Arizona State Tree

Foothill Palo Verde and Blue Palo Verde

In 1954, the State of Arizona named the Palo Verde as its State Tree. The legislature did not distinguish between the two species of this tree that are native to the State; therefore both the Foothill and the Blue share the honor.

Both the Blue and Foothill Palo Verdes have a number of characteristics in common, starting with the reason they are called Palo Verdes (Spanish for green stick). The name indicates that these trees can use the chlorophyll in their bark to photosynthesize and produce sugar. In general, three quarters of their food is metabolized through the bark, whereas only a quarter is produced by the leaves.

In terms of growth form, they both start with short crooked trunks or multiple thinner trunks that branch close to the ground. The Foothill’s branches tend to be stiff and upright and its crown is open and irregular, maturing usually at less than 30’. The Blue tends to have large drooping branches, and maturing to 40 feet tall with a round crown.

Foothill Palo Verde

Blue Palo Verde

Leaves

These small leaves (Foothill smaller than Blue) are twice-pinnate and grow and drop throughout the year in response to rainfall, drought, or cold. In prolonged droughts, trees may drop branches to conserve water.

The Blue blooms first in March/April with brilliant bright yellow flowers of 5 petals; the banner petal showing red-orange spots.

The Foothill bloom begins later, in April/May. Its 5-petaled flower is more subdued in pale yellow with a white banner petal.

The Blue Palo Verde (so named for the bluish-green bark and foliage) grows faster than the Foothill. Requiring more water to support its speedy growth, its life is fairly short (30-50 years in the wild). It is a dry riparian species and is found over its range primarily in desert washes, but is also used as a landscape tree.

The Foothill Palo Verde (sometimes called Yellow because of the yellow-green bark and foliage), prefers rocky slopes and gravelly flats. The Foothill grows more slowly but lives longer, up to 200 years or more in the wild. The range for both includes southern Arizona, Sonora and Baja, Mexico.

Pollination

Pollination results in seed pods that have provided centuries of sustenance for humans and animals. The pods of Foothill Palo Verde contain seeds separated by constrictions. These beans are very sweet and can be eaten raw, tasting much like snow peas. The Blue Palo Verde pods are flatter with fewer seeds per pod. These beans have a slightly bitter taste.

Blue Palo Verdes are much harder and require scarification for germination; the Foothill tree produces more abundant and tender seed crops that do not require scarification. Germination being easier, Foothill Palo Verdes survival on bajadas, plains, and rocky hillsides is more assured. Palo Verde seeds were a food source for the Hohokam people. They ground dry seeds in mortars to make flour. Today dry seeds can be sprouted and then roasted. When green, they can be blanched and eaten raw.

Foothill Palo Verde

Blue Palo Verde

Palo Verdes serve as the nurse plants for the saguaro, giving shade and protection to seedlings in their slow growth to becoming giants. Plant diversity encourages diversity of animals attracted to the food, shelter, and nesting sites provided by Palo Verde trees.

Palo Verde root borer beetles are three inches long, have wings and can fly. After mating, the adult females lay eggs in the soil near the roots of the Palo Verde tree. The larva or grub bore into the roots of the trees and chew on the live tissue. They may take 2-4 years to transform into an adult beetle.

The Foothill Palo Verde is a keystone species, especially in the Arizona Uplands subdivision, (known as a Palo Verde/Saguaro forest) of the Sonoran desert. As a hub of life, these trees create microhabitats that are critical to the structure and function of the Sonoran desert scrub biotic community.

 

Photos courtesy of Katherine Darrow, Carole DeAngeli, Buzz Hoffmann, Marianne Skov Jensen, Hank Jorgensen and Michael McNulty and the ASDM Digital Library. Text by Gay Gilbert.

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