Palo Verdes “Green Sticks” of the Ajo Area
Story and Photos by Hank Jorgenson
When we first came down to Ajo after retiring I knew nothing about Sonoran Desert plants except for the Saguaro and Organ Pipe cacti. That suddenly changed as we bought a house with a giant Blue Palo Verde (Parkinsonia florida) in the back yard. As I became more acquainted with desert plants, I learned there was another native paloverde, the Foothill Palo Verde (Parkinsonia microphylla).
The two Palo Verdes native to the United States are so similar that when those who see them at first usually have trouble with identification. However, there are distinct differences that facilitate accurate identification. Blue Palo Verde is the larger of the two. A mature Blue Palo Verde often reaches 30 to 40 feet, whereas the Foothill Palo Verde is 15 to rarely 30 feet tall.
Branches of the mature Blue Palo Verde emerge from a trunk that is at least several feet high. Trunks of the Foothill Palo Verde are rarely more than one or two feet long. Trunks of the Blue Palo Verde eventually develop a rough gray bark; whereas the trunks of the Foothill Palo Verde usually remain smooth and green through most of their lifetime (Figure 1).
Figure 1. Foothill Palo Verde Bark
The green color in trunks and branches of Palo Verde is due to chlorophyll which allows some photosynthesis to occur when leaves are dropped or scarce. Actually it is estimated that ¾ of the plant’s food for growth and respiration is produced by photosynthesis in the stems rather than the tiny leaves.
Blue Palo Verde develops long slender terminal branches that give the tree a drooping habit. Foothill Palo Verdes develop more of an upright “normal” tree habit (Figure 2).
Figure 2. Structure of two species of Palo Verde
Like all of the species in this family, leaves on the Palo Verdes are compound. The leaves are bipinnate, but the petiole is so short as to almost not be there (Figure 3). Individual leaflets on the Foothill Palo Verde are tiny, 1 to 2 mm long and 6 to 7 mm on the Blue Palo Verde. If the trees have leaves, using leaflet measurements is the most foolproof means of identification. The leaflets lack hairs.
Figure 3. Leaf struture of Foothill and Blue Palo Verde
Flowers on both species are about ¾ to 1 inch across with five petals. Petals on Blue Foothill flowers are uniformly bright yellow with bright orange anthers. Petals on Foothill Palo Verde trees are pale yellow with white areas, and anthers are also bright orange (Figure 4).
Figure 4. Flowers of Foothill and Blue Palo Verde
Flowering of Blue Palo Verde trees normally begins from late March into April while Foothill Palo Verde starts about 2 weeks later than the Blue. This year Blue Palo Verde flowering started extremely early, with some blossoms opening in early March.
Temperatures this spring have been unusually warm to hot, presumably due to global climate change, and we are seeing the effects of this on plant phenology. Long term changes in plant phenology have profound effects on the timing of arrival of pollinating insects and birds.
Palo Verde blossoms are pollinated mostly by certain bee species (Centris pallida (Figure 5), other Centris species and other bee species), but I have seen very few bees (mostly honeybees) so far this spring to pollinate the blossoms that are present now on the Blue Palo Verde tree in our yard.
Figure 5. Native Bee Pollinator, Centris pallida
There have been some Funereal Duskywing and Painted Lady butterflies (Figure 6) going to the blossoms.
Figure 6. Funereal Duskywing Butterfly
Fruits of both Palo Verdes are typical pods of the legume family. Those of the Blue Palo Verde are short (2-3 inches long) typically containing 2 flattened very hard seeds (Figure 7).
Figure 7. Seed pod and seeds of Palo Verde
These seeds will not germinate without scarification by animals or by weathering of the seed coat after long exposure to the elements. Pods of the Foothill species are 2 to 4 inches long and constricted between the seeds. The seeds are somewhat football shaped and are easily broken.
Consumption of Blue Palo Verde seeds is accomplished mainly by Javelinas, whose loud crunching of the hard seeds can be heard for great distances. The only natives who made use of the seeds were the Seri who developed a way to grind them into flour.
Both Palo Verdes are distributed across pretty much the whole American Sonoran Desert and south along Baja and the west coast of Mexico. Blue Palo Verde is absent in the northern half of Baja, while Foothill Palo Verde is found along the east coast of that area.
Of the two species, Blue Palo Verde has higher moisture requirements than Foothill Palo Verde and is mostly found along medium and large washes that receive run-in water during storms. Foothill Palo Verde is the dominant tree on steep slopes, and is also common on bajadas away from watercourses. |
