Oak Trees in the Desert
In the deserts of the American Southwest, some species of oak trees handle the summertime heat, seasonal droughts and native soils well. Mary Irish, author of "Arizona Gardener's Guide" mentions that established oak trees -- those with expansive root systems -- tolerate drought but benefit from deep waterings weekly in summer and monthly in winter. Native oak species need less irrigation, perhaps none, in the cooler high-elevation deserts except in prolonged drought and heat in summer.
The Emory's oak (Quercus emoryi) is an evergreen tree native from Arizona to Texas and northern Mexico. Its leafy canopy becomes rounded, 40 feet wide and 50 feet tall. The leathery, dark green leaves are oval and sharply teethed, resembling holly foliage. Its acorns are edible. "Sunset Western Garden Book" deems the Emory's oak best suited to the low desert elevations and tolerant of a wide array of well-drained soils. It appreciates periodic deep irrigation in summertime.
Escarpment Live Oak
A closely related cousin to the southern live oak, escarpment live oak (Q. fusiformis) hails from a more westerly range on the plains of New Mexico and western and central Texas. An evergreen oak with a dense crown of leathery dark green leaves, the foliage yellows and briefly drops in spring before the new leaves emerge. This species grows 30 to 40 feet tall and 25 to 35 feet wide and thrives in alkaline soils (pH above 7.0). "Sunset Western Garden Book" states: "easily endures heat, winter cold from Phoenix to Albuquerque." In "Arizona Gardener's Guide," Irish mentions that the cultivar 'Heritage' is worthwhile to grow in Arizona landscapes.
A slow growing plant, the Gambell oak (Q. gambellii) is also known as the Rocky Mountain white oak. It attains a large shrub to small tree form 15 to 30 feet tall and half as wide, rarely ever growing up to 50 feet tall although possible. If grown in a deep, fertile soil, Gambell oak grows much more quickly. The green leaves turn yellow, orange or red in autumn before dropping off and its sharply tipped acorns look woolly. An adaptable tree to various soils and moisture, Gambell oak is the "characteristic oak of Arizona's Oak Creek Canyon and the Colorado foothills south of Denver" according to the "Sunset Western Garden Book."
Look for green leaves with three to five bristle-tipped lobes on the Texas or Spanish oak (Q. texana). Low branches near the ground create a tree up to 35 feet tall and up to 45 feet wide in its native landscapes in dry parts of central and western Texas. It may look shrub-like and it loses its leaves in autumn after becoming scarlet red. The acorns from Texas oak occur in a densely felted cap colored rust red. Grow this oak species all across the American Southwest, especially in alkaline soils.
Desert Scrub Oak
More like a plump shrub, desert scrub oak (Q. turbinella) remains evergreen but typically gets no larger than 10 feet tall and 10 to 12 feet wide. If lowest branches are pruned away, it looks more like a small tree. Dull, yellow-green leaves that resemble holly foliage line the gray-barked branches. This oak species is tough, tolerating drought, heat and substantial winter cold all across the American West. It's native to the desert mountains from southern California to Colorado and western Texas. It grows alongside pinyon junipers.