Shade is welcome most of the year in the Southwest, but too much shade can limit other options. Trees with large leaves can cast dense shade, and when their leaves drop, they can be difficult to remove from their desert companions. However, many ornamental plants that thrive in the Southwest actually appreciate the dappled shade cast by the light open canopy of desert trees. This provides ideal conditions for succulents to breed. Hello, Avonim, AquariaAnd Agaveas well as flowers like cacti Echinopsis Hybrid
The best trees for underplanting are those with small leaves that die off as they fall. Increased winter sunlight through their open branches is ideal for understory plants, but a light canopy can also moderate winds and provide some protection from light frost.
Here are some favorites.
Velvet mesquite provides pollinator-friendly flowers in the spring.
From native New Mexico deep into central California and Mexico, velvety mesquite (Prosops velotina, zones 9–11) may form widespread bosques where groundwater is accessible, or may be found as scattered individuals in small desert washes. A prized wildlife tree and a reliable source of nectar for bees, it has a sweet scent that wafts through the garden in spring. The pods fall in mid-summer and are easy to pick. The canopy is not dense, and most leaves fall in mid-winter, allowing winter sun to keep the understory plants happy. Long-lived and deep-rooted, its strong arching branches can host native desert mistletoe (Phoradendron californicum, zones 8–11), a favorite food of the glossy black Phaenopapula bird. A velvet mesquite tree in zones 9 through 11 is expected to grow to about 25 feet tall and wide and will not require irrigation once established.
The small-leaved palo verde is an excellent tree for small gardens.
Small Leaf Palo Verde (Parkinsonia microphylla.Zones 9–10) is a small tree that grows on rocky hillsides in Arizona, Baja, Sonora, and small parts of California and is easily recognized by its smooth shiny green bark. During severe drought its young leaves disappear, but its bark continues to photosynthesize. Stronger and longer-lived than the related blue palo verde (Parkinsonia FloridaZones 8-11), it grows slowly to a height of 12 feet or more and is suitable for small gardens. Spring flowers turn pale lemon yellow each April in zones 9 and 10. It is the perfect complement to native desert plants.
‘Maverick’ Thornless Honey Mesquite is an excellent drought tolerant tree
This species is widespread throughout the Southwest, as a small tree in good condition, becoming more shrubby at the edge of its range. ‘Maverick’ thornless honey mesquite (Prosopis glandulosa ‘Maverick’, zones 7-11) is a selection that retains all the desirable characteristics of the species: fragrant spring flowers, gracefully weeping branch tips, light green pinnate leaves, and an airy canopy. About 30 feet tall and wide, durable, and very drought tolerant, it thrives in our area. Once the young leaves drop in late autumn, the plants below are bathed in mild winter sun.
Desert willow serves up waves of flowers in summer.
If you live in the Southwest, you’re likely familiar with desert willow (Chilopsis linearis, zones 7-11) already. From Texas to southern Utah and central California, this species is often found along desert washes, where it tolerates drought, and many desert landscapes use this tree to full advantage. Numerous cultivars have been named over the years, and most were selected for low seed production, but the flowers are what win people over. Fragrant trumpet clusters appear in waves during the summer in burgundy and white to pink colors, a favorite of hummingbirds and other pollinators. The leaves are narrow and willow-like, disappearing into the landscape when they fall in late fall. The canopy is usually straight and open, up to 18 feet long and 12 feet wide, providing downlight to plants during the summer and allowing more sun in the winter. Naturally occurring in zones 7-11, some are known to thrive in the sheltered microclimates of zones 5 and 6, where they are worth the extra effort.
Palo Blanco is a beautiful tree with curling papery bark.
Palo Blanco (Mariusosa Willardianasyn. Acacia willardiana(zones 9–11) is one of my favorite trees, with a graceful form that provides the most delicate shade—just enough so that choice cacti and succulents, and even flowering shrubs like the Texas Ranger (Leucophyllum frutescenszones 7–11) and emu bush (Eremophila maculata, zones 8–11). Its modified leaves (phyllodes) are long and slender, sparsely arranged, and flutter in a light breeze. The 2-inch flowers are fuzzy, cream-colored catkins near the ends of drooping branches in spring. These beautiful features complement ghost bark, smooth white and tan, curling into papery folds and oddly reminiscent of paper birch (Betula papyrifera, zones 2–7). Looking especially good when planted in small groups, slender stems can reach 20 feet or more tall, with a loosely spreading canopy 10 or 12 feet wide. Resistant to extreme drought but somewhat tender to severe frost, this tree thrives in warm microclimates.
—Dan Johnson lives and gardens in Denver and Tucson, Arizona. He is the associate director of horticulture at the Denver Botanic Gardens.