Three species of green-barked trees, the Palo Verdes, are common in Southern Arizona. Their yellow flowers turn the desert golden in the Spring. (See: Palo Verde trees about to turn the desert golden) There is a fourth green-barked tree called the Palo Brea (Parkinsonia praecox) which is native to central Sonora, Mexico. It does not occur naturally in Arizona, but it is widely used as an ornamental plant, especially in Phoenix. See photos from the Arizona-Sonora Desert Museum: https://www.desertmuseum.org/programs/alamos_trees_parpra.php
This tree is called “brea” rather than “verde” because its bark exudes a light-colored pitch or tar, a waxy coating, which is used as glue and can provide a sweet snack.
The ethnobotany of the Palo Brea is described in the book: “Mayo Ethnobotany: Land, History, and Traditional Knowledge in Northwest Mexico” by David Yetman, Thomas Van Devender, and Thomas R. Van Devender.
The Palo Brea is “A green-barked, bushy tree up to 8 m tall with numerous scraggly branches. It has a rather longer trunk and longer, more irregular branches than P. florida [Blue Palo Verde] and is the leafiest of the palo verdes. The bark is a distinctive light green that is often speckled with a whitish crystalline exudate.” The Palo Brea is dominant in coastal and foothills thornscrub of the tropical deciduous forest of Mexico. A subspecies also grows in Argentina where is commonly gets up to 20 feet tall and is more frost tolerant than the Mexican variety.
“It flowers profusely in March and April, the petals a brilliant dark yellow, the stamens red. When the persistent pods of the fruits ripen, they give the tree a reddish orange hue.”
According to Yetman et al., “The sap is used as a remedy for arthritis, diabetes, bronchitis, and asthma, and is eaten as a sweet as well….A tea brewed from the bark is said to be good for bruises, sprains, and strains but supposedly turns urine dark. ”
Soap was made by burning the roots and mixing the ash in a tub with a pig carcass and water. After a few days of cooking, the liquid was poured into clay molds where it would harden.
Domestic animals, especially goats, ate the beans and bark.
Palo Breas can form natural hybrids with the other Palo Verde species.
Palo Brea gum has been investigated as a possible food additive for humans. (Source) Palo Brea gum is chemically similar to gum arabic which is widely used as a food stabilizer.
For stories about other desert plants, see: