Mexican Palo Verde

Parkinsonia aculeata

The Mexican Palo Verde could well be the best tree for our future, given the increasingly hotter and drier climate that is predicted for Santa Barbara. It is a remarkably hardy tree that can tolerate heat of 120 degrees and cold down to 18 degrees. For water-conserving landscapes, it provides the quintessential look of a desert tree. In gardens that feature drought-tolerant succulents and cacti, it serves as a perfect companion.

This tree’s most distinctive features are its gorgeous flower display, its young green bark, and its interesting foliage.

Bright-yellow and pleasantly fragrant flowers bloom profusely in late spring and early summer; these can continue, intermittently, through October. The small flowers (¾ inch) form near the ends of branches, in clusters of 2 to 15. Each flower has five petals, with one that can either be orange or have orange spots. The sweet nectar attracts bees. After pollination, leathery fruit forms in the shape of narrow seed pods (1 to 4 inches long) that turn light brown with maturity. Each pod is constricted around 1 to 6 seeds, which birds and mammals find tasty and nutritious.

The tree’s young bark is unusual, in that it is green in color and can perform photosynthesis. When it ages, the bark will turn gray and could develop rough bumps.

The flat leaves are pendulous and 4 to 12 inches long. They are technically bipinnately compound, having 30 to 60 tiny (1/8 inch long) oval leaflets arranged in 2 rows. The yellow-green twigs zigzag and, at each node, bear 1 to 3 small (¼- to ¾-inch long) spines that will persist on the twigs when the leaflets drop off during extremes of hot, dry, or cold weather.

Without human intervention, it will develop naturally as a large shrub. However, it can be pruned to develop into a small- to medium-sized tree that will carry an open, airy, and rounded crown (15 to 25 feet tall with an equal spread), making it ideal for residential gardens. Its wispy and gracefully arching branches, particularly when covered with its lovely flowers, create a delightful accent in informal gardens.

The Mexican Palo Verde is native to regions of Mexico, Arizona, South America, and even the Galapagos Islands. It is now widespread throughout the warmer regions of the world. While it does seed out vigorously and, as a result, has been declared an invasive pest in some countries in the Southern Hemisphere, this problem has not been observed in our Mediterranean climate. The common name, Palo Verde, means “green stick” in Spanish, referring to the green bark on young trunks and stems. Other common names include “Paloverde”, “Jerusalem Thorn”, “Horsebean”, “Retama”, and “Barbados Fence Flower”.

Its botanical name is Parkinsonia aculeata. The genus name, Parkinsonia, honors John Parkinson (1567-1650), an English botanical author, who was Apothecary to King James I and Royal Botanist to King Charles I. The specific epithet, aculeata, is derived from the Latin word, “aculeatus”, meaning “prickly”, referring to the all-too-apparent spines.

The Mexican Palo Verde is easy to grow and requires little maintenance. It prefers a hot location with full sun. It can be planted in a wide range of soil types, including clay, alkaline and saline soils, though it does better in sandy well-drained soils. It will grow faster and larger with regular irrigation but, once established, it does quite well with our sparse annual rainfall and with drought. It can be trained as a multiple- or single-trunked tree. Only minor pruning is needed for thinning and shaping. It is essentially pest- and disease-free and does not require fertilizing. In moister coastal areas, it can suffer from powdery mildew on its foliage—though this usually clears up with warmer weather.

A thornless and sterile hybrid of Parkinsonia, named ‘Desert Museum’, is commercially available but, due to its susceptibility to mildew, should only be planted in hotter foothill and interior valley locations. Mexican Palo Verde has historically been harvested and used in many ways, including for firewood, charcoal, livestock fodder, and mulch. In Mexico, the leaves are brewed as a tea to treat fever and epilepsy. It has been planted and trained to become a living fence, a windbreak, an effective soil stabilizer, or a privacy screen. Its lovely flowering branches are used in dramatic floral arrangements.

In our community, the Mexican Palo Verde’s beauty, size, and sustainability make it an excellent choice to plant as a street tree, a public park tree, a home garden or patio tree, a tree for desert gardens, and as a tree for locations where other trees will not flourish—or even survive. Mature specimens of Mexican Palo Verde can be seen near the northeast corner of Alice Keck Park Memorial Garden, in the 2900 block of State Street, 1543 Portesuello Avenue, and in the 100 block of East De La Guerra Street.

Those who wish to honor a special someone can do so with an attractive commemorative marker that will be installed at the base of an existing street tree in the City of Santa Barbara. Because Santa Barbara Beautiful has participated in the planting of over 13,000 street trees, there are plenty of trees from which to choose! Application forms are available on the Santa Barbara Beautiful website.

Tree-of-the-Month articles are sponsored by Santa Barbara Beautiful, whose many missions include the increase of public awareness and appreciation of Santa Barbara’s many outstanding trees and, in a long-time partnership with the City Parks & Recreation Department, the funding and planting of trees along the City’s streets – a project which has resulted in the planting, to date, of more than 13,000 street trees.

Article and photos by David Gress