There are four species of green-barked trees: the Blue palo verde, the Foothills palo verde, the Mexican palo verde, and the Palo brea.
The Blue and Foothills palo verdes are considered native to southern Arizona; the Mexican palo verde inhabits the border region with Mexico, but there are plenty of them in the Tucson area, probably initially introduced. The Palo brea is native to central Sonora, Mexico, but it is used for landscaping, especially in Phoenix (watch out for the low-hanging spiny branches). All of these trees are drought deciduous which means they are leafless in dry times. Chlorophyll in the bark enables them to carry out photosynthesis even while leafless.
The first to bloom will be the Blue palo verdes. They are beginning to flower now. The Blues tend to inhabit dry washes because they need just a little more water than the Foothills variety. The Blues are multi-branched trees that can grow up to 40 feet high. The branches tend to droop. The bark, especially young bark, is bluish-green. The leaves are about one-quarter inch long and occur as two to four leaves along the two feather-like stalks or veins. These trees stop growing after about 30 years and may die long before reaching 100.
The Foothills palo verdes will bloom about two weeks later than the Blues. They tend to have straight upright branches with a yellowish-green color. The trees are usually about 15 feet high but can get up to 40 feet. The leaves are about one-tenth of an inch long with about three to five pairs of leaves along leaf stem. The life span is 100 years or more.
The Mexican palo verdes bloom as the Foothills are finishing and can bloom through August. The Mexican palo verdes also have upright branches, but have drooping long stems which hold tiny leaves. The trees are very fast growing (up to 8 feet per year) and can get up to 40 feet high. Leaves are about the same size as the Foothills and consist of 10 to 40 pairs along the leaf stem which can be up to a foot long. The life span is about 30 years.
Palo breas are small trees (25 feet) like the Foothills, but with leaves similar in size to the Blue palo verde. The green bark is relatively dark and granular.
The yellow flowers of the Palo verde trees are five-petalled. One petal, called the banner petal, stands out above the plane of the other four petals. In the Blue palo verde, the banner petal is yellow and sometimes has orange spots. The Foothills palo verde has a white banner petal. The Mexican palo verde may have a yellow to red banner petal or it may have red or orange spots. The Palo brea flowers are similar to those of the Blue palo verde, but form close to the branch making the branches look like yellow rods.
With a little practice, you can learn to identify the trees by the flowers and leaf size. But just to make things interesting, they form natural hybrids with each other and so sport a combination of characteristics.
The bean pods of the Blue palo verde are flat and the beans resemble small lima beans. They have a slight bitter taste when green. The pods generally contain one or two beans. The pods of the Foothills palo verde contain two to four navy-bean sized beans separated by a very narrow pod neck. These beans are very sweet when picked fresh and eaten raw; the taste is similar to snow peas. The pods of the Mexican palo verde usually contain three to five beans or more. The beans have a slight bitter taste when raw. After a few weeks, the beans of all three species dry out and become hard. They were often harvested in this condition and later ground into meal.
Palo brea pods resemble those of the Blue palo verde, flat. The Palo brea (tar stick) exudes a waxy substance from its trunk. Native Americans used it for glue.
These trees are bee pollinated and you can sometimes see as many as 20 species of bees at the tree at one time. (There are more than 1,000 species of bees in southern Arizona.) The staggered schedule of blooms lessens the competition for pollinators.
Now for a physics lesson.
Ever wonder why desert plants, especially palo verde trees, have such tiny leaves compared to broad leaf trees such as maples and eastern oaks? That trait is called microphylly and the Foothills palo verde (Parkinsonia mycrophyllum) is the extreme example. Desert plants must maintain an internal temperature of less than 115 degrees Fahrenheit to survive. The size of their leaves helps them do that. A group of many small leaves have a larger surface area than a single large leaf of equal volume. This large surface area helps the plants lose heat by radiation. Also, in the wind, a large leaf will tend to develop a boundary layer of stagnant air. With small leaves, the boundary layer air does not develop, so the leaves can radiate heat without transpiration, even in relatively calm air.
And, by the way, the Palo Verde is the Arizona State Tree.
You can see many good photographs of all the palo verdes (and many other things) at the Digital Library maintained by the Arizona-Sonora Desert Museum.
Copyrighted by Jonathan DuHamel. Reprint is permitted provided that credit of authorship is provided and linked back to the source.