What it means
Palo verde is Spanish for green stick, and that is the tree: a lacy, many-branched desert legume whose trunk and limbs are green because they photosynthesize, with leaves so small and so often absent that the bark does most of the work. It is Arizona’s state tree, the most common tree in Scottsdale’s natural desert and its medians alike, and the reason the whole Valley goes gold for three weeks every April. The desert plants field guide calls it “the green one,” which is how newcomers learn it, and warns pool owners about April, which is how they remember it.
Locals say the name two ways, PAH-lo VAIR-day with a Spanish ear or palo VER-dee with an Arizona one. The word also sits on a nuclear power plant west of Phoenix, a dozen streets and a hospital, which can make a sentence about “palo verde” briefly confusing.
Where you’ll meet it
Everywhere. Blue palo verdes line the washes and the irrigated medians of McCormick Ranch and Shea; foothill palo verdes cover the slopes of the Preserve and shade the young saguaros on the flats out of Brown’s Ranch. Both are labeled at the Xeriscape Garden, and both are in your skimmer basket in April.
Where the word came from
Spanish-speaking travelers named the tree for the one thing you cannot miss about it, and the name crossed into English unchanged, sometimes squeezed into one word, paloverde, by botanists. The scientific names have been less settled. For most of the twentieth century the trees were Cercidium; they are now Parkinsonia, and the two Arizona natives are Parkinsonia florida, the blue palo verde, and Parkinsonia microphylla, the foothill or littleleaf palo verde. Nursery tags still say Cercidium.
The state tree designation came in 1954, and the legislature, as the Arizona Native Plant Society likes to point out, did not say which palo verde it meant. The two species are easy to tell apart once you look. The blue palo verde has blue-green bark, larger leaves, big bright yellow flowers with a red-orange spotted banner petal, and a drooping, rounded crown that can reach 40 feet. It grows fast, wants the extra water of a wash or a sprinkler line, and lives 30 to 50 years. The foothill palo verde has yellow-green bark, smaller leaves and flowers with a white banner petal, stiff upright branches under 30 feet, and a slow life on rocky slopes that can pass 200 years. Blue blooms first, in March and April; foothill follows in April and May. In Scottsdale that means the yards and medians go yellow a couple of weeks before the mountainsides do.
The bark is the biology lesson. About three quarters of the tree’s food is made in its green stems, which lets it shed its leaves at the first sign of drought and keep working. The Akimel O’odham and other desert peoples ate the beans fresh and ground the dried seeds into flour, and the tree’s shade is where saguaros begin: a bird eats the fruit, perches in a palo verde, and the seed that falls there has the only chance it will get.
What it is not
Not a mesquite, which has dark bark and real shade, and not an ironwood, which is gray and evergreen. The thornless “Desert Museum” palo verde in resort courtyards is a hybrid bred for patios, not a wild species. And the yellow tree blooming in February is probably an acacia or a cassia, a month early for the real thing.
The fine print
The pool problem is real and seasonal. A mature blue palo verde drops thousands of small flowers over three weeks, then seed pods in early summer, and both float. The first-year guide is right that the cure is placement: plant over gravel, away from the water, and accept skimmer duty on the ones the builder put there. Allergy sufferers can relax somewhat; the pollen is heavy and mostly bee-carried, and the yellow film on the car is petals.
Pruning is where most yard palo verdes are ruined. The trees grow fast on irrigation and develop long, heavy limbs; cheap crews respond by topping them, which produces a thicket of weak shoots that snap in July outflow winds. The right move is light thinning after bloom and deep, infrequent watering so the roots go down instead of out. A palo verde on a lawn sprinkler schedule grows twice as fast and falls over twice as often.
In the wild the foothill palo verde is the workhorse of the saguaro country on the McDowell slopes, and its nurse role is why saguaro stands are thickest where the palo verdes are. The blue palo verde keeps to washes and bajadas where its roots reach water. Both go leafless in a dry spring and look dead, then leaf out and bloom on the first good rain; a wet December and January means a heavy April, the same rule that governs the wildflowers. Neither species climbs much above 4,000 feet; cold, not heat, draws the line.