and has grown the way I had hoped for:

In fact, it’s become the centerpiece of the front yard even though it’s so light and airy:

The trunk does justice to its name, “white stick:”

The bark peels off in scrolls, more so in the fall and winter than now:

Catkin-like flowers appear any time between late spring and summer (I took this photo a couple of days ago):

The palo blanco drops its leaves in response to cold and drought. Since our tree is in a bed that receives drip irrigation every 10 days, it retains its leaves throughout the summer.

I love our palo blanco as much now as I did originally. That says a lot, considering how often my favorites change.

Most sources list the palo blanco as hardy to 25°F, but renowned Tucson landscape designer Scott Calhoun says on the University of Arizona Campus Arboretum website that “in my experience, individual seed-grown trees exhibit variations in hardiness with several specimens I’ve planted enduring a handful of nights in the mid-teens.” Our palo blanco hasn’t been subjected to temperatures below 26°F and it’s been perfectly fine, other than dropping its leaves.

Here are some photos of palo blancos in Arizona. You’ll see why I became so enamored with this tree in the first place:

Arizona-Sonora Desert Museum, Tucson

Arizona-Sonora Desert Museum, Tucson

Scottsdale Xeriscape Demonstration Garden, Scottsdale

Desert Botanical Garden, Phoenix

Leafless against the winter sky

If you want to see more photos of palo blancos, check out these posts on my blog, especially this one.

In contrast to palo verdes, which are now available in nurseries in Northern California (Green Acres in Sacramento carries the ‘Desert Museum’ hybrid), palo blancos are still hard to find up here. If you’re interested, you may have to do some looking. Folks in Arizona are lucky; there, pretty much every nursery has them. Note that in the nursery trade the palo blanco still goes by its old name, Acacia willardiana.

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Note on taxonomy for readers interesting in that sort of thing: For 150 years, the botanical name of the palo blanco was Acacia willardiana. However, in the wake of the 17th International Botanical Congress held in Vienna, Austria in 2005, the genus Acacia underwent a fiercely contested split that still ruffles feathers now. The 1,000+ Australian species (usually referred to as “wattles”) retained the genus name Acacia while the approx. 160 thorny species from Africa, Asia and Latin America became the genus Vachellia, and the remaining 200 species from Africa, Asia and Latin America were split into the genera Senegalia, Mariosousa and Acaciella. For a detailed accounting of these developments, see Kull, C.A. and Rangan, H. (2012). “Science, sentiment and territorial chauvinism in the acacia name change debate” (PDF here).

© Gerhard Bock, 2023. All rights reserved. To receive all new posts by email, please subscribe here.